HotFreeBooks.com
The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
by Charles Darwin
1  2  3  4  5  6  7  8  9  10  11     Next Part
Home - Random Browse

THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM.

BY

CHARLES DARWIN, M.A., F.R.S., ETC.



CONTENTS.

CHAPTER I.

INTRODUCTORY REMARKS.

Various means which favour or determine the cross-fertilisation of plants.—Benefits derived from cross-fertilisation.—Self-fertilisation favourable to the propagation of the species.—Brief history of the subject.—Object of the experiments, and the manner in which they were tried.—Statistical value of the measurements.—The experiments carried on during several successive generations.—Nature of the relationship of the plants in the later generations.—Uniformity of the conditions to which the plants were subjected.—Some apparent and some real causes of error.—Amount of pollen employed.—Arrangement of the work.—Importance of the conclusions.

CHAPTER II.

CONVOLVULACEAE.

Ipomoea purpurea, comparison of the height and fertility of the crossed and self-fertilised plants during ten successive generations.—Greater constitutional vigour of the crossed plants.—The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals.—The effects of a cross with a fresh stock.—The descendants of the self-fertilised plant named Hero.—Summary on the growth, vigour, and fertility of the successive crossed and self-fertilised generations.—Small amount of pollen in the anthers of the self-fertilised plants of the later generations, and the sterility of their first-produced flowers.—Uniform colour of the flowers produced by the self-fertilised plants.—The advantage from a cross between two distinct plants depends on their differing in constitution.

CHAPTER III.

SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.

Mimulus luteus; height, vigour, and fertility of the crossed and self-fertilised plants of the first four generations.—Appearance of a new, tall, and highly self-fertile variety.—Offspring from a cross between self-fertilised plants.—Effects of a cross with a fresh stock.—Effects of crossing flowers on the same plant.—Summary on Mimulus luteus.—Digitalis purpurea, superiority of the crossed plants.—Effects of crossing flowers on the same plant.—Calceolaria.—Linaria vulgaris.—Verbascum thapsus.—Vandellia nummularifolia.—Cleistogene flowers.—Gesneria pendulina.—Salvia coccinea.—Origanum vulgare, great increase of the crossed plants by stolons.—Thunbergia alata.

CHAPTER IV.

CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.

Brassica oleracea, crossed and self-fertilised plants.—Great effect of a cross with a fresh stock on the weight of the offspring.—Iberis umbellata.—Papaver vagum.—Eschscholtzia californica, seedlings from a cross with a fresh stock not more vigorous, but more fertile than the self-fertilised seedlings.—Reseda lutea and odorata, many individuals sterile with their own pollen.—Viola tricolor, wonderful effects of a cross.—Adonis aestivalis.—Delphinium consolida.—Viscaria oculata, crossed plants hardly taller, but more fertile than the self-fertilised.—Dianthus caryophyllus, crossed and self-fertilised plants compared for four generations.—Great effects of a cross with a fresh stock.—Uniform colour of the flowers on the self-fertilised plants.—Hibiscus africanus.

CHAPTER V.

GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.

Pelargonium zonale, a cross between plants propagated by cuttings does no good.—Tropaeolum minus.—Limnanthes douglasii.—Lupinus luteus and pilosus.—Phaseolus multiflorus and vulgaris.—Lathyrus odoratus, varieties of, never naturally intercross in England.—Pisum sativum, varieties of, rarely intercross, but a cross between them highly beneficial.—Sarothamnus scoparius, wonderful effects of a cross.—Ononis minutissima, cleistogene flowers of.—Summary on the Leguminosae.—Clarkia elegans.—Bartonia aurea.—Passiflora gracilis.—Apium petroselinum.—Scabiosa atropurpurea.—Lactuca sativa.—Specularia speculum.—Lobelia ramosa, advantages of a cross during two generations.—Lobelia fulgens.—Nemophila insignis, great advantages of a cross.—Borago officinalis.—Nolana prostrata.

CHAPTER VI.

SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.

Petunia violacea, crossed and self-fertilised plants compared for four generations.—Effects of a cross with a fresh stock.—Uniform colour of the flowers on the self-fertilised plants of the fourth generation.—Nicotiana tabacum, crossed and self-fertilised plants of equal height.—Great effects of a cross with a distinct sub-variety on the height, but not on the fertility, of the offspring.—Cyclamen persicum, crossed seedlings greatly superior to the self-fertilised.—Anagallis collina.—Primula veris.—Equal-styled variety of Primula veris, fertility of, greatly increased by a cross with a fresh stock.—Fagopyrum esculentum.—Beta vulgaris.—Canna warscewiczi, crossed and self-fertilised plants of equal height.—Zea mays.—Phalaris canariensis.

CHAPTER VII.

SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED PLANTS.

Number of species and plants measured.—Tables given.—Preliminary remarks on the offspring of plants crossed by a fresh stock.—Thirteen cases specially considered.—The effects of crossing a self-fertilised plant either by another self-fertilised plant or by an intercrossed plant of the old stock.—Summary of the results.—Preliminary remarks on the crossed and self-fertilised plants of the same stock.—The twenty-six exceptional cases considered, in which the crossed plants did not exceed greatly in height the self-fertilised.—Most of these cases shown not to be real exceptions to the rule that cross-fertilisation is beneficial.—Summary of results.—Relative weights of the crossed and self-fertilised plants.

CHAPTER VIII.

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS.

Greater constitutional vigour of crossed plants.—The effects of great crowding.—Competition with other kinds of plants.—Self-fertilised plants more liable to premature death.—Crossed plants generally flower before the self-fertilised.—Negative effects of intercrossing flowers on the same plant.—Cases described.—Transmission of the good effects of a cross to later generations.—Effects of crossing plants of closely related parentage.—Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions.

CHAPTER IX.

THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS.

Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner.—Fertility of the parent-plants when first crossed and self-fertilised, and of their crossed and self-fertilised offspring when again crossed and self-fertilised.—Comparison of the fertility of flowers fertilised with their own pollen and with that from other flowers on the same plant.—Self-sterile plants.—Causes of self-sterility.—The appearance of highly self-fertile varieties.—Self-fertilisation apparently in some respects beneficial, independently of the assured production of seeds.—Relative weights and rates of germination of seeds from crossed and self-fertilised flowers.

CHAPTER X.

MEANS OF FERTILISATION.

Sterility and fertility of plants when insects are excluded.—The means by which flowers are cross-fertilised.—Structures favourable to self-fertilisation.—Relation between the structure and conspicuousness of flowers, the visits of insects, and the advantages of cross-fertilisation.—The means by which flowers are fertilised with pollen from a distinct plant.—Greater fertilising power of such pollen.—Anemophilous species.—Conversion of anemophilous species into entomophilous.—Origin of nectar.—Anemophilous plants generally have their sexes separated.—Conversion of diclinous into hermaphrodite flowers.—Trees often have their sexes separated.

CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they can.—Cause of this habit.—Means by which bees recognise the flowers of the same species.—Sudden secretion of nectar.—Nectar of certain flowers unattractive to certain insects.—Industry of bees, and the number of flowers visited within a short time.—Perforation of the corolla by bees.—Skill shown in the operation.—Hive-bees profit by the holes made by humble-bees.—Effects of habit.—The motive for perforating flowers to save time.—Flowers growing in crowded masses chiefly perforated.

CHAPTER XII.

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation injurious.—Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation avoided.—The benefits and evils of the two processes depend on the degree of differentiation in the sexual elements.—The evil effects not due to the combination of morbid tendencies in the parents.—Nature of the conditions to which plants are subjected when growing near together in a state of nature or under culture, and the effects of such conditions.—Theoretical considerations with respect to the interaction of differentiated sexual elements.—Practical lessons.—Genesis of the two sexes.—Close correspondence between the effects of cross-fertilisation and self-fertilisation, and of the legitimate and illegitimate unions of heterostyled plants, in comparison with hybrid unions.

INDEX.

...

THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.

CHAPTER I.

INTRODUCTORY REMARKS.

Various means which favour or determine the cross-fertilisation of plants. Benefits derived from cross-fertilisation. Self-fertilisation favourable to the propagation of the species. Brief history of the subject. Object of the experiments, and the manner in which they were tried. Statistical value of the measurements. The experiments carried on during several successive generations. Nature of the relationship of the plants in the later generations. Uniformity of the conditions to which the plants were subjected. Some apparent and some real causes of error. Amount of pollen employed. Arrangement of the work. Importance of the conclusions.

There is weighty and abundant evidence that the flowers of most kinds of plants are constructed so as to be occasionally or habitually cross-fertilised by pollen from another flower, produced either by the same plant, or generally, as we shall hereafter see reason to believe, by a distinct plant. Cross-fertilisation is sometimes ensured by the sexes being separated, and in a large number of cases by the pollen and stigma of the same flower being matured at different times. Such plants are called dichogamous, and have been divided into two sub-classes: proterandrous species, in which the pollen is mature before the stigma, and proterogynous species, in which the reverse occurs; this latter form of dichogamy not being nearly so common as the other. Cross-fertilisation is also ensured, in many cases, by mechanical contrivances of wonderful beauty, preventing the impregnation of the flowers by their own pollen. There is a small class of plants, which I have called dimorphic and trimorphic, but to which Hildebrand has given the more appropriate name of heterostyled; this class consists of plants presenting two or three distinct forms, adapted for reciprocal fertilisation, so that, like plants with separate sexes, they can hardly fail to be intercrossed in each generation. The male and female organs of some flowers are irritable, and the insects which touch them get dusted with pollen, which is thus transported to other flowers. Again, there is a class, in which the ovules absolutely refuse to be fertilised by pollen from the same plant, but can be fertilised by pollen from any other individual of the same species. There are also very many species which are partially sterile with their own pollen. Lastly, there is a large class in which the flowers present no apparent obstacle of any kind to self-fertilisation, nevertheless these plants are frequently intercrossed, owing to the prepotency of pollen from another individual or variety over the plant's own pollen.

As plants are adapted by such diversified and effective means for cross-fertilisation, it might have been inferred from this fact alone that they derived some great advantage from the process; and it is the object of the present work to show the nature and importance of the benefits thus derived. There are, however, some exceptions to the rule of plants being constructed so as to allow of or to favour cross-fertilisation, for some few plants seem to be invariably self-fertilised; yet even these retain traces of having been formerly adapted for cross-fertilisation. These exceptions need not make us doubt the truth of the above rule, any more than the existence of some few plants which produce flowers, and yet never set seed, should make us doubt that flowers are adapted for the production of seed and the propagation of the species.

We should always keep in mind the obvious fact that the production of seed is the chief end of the act of fertilisation; and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation, than by the union of the sexual elements belonging to two distinct flowers or plants. Yet it is as unmistakably plain that innumerable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are adapted for catching prey; or that the plumes, wings, and hooks of a seed are adapted for its dissemination. Flowers, therefore, are constructed so as to gain two objects which are, to a certain extent, antagonistic, and this explains many apparent anomalies in their structure. The close proximity of the anthers to the stigma in a multitude of species favours, and often leads, to self-fertilisation; but this end could have been gained far more safely if the flowers had been completely closed, for then the pollen would not have been injured by the rain or devoured by insects, as often happens. Moreover, in this case, a very small quantity of pollen would have been sufficient for fertilisation, instead of millions of grains being produced. But the openness of the flower and the production of a great and apparently wasteful amount of pollen are necessary for cross-fertilisation. These remarks are well illustrated by the plants called cleistogene, which bear on the same stock two kinds of flowers. The flowers of the one kind are minute and completely closed, so that they cannot possibly be crossed; but they are abundantly fertile, although producing an extremely small quantity of pollen. The flowers of the other kind produce much pollen and are open; and these can be, and often are, cross-fertilised. Hermann Muller has also made the remarkable discovery that there are some plants which exist under two forms; that is, produce on distinct stocks two kinds of hermaphrodite flowers. The one form bears small flowers constructed for self-fertilisation; whilst the other bears larger and much more conspicuous flowers plainly constructed for cross-fertilisation by the aid of insects; and without their aid these produce no seed.

The adaptation of flowers for cross-fertilisation is a subject which has interested me for the last thirty-seven years, and I have collected a large mass of observations, but these are now rendered superfluous by the many excellent works which have been lately published. In the year 1857 I wrote a short paper on the fertilisation of the kidney bean (1/1. 'Gardeners' Chronicle' 1857 page 725 and 1858 pages 824 and 844. 'Annals and Magazine of Natural History' 3rd series volume 2 1858 page 462.); and in 1862 my work 'On the Contrivances by which British and Foreign Orchids are Fertilised by Insects' appeared. It seemed to me a better plan to work out one group of plants as carefully as I could, rather than to publish many miscellaneous and imperfect observations. My present work is the complement of that on Orchids, in which it was shown how admirably these plants are constructed so as to permit of, or to favour, or to necessitate cross-fertilisation. The adaptations for cross-fertilisation are perhaps more obvious in the Orchideae than in any other group of plants, but it is an error to speak of them, as some authors have done, as an exceptional case. The lever-like action of the stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by which the anthers are depressed and rubbed on the backs of bees, shows as perfect a structure as can be found in any orchid. Papilionaceous flowers, as described by various authors—for instance, by Mr. T.H. Farrer—offer innumerable curious adaptations for cross-fertilisation. The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful as that of the most wonderful orchid. The stamens, according to Fritz Muller, are irritable, so that as soon as a moth visits a flower, the anthers explode and cover the insect with pollen; one of the filaments which is broader than the others then moves and closes the flower for about twelve hours, after which time it resumes its original position. (1/2. 'Botanische Zeitung' 1866 page 129.) Thus the stigma cannot be fertilised by pollen from the same flower, but only by that brought by a moth from some other flower. Endless other beautiful contrivances for this same purpose could be specified.

Long before I had attended to the fertilisation of flowers, a remarkable book appeared in 1793 in Germany, 'Das Entdeckte Geheimniss der Natur,' by C.K. Sprengel, in which he clearly proved by innumerable observations, how essential a part insects play in the fertilisation of many plants. But he was in advance of his age, and his discoveries were for a long time neglected. Since the appearance of my book on Orchids, many excellent works on the fertilisation of flowers, such as those by Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter papers, have been published. (1/3. Sir John Lubbock has given an interesting summary of the whole subject in his 'British Wild Flowers considered in relation to Insects' 1875. Hermann Muller's work 'Die Befruchtung der Blumen durch Insekten' 1873, contains an immense number of original observations and generalisations. It is, moreover, invaluable as a repertory with references to almost everything which has been published on the subject. His work differs from that of all others in specifying what kinds of insects, as far as known, visit the flowers of each species. He likewise enters on new ground, by showing not only that flowers are adapted for their own good to the visits of certain insects; but that the insects themselves are excellently adapted for procuring nectar or pollen from certain flowers. The value of H. Muller's work can hardly be over-estimated, and it is much to be desired that it should be translated into English. Severin Axell's work is written in Swedish, so that I have not been able to read it.) A list would occupy several pages, and this is not the proper place to give their titles, as we are not here concerned with the means, but with the results of cross-fertilisation. No one who feels interest in the mechanism by which nature effects her ends, can read these books and memoirs without the most lively interest.

From my own observations on plants, guided to a certain extent by the experience of the breeders of animals, I became convinced many years ago that it is a general law of nature that flowers are adapted to be crossed, at least occasionally, by pollen from a distinct plant. Sprengel at times foresaw this law, but only partially, for it does not appear that he was aware that there was any difference in power between pollen from the same plant and from a distinct plant. In the introduction to his book (page 4) he says, as the sexes are separated in so many flowers, and as so many other flowers are dichogamous, "it appears that nature has not willed that any one flower should be fertilised by its own pollen." Nevertheless, he was far from keeping this conclusion always before his mind, or he did not see its full importance, as may be perceived by anyone who will read his observations carefully; and he consequently mistook the meaning of various structures. But his discoveries are so numerous and his work so excellent, that he can well afford to bear a small amount of blame. A most capable judge, H. Muller, likewise says: "It is remarkable in how very many cases Sprengel rightly perceived that pollen is necessarily transported to the stigmas of other flowers of the same species by the insects which visit them, and yet did not imagine that this transportation was of any service to the plants themselves." (1/4. 'Die Befruchtung der Blumen' 1873 page 4. His words are: "Es ist merkwurdig, in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur die Pflanzen selbst gesucht werden musse.")

Andrew Knight saw the truth much more clearly, for he remarks, "Nature intended that a sexual intercourse should take place between neighbouring plants of the same species." (1/5. 'Philosophical Transactions' 1799 page 202.) After alluding to the various means by which pollen is transported from flower to flower, as far as was then imperfectly known, he adds, "Nature has something more in view than that its own proper males would fecundate each blossom." In 1811 Kolreuter plainly hinted at the same law, as did afterwards another famous hybridiser of plants, Herbert. (1/6. Kolreuter 'Mem. de l'Acad. de St. Petersbourg' tome 3 1809 published 1811 page 197. After showing how well the Malvaceae are adapted for cross-fertilisation, he asks, "An id aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo pulvere, sed semper eo aliarum suae speciei impregnentur, merito quaeritur? Certe natura nil facit frustra." Herbert 'Amaryllidaceae, with a Treatise on Cross-bred Vegetables' 1837.) But none of these distinguished observers appear to have been sufficiently impressed with the truth and generality of the law, so as to insist on it and impress their beliefs on others.

In 1862 I summed up my observations on Orchids by saying that nature "abhors perpetual self-fertilisation." If the word perpetual had been omitted, the aphorism would have been false. As it stands, I believe that it is true, though perhaps rather too strongly expressed; and I should have added the self-evident proposition that the propagation of the species, whether by self-fertilisation or by cross-fertilisation, or asexually by buds, stolons, etc. is of paramount importance. Hermann Muller has done excellent service by insisting repeatedly on this latter point.

It often occurred to me that it would be advisable to try whether seedlings from cross-fertilised flowers were in any way superior to those from self-fertilised flowers. But as no instance was known with animals of any evil appearing in a single generation from the closest possible interbreeding, that is between brothers and sisters, I thought that the same rule would hold good with plants; and that it would be necessary at the sacrifice of too much time to self-fertilise and intercross plants during several successive generations, in order to arrive at any result. I ought to have reflected that such elaborate provisions favouring cross-fertilisation, as we see in innumerable plants, would not have been acquired for the sake of gaining a distant and slight advantage, or of avoiding a distant and slight evil. Moreover, the fertilisation of a flower by its own pollen corresponds to a closer form of interbreeding than is possible with ordinary bi-sexual animals; so that an earlier result might have been expected.

I was at last led to make the experiments recorded in the present volume from the following circumstance. For the sake of determining certain points with respect to inheritance, and without any thought of the effects of close interbreeding, I raised close together two large beds of self-fertilised and crossed seedlings from the same plant of Linaria vulgaris. To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilised ones. Bees incessantly visit the flowers of this Linaria and carry pollen from one to the other; and if insects are excluded, the flowers produce extremely few seeds; so that the wild plants from which my seedlings were raised must have been intercrossed during all previous generations. It seemed therefore quite incredible that the difference between the two beds of seedlings could have been due to a single act of self-fertilisation; and I attributed the result to the self-fertilised seeds not having been well ripened, improbable as it was that all should have been in this state, or to some other accidental and inexplicable cause. During the next year, I raised for the same purpose as before two large beds close together of self-fertilised and crossed seedlings from the carnation, Dianthus caryophyllus. This plant, like the Linaria, is almost sterile if insects are excluded; and we may draw the same inference as before, namely, that the parent-plants must have been intercrossed during every or almost every previous generation. Nevertheless, the self-fertilised seedlings were plainly inferior in height and vigour to the crossed.

My attention was now thoroughly aroused, for I could hardly doubt that the difference between the two beds was due to the one set being the offspring of crossed, and the other of self-fertilised flowers. Accordingly I selected almost by hazard two other plants, which happened to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea purpurea, both of which, unlike the Linaria and Dianthus, are highly self-fertile if insects are excluded. Some flowers on a single plant of both species were fertilised with their own pollen, and others were crossed with pollen from a distinct individual; both plants being protected by a net from insects. The crossed and self-fertilised seeds thus produced were sown on opposite sides of the same pots, and treated in all respects alike; and the plants when fully grown were measured and compared. With both species, as in the cases of the Linaria and Dianthus, the crossed seedlings were conspicuously superior in height and in other ways to the self-fertilised. I therefore determined to begin a long series of experiments with various plants, and these were continued for the following eleven years; and we shall see that in a large majority of cases the crossed beat the self-fertilised plants. Several of the exceptional cases, moreover, in which the crossed plants were not victorious, can be explained.

It should be observed that I have spoken for the sake of brevity, and shall continue to do so, of crossed and self-fertilised seeds, seedlings, or plants; these terms implying that they are the product of crossed or self-fertilised flowers. Cross-fertilisation always means a cross between distinct plants which were raised from seeds and not from cuttings or buds. Self-fertilisation always implies that the flowers in question were impregnated with their own pollen.

My experiments were tried in the following manner. A single plant, if it produced a sufficiency of flowers, or two or three plants were placed under a net stretched on a frame, and large enough to cover the plant (together with the pot, when one was used) without touching it. This latter point is important, for if the flowers touch the net they may be cross-fertilised by bees, as I have known to happen; and when the net is wet the pollen may be injured. I used at first "white cotton net," with very fine meshes, but afterwards a kind of net with meshes one-tenth of an inch in diameter; and this I found by experience effectually excluded all insects excepting Thrips, which no net will exclude. On the plants thus protected several flowers were marked, and were fertilised with their own pollen; and an equal number on the same plants, marked in a different manner, were at the same time crossed with pollen from a distinct plant. The crossed flowers were never castrated, in order to make the experiments as like as possible to what occurs under nature with plants fertilised by the aid of insects. Therefore, some of the flowers which were crossed may have failed to be thus fertilised, and afterwards have been self-fertilised. But this and some other sources of error will presently be discussed. In some few cases of spontaneously self-fertile species, the flowers were allowed to fertilise themselves under the net; and in still fewer cases uncovered plants were allowed to be freely crossed by the insects which incessantly visited them. There are some great advantages and some disadvantages in my having occasionally varied my method of proceeding; but when there was any difference in the treatment, it is always so stated under the head of each species.

Care was taken that the seeds were thoroughly ripened before being gathered. Afterwards the crossed and self-fertilised seeds were in most cases placed on damp sand on opposite sides of a glass tumbler covered by a glass plate, with a partition between the two lots; and the glass was placed on the chimney-piece in a warm room. I could thus observe the germination of the seeds. Sometimes a few would germinate on one side before any on the other, and these were thrown away. But as often as a pair germinated at the same time, they were planted on opposite sides of a pot, with a superficial partition between the two; and I thus proceeded until from half-a-dozen to a score or more seedlings of exactly the same age were planted on the opposite sides of several pots. If one of the young seedlings became sickly or was in any way injured, it was pulled up and thrown away, as well as its antagonist on the opposite side of the same pot.

As a large number of seeds were placed on the sand to germinate, many remained after the pairs had been selected, some of which were in a state of germination and others not so; and these were sown crowded together on the opposite sides of one or two rather larger pots, or sometimes in two long rows out of doors. In these cases there was the most severe struggle for life among the crossed seedlings on one side of the pot, and the self-fertilised seedlings on the other side, and between the two lots which grew in competition in the same pot. A vast number soon perished, and the tallest of the survivors on both sides when fully grown were measured. Plants treated in this manner, were subjected to nearly the same conditions as those growing in a state of nature, which have to struggle to maturity in the midst of a host of competitors.

On other occasions, from the want of time, the seeds, instead of being allowed to germinate on damp sand, were sown on the opposite sides of pots, and the fully grown plants measured. But this plan is less accurate, as the seeds sometimes germinated more quickly on one side than on the other. It was however necessary to act in this manner with some few species, as certain kinds of seeds would not germinate well when exposed to the light; though the glasses containing them were kept on the chimney-piece on one side of a room, and some way from the two windows which faced the north-east. (1/7. This occurred in the plainest manner with the seeds of Papaver vagum and Delphinium consolida, and less plainly with those of Adonis aestivalis and Ononis minutissima. Rarely more than one or two of the seeds of these four species germinated on the bare sand, though left there for some weeks; but when these same seeds were placed on earth in pots, and covered with a thin layer of sand, they germinated immediately in large numbers.)

The soil in the pots in which the seedlings were planted, or the seeds sown, was well mixed, so as to be uniform in composition. The plants on the two sides were always watered at the same time and as equally as possible; and even if this had not been done, the water would have spread almost equally to both sides, as the pots were not large. The crossed and self-fertilised plants were separated by a superficial partition, which was always kept directed towards the chief source of the light, so that the plants on both sides were equally illuminated. I do not believe it possible that two sets of plants could have been subjected to more closely similar conditions, than were my crossed and self-fertilised seedlings, as grown in the above described manner.

In comparing the two sets, the eye alone was never trusted. Generally the height of every plant on both sides was carefully measured, often more than once, namely, whilst young, sometimes again when older, and finally when fully or almost fully grown. But in some cases, which are always specified, owing to the want of time, only one or two of the tallest plants on each side were measured. This plan, which is not a good one, was never followed (except with the crowded plants raised from the seeds remaining after the pairs had been planted) unless the tallest plants on each side seemed fairly to represent the average difference between those on both sides. It has, however, some great advantages, as sickly or accidentally injured plants, or the offspring of ill-ripened seeds, are thus eliminated. When the tallest plants alone on each side were measured, their average height of course exceeds that of all the plants on the same side taken together. But in the case of the much crowded plants raised from the remaining seeds, the average height of the tallest plants was less than that of the plants in pairs, owing to the unfavourable conditions to which they were subjected from being greatly crowded. For our purpose, however, of the comparison of the crossed and self-fertilised plants, their absolute height signifies little.

As the plants were measured by an ordinary English standard divided into inches and eighths of an inch, I have not thought it worth while to change the fractions into decimals. The average or mean heights were calculated in the ordinary rough method by adding up the measurements of all, and dividing the product by the number of plants measured; the result being here given in inches and decimals. As the different species grow to various heights, I have always for the sake of easy comparison given in addition the average height of the crossed plants of each species taken as 100, and have calculated the average height of the self-fertilised plant in relation to this standard. With respect to the crowded plants raised from the seeds remaining after the pairs had been planted, and of which only some of the tallest on each side were measured, I have not thought it worth while to complicate the results by giving separate averages for them and for the pairs, but have added up all their heights, and thus obtained a single average.

I long doubted whether it was worth while to give the measurements of each separate plant, but have decided to do so, in order that it may be seen that the superiority of the crossed plants over the self-fertilised, does not commonly depend on the presence of two or three extra fine plants on the one side, or of a few very poor plants on the other side. Although several observers have insisted in general terms on the offspring from intercrossed varieties being superior to either parent-form, no precise measurements have been given (1/8. A summary of these statements, with references, may be found in my 'Variation of Animals and Plants under Domestication' chapter 17 2nd edition 1875 volume 2 page 109.); and I have met with no observations on the effects of crossing and self-fertilising the individuals of the same variety. Moreover, experiments of this kind require so much time—mine having been continued during eleven years—that they are not likely soon to be repeated.

As only a moderate number of crossed and self-fertilised plants were measured, it was of great importance to me to learn how far the averages were trustworthy. I therefore asked Mr. Galton, who has had much experience in statistical researches, to examine some of my tables of measurements, seven in number, namely, those of Ipomoea, Digitalis, Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if we took by chance a dozen or score of men belonging to two nations and measured them, it would I presume be very rash to form any judgment from such small numbers on their average heights. But the case is somewhat different with my crossed and self-fertilised plants, as they were of exactly the same age, were subjected from first to last to the same conditions, and were descended from the same parents. When only from two to six pairs of plants were measured, the results are manifestly of little or no value, except in so far as they confirm and are confirmed by experiments made on a larger scale with other species. I will now give the report on the seven tables of measurements, which Mr. Galton has had the great kindness to draw up for me.

["I have examined the measurements of the plants with care, and by many statistical methods, to find out how far the means of the several sets represent constant realities, such as would come out the same so long as the general conditions of growth remained unaltered. The principal methods that were adopted are easily explained by selecting one of the shorter series of plants, say of Zea mays, for an example."

TABLE 1/1. Zea mays (young plants). (Mr. Galton.)

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed, as recorded by Mr. Darwin.

Column 3: Self-fertilised, as recorded by Mr. Darwin.

Column 4: Crossed, in Separate Pots, arranged in order of magnitude.

Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.

Column 6: Crossed, in a Single Series, arranged in order of magnitude.

Column 7: Self-fertilised, in a Single Series, arranged in order of magnitude.

Column 8: Difference, in a Single Series, arranged in order of magnitude.

Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8. Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8. Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3. Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8. Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8.

Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22 : 18 3/8 : -3 5/8. Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18 : -3 5/8. Pot 2 : - : - :: - : - :: 21 4/8 : 18 : -3 4/8. Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21 : 18 : -3. Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18 :: 21 : 17 3/8 : -3 5/8.

Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8. Pot 3 : 21 5/8 : 18 :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8. Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8. Pot 3 : - : - :: - : - :: 12 : 15 2/8 : +3 2/8. Pot 3 : 21 : 18 :: 23 : 18 :: 12 : 12 6/8 : +0 6/8.

Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18. Pot 4 : 23 : 15 4/8 :: 21 : 15 4/8. Pot 4 : 12 : 18 :: 12 : 12 6/8.

"The observations as I received them are shown in Table 1/1, Columns 2 and 3, where they certainly have no prima facie appearance of regularity. But as soon as we arrange them the in order of their magnitudes, as in columns 4 and 5, the case is materially altered. We now see, with few exceptions, that the largest plant on the crossed side in each pot exceeds the largest plant on the self-fertilised side, that the second exceeds the second, the third the third, and so on. Out of the fifteen cases in the table, there are only two exceptions to this rule. We may therefore confidently affirm that a crossed series will always be found to exceed a self-fertilised series, within the range of the conditions under which the present experiment has been made."

TABLE 1/2.

Column 1: Number (Name) of Pot.

Column 2: Crossed.

Column 3: Self-fertilised.

Column 4: Difference.

Pot 1 : 18 7/8 : 19 2/8 : +0 3/8. Pot 2 : 20 7/8 : 19 : -1 7/8. Pot 3 : 21 1/8 : 16 7/8 : -4 2/8. Pot 4 : 19 6/8 : 16 : -3 6/8.

"Next as regards the numerical estimate of this excess. The mean values of the several groups are so discordant, as is shown in Table 1/2, that a fairly precise numerical estimate seems impossible. But the consideration arises, whether the difference between pot and pot may not be of much the same order of importance as that of the other conditions upon which the growth of the plants has been modified. If so, and only on that condition, it would follow that when all the measurements, either of the crossed or the self-fertilised plants, were combined into a single series, that series would be statistically regular. The experiment is tried in Table 1/1, columns 7 and 8, where the regularity is abundantly clear, and justifies us in considering its mean as perfectly reliable. I have protracted these measurements, and revised them in the usual way, by drawing a curve through them with a free hand, but the revision barely modifies the means derived from the original observations. In the present, and in nearly all the other cases, the difference between the original and revised means is under 2 per cent of their value. It is a very remarkable coincidence that in the seven kinds of plants, whose measurements I have examined, the ratio between the heights of the crossed and of the self-fertilised ranges in five cases within very narrow limits. In Zea mays it is as 100 to 84, and in the others it ranges between 100 to 76 and 100 to 86."

"The determination of the variability (measured by what is technically called the 'probable error') is a problem of more delicacy than that of determining the means, and I doubt, after making many trials, whether it is possible to derive useful conclusions from these few observations. We ought to have measurements of at least fifty plants in each case, in order to be in a position to deduce fair results. One fact, however, bearing on variability, is very evident in most cases, though not in Zea mays, namely, that the self-fertilised plants include the larger number of exceptionally small specimens, while the crossed are more generally full grown."

"Those groups of cases in which measurements have been made of a few of the tallest plants that grew in rows, each of which contained a multitude of plants, show very clearly that the crossed plants exceed the self-fertilised in height, but they do not tell by inference anything about their respective mean values. If it should happen that a series is known to follow the law of error or any other law, and if the number of individuals in the series is known, it would be always possible to reconstruct the whole series when a fragment of it has been given. But I find no such method to be applicable in the present case. The doubt as to the number of plants in each row is of minor importance; the real difficulty lies in our ignorance of the precise law followed by the series. The experience of the plants in pots does not help us to determine that law, because the observations of such plants are too few to enable us to lay down more than the middle terms of the series to which they belong with any sort of accuracy, whereas the cases we are now considering refer to one of its extremities. There are other special difficulties which need not be gone into, as the one already mentioned is a complete bar."]

Mr. Galton sent me at the same time graphical representations which he had made of the measurements, and they evidently form fairly regular curves. He appends the words "very good" to those of Zea and Limnanthes. He also calculated the average height of the crossed and self-fertilised plants in the seven tables by a more correct method than that followed by me, namely, by including the heights, as estimated in accordance with statistical rules, of a few plants which died before they were measured; whereas I merely added up the heights of the survivors, and divided the sum by their number. The difference in our results is in one way highly satisfactory, for the average heights of the self-fertilised plants, as deduced by Mr. Galton, is less than mine in all the cases excepting one, in which our averages are the same; and this shows that I have by no means exaggerated the superiority of the crossed over the self-fertilised plants.

After the heights of the crossed and self-fertilised plants had been taken, they were sometimes cut down close to the ground, and an equal number of both weighed. This method of comparison gives very striking results, and I wish that it had been oftener followed. Finally a record was often kept of any marked difference in the rate of germination of the crossed and self-fertilised seeds,—of the relative periods of flowering of the plants raised from them,—and of their productiveness, that is, of the number of seed-capsules which they produced and of the average number of seeds which each capsule contained.

When I began my experiments I did not intend to raise crossed and self-fertilised plants for more than a single generation; but as soon as the plants of the first generation were in flower I thought that I would raise one more generation, and acted in the following manner. Several flowers on one or more of the self-fertilised plants were again self-fertilised; and several flowers on one or more of the crossed plants were fertilised with pollen from another crossed plant of the same lot. Having thus once begun, the same method was followed for as many as ten successive generations with some of the species. The seeds and seedlings were always treated in exactly the same manner as already described. The self-fertilised plants, whether originally descended from one or two mother-plants, were thus in each generation as closely interbred as was possible; and I could not have improved on my plan. But instead of crossing one of the crossed plants with another crossed plant, I ought to have crossed the self-fertilised plants of each generation with pollen taken from a non-related plant—that is, one belonging to a distinct family or stock of the same species and variety. This was done in several cases as an additional experiment, and gave very striking results. But the plan usually followed was to put into competition and compare intercrossed plants, which were almost always the offspring of more or less closely related plants, with the self-fertilised plants of each succeeding generation;—all having been grown under closely similar conditions. I have, however, learnt more by this method of proceeding, which was begun by an oversight and then necessarily followed, than if I had always crossed the self-fertilised plants of each succeeding generation with pollen from a fresh stock.

I have said that the crossed plants of the successive generations were almost always inter-related. When the flowers on an hermaphrodite plant are crossed with pollen taken from a distinct plant, the seedlings thus raised may be considered as hermaphrodite brothers or sisters; those raised from the same capsule being as close as twins or animals of the same litter. But in one sense the flowers on the same plant are distinct individuals, and as several flowers on the mother-plant were crossed by pollen taken from several flowers on the father-plant, such seedlings would be in one sense half-brothers or sisters, but more closely related than are the half-brothers and sisters of ordinary animals. The flowers on the mother-plant were, however, commonly crossed by pollen taken from two or more distinct plants; and in these cases the seedlings might be called with more truth half-brothers or sisters. When two or three mother-plants were crossed, as often happened, by pollen taken from two or three father-plants (the seeds being all intermingled), some of the seedlings of the first generation would be in no way related, whilst many others would be whole or half-brothers and sisters. In the second generation a large number of the seedlings would be what may be called whole or half first-cousins, mingled with whole and half-brothers and sisters, and with some plants not at all related. So it would be in the succeeding generations, but there would also be many cousins of the second and more remote degrees. The relationship will thus have become more and more inextricably complex in the later generations; with most of the plants in some degree and many of them closely related.

I have only one other point to notice, but this is one of the highest importance; namely, that the crossed and self-fertilised plants were subjected in the same generation to as nearly similar and uniform conditions as was possible. In the successive generations they were exposed to slightly different conditions as the seasons varied, and they were raised at different periods. But in other respects all were treated alike, being grown in pots in the same artificially prepared soil, being watered at the same time, and kept close together in the same greenhouse or hothouse. They were therefore not exposed during successive years to such great vicissitudes of climate as are plants growing out of doors.

ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS.

It has been objected to such experiments as mine, that covering plants with a net, although only for a short time whilst in flower, may affect their health and fertility. I have seen no such effect except in one instance with a Myosotis, and the covering may not then have been the real cause of injury. But even if the net were slightly injurious, and certainly it was not so in any high degree, as I could judge by the appearance of the plants and by comparing their fertility with that of neighbouring uncovered plants, it would not have vitiated my experiments; for in all the more important cases the flowers were crossed as well as self-fertilised under a net, so that they were treated in this respect exactly alike.

As it is impossible to exclude such minute pollen-carrying insects as Thrips, flowers which it was intended to fertilise with their own pollen may sometimes have been afterwards crossed with pollen brought by these insects from another flower on the same plant; but as we shall hereafter see, a cross of this kind does not produce any effect, or at most only a slight one. When two or more plants were placed near one another under the same net, as was often done, there is some real though not great danger of the flowers which were believed to be self-fertilised being afterwards crossed with pollen brought by Thrips from a distinct plant. I have said that the danger is not great because I have often found that plants which are self-sterile, unless aided by insects, remained sterile when several plants of the same species were placed under the same net. If, however, the flowers which had been presumably self-fertilised by me were in any case afterwards crossed by Thrips with pollen brought from a distinct plant, crossed seedlings would have been included amongst the self-fertilised; but it should be especially observed that this occurrence would tend to diminish and not to increase any superiority in average height, fertility, etc., of the crossed over the self-fertilised plants.

As the flowers which were crossed were never castrated, it is probable or even almost certain that I sometimes failed to cross-fertilise them effectually, and that they were afterwards spontaneously self-fertilised. This would have been most likely to occur with dichogamous species, for without much care it is not easy to perceive whether their stigmas are ready to be fertilised when the anthers open. But in all cases, as the flowers were protected from wind, rain, and the access of insects, any pollen placed by me on the stigmatic surface whilst it was immature, would generally have remained there until the stigma was mature; and the flowers would then have been crossed as was intended. Nevertheless, it is highly probable that self-fertilised seedlings have sometimes by this means got included amongst the crossed seedlings. The effect would be, as in the former case, not to exaggerate but to diminish any average superiority of the crossed over the self-fertilised plants.

Errors arising from the two causes just named, and from others,—such as some of the seeds not having been thoroughly ripened, though care was taken to avoid this error—the sickness or unperceived injury of any of the plants,—will have been to a large extent eliminated, in those cases in which many crossed and self-fertilised plants were measured and an average struck. Some of these causes of error will also have been eliminated by the seeds having been allowed to germinate on bare damp sand, and being planted in pairs; for it is not likely that ill-matured and well-matured, or diseased and healthy seeds, would germinate at exactly the same time. The same result will have been gained in the several cases in which only a few of the tallest, finest, and healthiest plants on each side of the pots were measured.

Kolreuter and Gartner have proved that with some plants several, even as many as from fifty to sixty, pollen-grains are necessary for the fertilisation of all the ovules in the ovarium. (1/9. 'Kentniss der Befruchtung' 1844 page 345. Naudin 'Nouvelles Archives du Museum' tome 1 page 27.) Naudin also found in the case of Mirabilis that if only one or two of its very large pollen-grains were placed on the stigma, the plants raised from such seeds were dwarfed. I was therefore careful to give an amply sufficient supply of pollen, and generally covered the stigma with it; but I did not take any special pains to place exactly the same amount on the stigmas of the self-fertilised and crossed flowers. After having acted in this manner during two seasons, I remembered that Gartner thought, though without any direct evidence, that an excess of pollen was perhaps injurious; and it has been proved by Spallanzani, Quatrefages, and Newport, that with various animals an excess of the seminal fluid entirely prevents fertilisation. (1/10. 'Transactions of the Philosophical Society' 1853 pages 253-258.) It was therefore necessary to ascertain whether the fertility of the flowers was affected by applying a rather small and an extremely large quantity of pollen to the stigma. Accordingly a very small mass of pollen-grains was placed on one side of the large stigma in sixty-four flowers of Ipomoea purpurea, and a great mass of pollen over the whole surface of the stigma in sixty-four other flowers. In order to vary the experiment, half the flowers of both lots were on plants produced from self-fertilised seeds, and the other half on plants from crossed seeds. The sixty-four flowers with an excess of pollen yielded sixty-one capsules; and excluding four capsules, each of which contained only a single poor seed, the remainder contained on an average 5.07 seeds per capsule. The sixty-four flowers with only a little pollen placed on one side of the stigma yielded sixty-three capsules, and excluding one from the same cause as before, the remainder contained on an average 5.129 seeds. So that the flowers fertilised with little pollen yielded rather more capsules and seeds than did those fertilised with an excess; but the difference is too slight to be of any significance. On the other hand, the seeds produced by the flowers with an excess of pollen were a little heavier of the two; for 170 of them weighed 79.67 grains, whilst 170 seeds from the flowers with very little pollen weighed 79.20 grains. Both lots of seeds having been placed on damp sand presented no difference in their rate of germination. We may therefore conclude that my experiments were not affected by any slight difference in the amount of pollen used; a sufficiency having been employed in all cases.

The order in which our subject will be treated in the present volume is as follows. A long series of experiments will first be given in Chapters 2 to 6. Tables will afterwards be appended, showing in a condensed form the relative heights, weights, and fertility of the offspring of the various crossed and self-fertilised species. Another table exhibits the striking results from fertilising plants, which during several generations had either been self-fertilised or had been crossed with plants kept all the time under closely similar conditions, with pollen taken from plants of a distinct stock and which had been exposed to different conditions. In the concluding chapters various related points and questions of general interest will be discussed.

Anyone not specially interested in the subject need not attempt to read all the details (marked []); though they possess, I think, some value, and cannot be all summarised. But I would suggest to the reader to take as an example the experiments on Ipomoea in Chapter 2; to which may be added those on Digitalis, Origanum, Viola, or the common cabbage, as in all these cases the crossed plants are superior to the self-fertilised in a marked degree, but not in quite the same manner. As instances of self-fertilised plants being equal or superior to the crossed, the experiments on Bartonia, Canna, and the common pea ought to be read; but in the last case, and probably in that of Canna, the want of any superiority in the crossed plants can be explained.

Species were selected for experiment belonging to widely distinct families, inhabiting various countries. In some few cases several genera belonging to the same family were tried, and these are grouped together; but the families themselves have been arranged not in any natural order, but in that which was the most convenient for my purpose. The experiments have been fully given, as the results appear to me of sufficient value to justify the details. Plants bearing hermaphrodite flowers can be interbred more closely than is possible with bisexual animals, and are therefore well-fitted to throw light on the nature and extent of the good effects of crossing, and on the evil effects of close interbreeding or self-fertilisation. The most important conclusion at which I have arrived is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution, owing to their progenitors having been subjected during several generations to slightly different conditions, or to what we call in our ignorance spontaneous variation. This conclusion, as we shall hereafter see, is closely connected with various important physiological problems, such as the benefit derived from slight changes in the conditions of life, and this stands in the closest connection with life itself. It throws light on the origin of the two sexes and on their separation or union in the same individual, and lastly on the whole subject of hybridism, which is one of the greatest obstacles to the general acceptance and progress of the great principle of evolution.

In order to avoid misapprehension, I beg leave to repeat that throughout this volume a crossed plant, seedling, or seed, means one of crossed PARENTAGE, that is, one derived from a flower fertilised with pollen from a distinct plant of the same species. And that a self-fertilised plant, seedling, or seed, means one of self-fertilised PARENTAGE, that is, one derived from a flower fertilised with pollen from the same flower, or sometimes, when thus stated, from another flower on the same plant.



CHAPTER II.

CONVOLVULACEAE.

Ipomoea purpurea, comparison of the height and fertility of the crossed and self-fertilised plants during ten successive generations. Greater constitutional vigour of the crossed plants. The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals. The effects of a cross with a fresh stock. The descendants of the self-fertilised plant named Hero. Summary on the growth, vigour, and fertility of the successive crossed and self-fertilised generations. Small amount of pollen in the anthers of the self-fertilised plants of the later generations, and the sterility of their first-produced flowers. Uniform colour of the flowers produced by the self-fertilised plants. The advantage from a cross between two distinct plants depends on their differing in constitution.

A plant of Ipomoea purpurea, or as it is often called in England the convolvulus major, a native of South America, grew in my greenhouse. Ten flowers on this plant were fertilised with pollen from the same flower; and ten other flowers on the same plant were crossed with pollen from a distinct plant. The fertilisation of the flowers with their own pollen was superfluous, as this convolvulus is highly self-fertile; but I acted in this manner to make the experiments correspond in all respects. Whilst the flowers are young the stigma projects beyond the anthers; and it might have been thought that it could not be fertilised without the aid of humble-bees, which often visit the flowers; but as the flower grows older the stamens increase in length, and their anthers brush against the stigma, which thus receives some pollen. The number of seeds produced by the crossed and self-fertilised flowers differed very little.

[Crossed and self-fertilised seeds obtained in the above manner were allowed to germinate on damp sand, and as often as pairs germinated at the same time they were planted in the manner described in the Introduction (Chapter 1), on the opposite sides of two pots. Five pairs were thus planted; and all the remaining seeds, whether or not in a state of germination, were planted on the opposite sides of a third pot, so that the young plants on both sides were here greatly crowded and exposed to very severe competition. Rods of iron or wood of equal diameter were given to all the plants to twine up; and as soon as one of each pair reached the summit both were measured. A single rod was placed on each side of the crowded pot, Number 3, and only the tallest plant on each side was measured.

TABLE 2/1. Ipomoea purpurea (First Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Seedlings from Crossed Plants.

Column 3: Seedlings from Self-fertilised Plants.

Pot 1 : 87 4/8 : 69. Pot 1 : 87 4/8 : 66. Pot 1 : 89 : 73.

Pot 2 : 88 : 68 4/8. Pot 2 : 87 : 60 4/8.

Pot 3 : 77 : 57. Plants crowded; the tallest one measured on each side.

Total : 516 : 394.

The average height of the six crossed plants is here 86 inches, whilst that of the six self-fertilised plants is only 65.66 inches, so that the crossed plants are to the self-fertilised in height as 100 to 76. It should be observed that this difference is not due to a few of the crossed plants being extremely tall, or to a few of the self-fertilised being extremely short, but to all the crossed plants attaining a greater height than their antagonists. The three pairs in Pot 1 were measured at two earlier periods, and the difference was sometimes greater and sometimes less than that at the final measuring. But it is an interesting fact, of which I have seen several other instances, that one of the self-fertilised plants, when nearly a foot in height, was half an inch taller than the crossed plant; and again, when two feet high, it was 1 3/8 of an inch taller, but during the ten subsequent days the crossed plant began to gain on its antagonist, and ever afterward asserted its supremacy, until it exceeded its self-fertilised opponent by 16 inches.

The five crossed plants in Pots 1 and 2 were covered with a net, and produced 121 capsules; the five self-fertilised plants produced eighty-four capsules, so that the numbers of capsules were as 100 to 69. Of the 121 capsules on the crossed plants sixty-five were the product of flowers crossed with pollen from a distinct plant, and these contained on an average 5.23 seeds per capsule; the remaining fifty-six capsules were spontaneously self-fertilised. Of the eighty-four capsules on the self-fertilised plants, all the product of renewed self-fertilisation, fifty-five (which were alone examined) contained on an average 4.85 seeds per capsule. Therefore the cross-fertilised capsules, compared with the self-fertilised capsules, yielded seeds in the proportion of 100 to 93. The crossed seeds were relatively heavier than the self-fertilised seeds. Combining the above data (i.e., number of capsules and average number of contained seeds), the crossed plants, compared with the self-fertilised, yielded seeds in the ratio of 100 to 64.

These crossed plants produced, as already stated, fifty-six spontaneously self-fertilised capsules, and the self-fertilised plants produced twenty-nine such capsules. The former contained on an average, in comparison with the latter, seeds in the proportion of 100 to 99.

In Pot 3, on the opposite sides of which a large number of crossed and self-fertilised seeds had been sown and the seedlings allowed to struggle together, the crossed plants had at first no great advantage. At one time the tallest crossed was 25 1/8 inches high, and the tallest self-fertilised plants 21 3/8. But the difference afterwards became much greater. The plants on both sides, from being so crowded, were poor specimens. The flowers were allowed to fertilise themselves spontaneously under a net; the crossed plants produced thirty-seven capsules, the self-fertilised plants only eighteen, or as 100 to 47. The former contained on an average 3.62 seeds per capsule; and the latter 3.38 seeds, or as 100 to 93. Combining these data (i.e., number of capsules and average number of seeds), the crowded crossed plants produced seeds compared with the self-fertilised as 100 to 45. These latter seeds, however, were decidedly heavier, a hundred weighing 41.64 grains, than those from the capsules on the crossed plants, of which a hundred weighed 36.79 grains; and this probably was due to the fewer capsules borne by the self-fertilised plants having been better nourished. We thus see that the crossed plants in this the first generation, when grown under favourable conditions, and when grown under unfavourable conditions from being much crowded, greatly exceeded in height, and in the number of capsules produced, and slightly in the number of seeds per capsule, the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Flowers on the crossed plants of the last generation (Table 2/1) were crossed by pollen from distinct plants of the same generation; and flowers on the self-fertilised plants were fertilised by pollen from the same flower. The seeds thus produced were treated in every respect as before, and we have in Table 2/2 the result.

TABLE 2/2. Ipomoea purpurea (Second Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 87 : 67 4/8. Pot 1 : 83 : 68 4/8. Pot 1 : 83 : 80 4/8.

Pot 2 : 85 4/8 : 61 4/8. Pot 2 : 89 : 79. Pot 2 : 77 4/8 : 41.

Total : 505 : 398.

Here again every single crossed plant is taller than its antagonist. The self-fertilised plant in Pot 1, which ultimately reached the unusual height of 80 4/8 inches, was for a long time taller than the opposed crossed plant, though at last beaten by it. The average height of the six crossed plants is 84.16 inches, whilst that of the six self-fertilised plants is 66.33 inches, or as 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Seeds from the crossed plants of the last generation (Table 2/2) again crossed, and from the self-fertilised plants again self-fertilised, were treated in all respects exactly as before, with the following result:—

TABLE 2/3. Ipomoea purpurea (Third Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 74 : 56 4/8. Pot 1 : 72 : 51 4/8. Pot 1 : 73 4/8 : 54.

Pot 2 : 82 : 59. Pot 2 : 81 : 30. Pot 2 : 82 : 66.

Total : 464.5 : 317.

Again all the crossed plants are higher than their antagonists: their average height is 77.41 inches, whereas that of the self-fertilised is 52.83 inches, or as 100 to 68.

I attended closely to the fertility of the plants of this third generation. Thirty flowers on the crossed plants were crossed with pollen from other crossed plants of the same generation, and the twenty-six capsules thus produced contained, on an average, 4.73 seeds; whilst thirty flowers on the self-fertilised plants, fertilised with the pollen from the same flower, produced twenty-three capsules, each containing 4.43 seeds. Thus the average number of seeds in the crossed capsules was to that in the self-fertilised capsules as 100 to 94. A hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of the self-fertilised seeds weighed only 37.63 grains. Many of these lighter self-fertilised seeds placed on damp sand germinated before the crossed; thus thirty-six of the former germinated whilst only thirteen of the latter or crossed seeds germinated. In Pot 1 the three crossed plants produced spontaneously under the net (besides the twenty-six artificially cross-fertilised capsules) seventy-seven self-fertilised capsules containing on an average 4.41 seeds; whilst the three self-fertilised plants produced spontaneously (besides the twenty-three artificially self-fertilised capsules) only twenty-nine self-fertilised capsules, containing on an average 4.14 seeds. Therefore the average number of seeds in the two lots of spontaneously self-fertilised capsules was as 100 to 94. Taking into consideration the number of capsules together with the average number of seeds, the crossed plants (spontaneously self-fertilised) produced seeds in comparison with the self-fertilised plants (spontaneously self-fertilised) in the proportion of 100 to 35. By whatever method the fertility of these plants is compared, the crossed are more fertile than the self-fertilised plants.

I tried in several ways the comparative vigour and powers of growth of the crossed and self-fertilised plants of this third generation. Thus, four self-fertilised seeds which had just germinated were planted on one side of a pot, and after an interval of forty-eight hours, four crossed seeds in the same state of germination were planted on the opposite side; and the pot was kept in the hothouse. I thought that the advantage thus given to the self-fertilised seedlings would have been so great that they would never have been beaten by the crossed ones. They were not beaten until all had grown to a height of 18 inches; and the degree to which they were finally beaten is shown in Table 2/4. We here see that the average height of the four crossed plants is 76.62, and of the four self-fertilised plants 65.87 inches, or as 100 to 86; therefore less than when both sides started fair.

TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised plants having had a start of forty-eight hours).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 3 : 78 4/8 : 73 4/8. Pot 3 : 77 4/8 : 53. Pot 3 : 73 : 61 4/8. Pot 3 : 77 4/8 : 75 4/8.

Total : 306.5 : 263.5.

Crossed and self-fertilised seeds of the third generation were also sown out of doors late in the summer, and therefore under unfavourable conditions, and a single stick was given to each lot of plants to twine up. The two lots were sufficiently separate so as not to interfere with each other's growth, and the ground was clear of weeds. As soon as they were killed by the first frost (and there was no difference in their hardiness), the two tallest crossed plants were found to be 24.5 and 22.5 inches, whilst the two tallest self-fertilised plants were only 15 and 12.5 inches in height, or as 100 to 59.

I likewise sowed at the same time two lots of the same seeds in a part of the garden which was shady and covered with weeds. The crossed seedlings from the first looked the most healthy, but they twined up a stick only to a height of 7 1/4 inches; whilst the self-fertilised were not able to twine at all; and the tallest of them was only 3 1/2 inches in height.

Lastly, two lots of the same seeds were sown in the midst of a bed of candy-tuft (Iberis) growing vigorously. The seedlings came up, but all the self-fertilised ones soon died excepting one, which never twined and grew to a height of only 4 inches. Many of the crossed seedlings, on the other hand, survived; and some twined up the stems of the Iberis to the height of 11 inches. These cases prove that the crossed seedlings have an immense advantage over the self-fertilised, both when growing isolated under very unfavourable conditions, and when put into competition with each other or with other plants, as would happen in a state of nature.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seedlings raised as before from the crossed and self-fertilised plants of the third generation in Table 2/3, gave results as follows:—

TABLE 2/5. Ipomoea purpurea (Fourth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 : 80. Pot 1 : 47 : 44 1/2.

Pot 2 : 83 : 73 1/2. Pot 2 : 59 : 51 1/2.

Pot 3 : 82 : 56 1/2. Pot 3 : 65 1/2 : 63. Pot 3 : 68 : 52.

Total : 488.5 : 421.0.

Here the average height of the seven crossed plants is 69.78 inches, and that of the seven self-fertilised plants 60.14; or as 100 to 86. This smaller difference relatively to that in the former generations, may be attributed to the plants having been raised during the depth of winter, and consequently to their not having grown vigorously, as was shown by their general appearance and from several of them never reaching the summits of the rods. In Pot 2, one of the self-fertilised plants was for a long time taller by two inches than its opponent, but was ultimately beaten by it, so that all the crossed plants exceeded their opponents in height. Of twenty-eight capsules produced by the crossed plants fertilised by pollen from a distinct plant, each contained on an average 4.75 seeds; of twenty-seven self-fertilised capsules on the self-fertilised plants, each contained on an average 4.47 seeds; so that the proportion of seeds in the crossed and self-fertilised capsules was as 100 to 94.

Some of the same seeds, from which the plants in Table 2/5 had been raised, were planted, after they had germinated on damp sand, in a square tub, in which a large Brugmansia had long been growing. The soil was extremely poor and full of roots; six crossed seeds were planted in one corner, and six self-fertilised seeds in the opposite corner. All the seedlings from the latter soon died excepting one, and this grew to the height of only 1 1/2 inches. Of the crossed plants three survived, and they grew to the height of 2 1/2 inches, but were not able to twine round a stick; nevertheless, to my surprise, they produced some small miserable flowers. The crossed plants thus had a decided advantage over the self-fertilised plants under this extremity of bad conditions.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

These were raised in the same manner as before, and when measured gave the following results:—

TABLE 2/6. Ipomoea purpurea (Fifth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 96 : 73. Pot 1 : 86 : 78. Pot 1 : 69 : 29.

Pot 2 : 84 : 51. Pot 2 : 84 : 84. Pot 2 : 76 1/4 : 59.

Total : 495.25 : 374.00.

The average height of the six crossed plants is 82.54 inches, and that of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every crossed plant exceeded its antagonist in height. In Pot 1 the middle plant on the crossed side was slightly injured whilst young by a blow, and was for a time beaten by its opponent, but ultimately recovered the usual superiority. The crossed plants produced spontaneously a vast number more capsules than did the self-fertilised plants; and the capsules of the former contained on an average 3.37 seeds, whilst those of the latter contained only 3.0 per capsule, or as 100 to 89. But looking only to the artificially fertilised capsules, those on the crossed plants again crossed contained on an average 4.46 seeds, whilst those on the self-fertilised plants again self-fertilised contained 4.77 seeds; so that the self-fertilised capsules were the more fertile of the two, and of this unusual fact I can offer no explanation.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

These were raised in the usual manner, with the following result. I should state that there were originally eight plants on each side; but as two of the self-fertilised became extremely unhealthy and never grew to near their full height, these as well as their opponents have been struck out of the list. If they had been retained, they would have made the average height of the crossed plants unfairly greater than that of the self-fertilised. I have acted in the same manner in a few other instances, when one of a pair plainly became very unhealthy.

TABLE 2/7. Ipomoea purpurea (Sixth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 93 : 50 1/2. Pot 1 : 91 : 65.

Pot 2 : 79 : 50. Pot 2 : 86 1/2 : 87. Pot 2 : 88 : 62.

Pot 3 : 87 1/2 : 64 1/2.

Total : 525 : 379.

The average height of the six crossed plants is here 87.5, and of the six self-fertilised plants 63.16, or as 100 to 72. This large difference was chiefly due to most of the plants, especially the self-fertilised ones, having become unhealthy towards the close of their growth, and they were severely attacked by aphides. From this cause nothing can be inferred with respect to their relative fertility. In this generation we have the first instance of a self-fertilised plant in Pot 2 exceeding (though only by half an inch) its crossed opponent. This victory was fairly won after a long struggle. At first the self-fertilised plant was several inches taller than its opponent, but when the latter was 4 1/2 feet high it had grown equal; it then grew a little taller than the self-fertilised plant, but was ultimately beaten by it to the extent of half an inch, as shown in Table 2/7. I was so much surprised at this case that I saved the self-fertilised seeds of this plant, which I will call the "Hero," and experimented on its descendants, as will hereafter be described.

Besides the plants included in Table 2/7, nine crossed and nine self-fertilised plants of the same lot were raised in two other pots, 4 and 5. These pots had been kept in the hothouse, but from want of room were, whilst the plants were young, suddenly moved during very cold weather into the coldest part of the greenhouse. They all suffered greatly, and never quite recovered. After a fortnight only two of the nine self-fertilised seedlings were alive, whilst seven of the crossed survived. The tallest of these latter plants when measured was 47 inches in height, whilst the tallest of the two surviving self-fertilised plants was only 32 inches. Here again we see how much more vigorous the crossed plants are than the self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.

These were raised as heretofore with the following result:—

TABLE 2/8. Ipomoea purpurea (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 4/8 : 74 6/8. Pot 1 : 84 6/8 : 84. Pot 1 : 76 2/8 : 55 4/8.

Pot 2 : 84 4/8 : 65. Pot 2 : 90 : 51 2/8. Pot 2 : 82 2/8 : 80 4/8.

Pot 3 : 83 : 67 6/8. Pot 3 : 86 : 60 2/8.

Pot 4 : 84 2/8 : 75 2/8.

Total : 755.50 : 614.25.

Each of these nine crossed plants is higher than its opponent, though in one case only by three-quarters of an inch. Their average height is 83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to 81. These plants, after growing to their full height, became very unhealthy and infested with aphides, just when the seeds were setting, so that many of the capsules failed, and nothing can be said on their relative fertility.

CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.

As just stated, the plants of the last generation, from which the present ones were raised, were very unhealthy and their seeds of unusually small size; and this probably accounts for the two lots behaving differently to what they did in any of the previous or succeeding generations. Many of the self-fertilised seeds germinated before the crossed ones, and these were of course rejected. When the crossed seedlings in Table 2/9 had grown to a height of between 1 and 2 feet, they were all, or almost all, shorter than their self-fertilised opponents, but were not then measured. When they had acquired an average height of 32.28 inches, that of the self-fertilised plants was 40.68, or as 100 to 122. Moreover, every one of the self-fertilised plants, with a single exception, exceeded its crossed opponent. When, however, the crossed plants had grown to an average height of 77.56 inches, they just exceeded (namely, by .7 of an inch) the average height of the self-fertilised plants; but two of the latter were still taller than their crossed opponents. I was so much astonished at this whole case, that I tied string to the summits of the rods; the plants being thus allowed to continue climbing upwards. When their growth was complete they were untwined, stretched straight, and measured. The crossed plants had now almost regained their accustomed superiority, as may be seen in Table 2/9.

The average height of the eight crossed plants is here 113.25 inches, and that of the self-fertilised plants 96.65, or as 100 to 85. Nevertheless two of the self-fertilised plants, as may be seen in Table 2/9, were still higher than their crossed opponents. The latter manifestly had much thicker stems and many more lateral branches, and looked altogether more vigorous than the self-fertilised plants, and generally flowered before them. The earlier flowers produced by these self-fertilised plants did not set any capsules, and their anthers contained only a small amount of pollen; but to this subject I shall return. Nevertheless capsules produced by two other self-fertilised plants of the same lot, not included in Table 2/9, which had been highly favoured by being grown in separate pots, contained the large average number of 5.1 seeds per capsule.

TABLE 2/9. Ipomoea purpurea (Eighth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 111 6/8 : 96. Pot 1 : 127 : 54. Pot 1 : 130 6/8 : 93 4/8.

Pot 2 : 97 2/8 : 94. Pot 2 : 89 4/8 : 125 6/8.

Pot 3 : 103 6/8 : 115 4/8. Pot 3 : 100 6/8 : 84 6/8. Pot 3 : 147 4/8 : 109 6/8.

Total : 908.25 : 773.25.

CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.

The plants of this generation were raised in the same manner as before, with the result shown in Table 2/10.

The fourteen crossed plants average in height 81.39 inches and the fourteen self-fertilised plants 64.07, or as 100 to 79. One self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in height, its opponent. The self-fertilised plants showed no sign of inheriting the precocious growth of their parents; this having been due, as it would appear, to the abnormal state of the seeds from the unhealthiness of their parents. The fourteen self-fertilised plants yielded only forty spontaneously self-fertilised capsules, to which must be added seven, the product of ten flowers artificially self-fertilised. On the other hand, the fourteen crossed plants yielded 152 spontaneously self-fertilised capsules; but thirty-six flowers on these plants were crossed (yielding thirty-three capsules), and these flowers would probably have produced about thirty spontaneously self-fertilised capsules. Therefore an equal number of the crossed and self-fertilised plants would have produced capsules in the proportion of about 182 to 47, or as 100 to 26. Another phenomenon was well pronounced in this generation, but I believe had occurred previously to a slight extent; namely, that most of the flowers on the self-fertilised plants were somewhat monstrous. The monstrosity consisted in the corolla being irregularly split so that it did not open properly, with one or two of the stamens slightly foliaceous, coloured, and firmly coherent to the corolla. I observed this monstrosity in only one flower on the crossed plants. The self-fertilised plants, if well nourished, would almost certainly, in a few more generations, have produced double flowers, for they had already become in some degree sterile. (2/1. See on this subject 'Variation of Animals and Plants under Domestication' chapter 18 2nd edition volume 2 page 152.)

TABLE 2/10. Ipomoea purpurea (Ninth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 83 4/8 : 57. Pot 1 : 85 4/8 : 71. Pot 1 : 83 4/8 : 48 3/8.

Pot 2 : 83 2/8 : 45. Pot 2 : 64 2/8 : 43 6/8. Pot 2 : 64 3/8 : 38 4/8.

Pot 3 : 79 : 63. Pot 3 : 88 1/8 : 71. Pot 3 : 61 : 89 4/8.

Pot 4 : 82 4/8 : 82 4/8. Pot 4 : 90 : 76 1/8.

Pot 5 : 89 4/8 : 67. Pot 5 : 92 4/8 : 74 2/8. Pot 5 : 92 4/8 : 70. Crowded plants.

Total : 1139.5 : 897.0.

CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.

Six plants were raised in the usual manner from the crossed plants of the last generation (Table 2/10) again intercrossed, and from the self-fertilised again self-fertilised. As one of the crossed plants in Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and producing hardly any capsules, it and its opponent have been struck out of the table.

TABLE 2/11. Ipomoea purpurea (Tenth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 92 3/8 : 47 2/8. Pot 1 : 94 4/8 : 34 6/8.

Pot 2 : 87 : 54 4/8. Pot 2 : 89 5/8 : 49 2/8. Pot 2 : 105 : 66 2/8.

Total : 468.5 : 252.0.

The five crossed plants average 93.7 inches, and the five self-fertilised only 50.4, or as 100 to 54. This difference, however, is so great that it must be looked at as in part accidental. The six crossed plants (the diseased one here included) yielded spontaneously 101 capsules, and the six self-fertilised plants 88, the latter being chiefly produced by one of the plants. But as the diseased plant, which yielded hardly any seed, is here included, the ratio of 101 to 88 does not fairly give the relative fertility of the two lots. The stems of the six crossed plants looked so much finer than those of the six self-fertilised plants, that after the capsules had been gathered and most of the leaves had fallen off, they were weighed. Those of the crossed plants weighed 2,693 grains, whilst those of the self-fertilised plants weighed only 1,173 grains, or as 100 to 44; but as the diseased and dwarfed crossed plant is here included, the superiority of the former in weight was really greater.]

THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

In all the foregoing experiments, seedlings from flowers crossed by pollen from a distinct plant (though in the later generations more or less closely related) were put into competition with, and almost invariably proved markedly superior in height to the offspring from self-fertilised flowers. I wished, therefore, to ascertain whether a cross between two flowers on the same plant would give to the offspring any superiority over the offspring from flowers fertilised with their own pollen. I procured some fresh seed and raised two plants, which were covered with a net; and several of their flowers were crossed with pollen from a distinct flower on the same plant. Twenty-nine capsules thus produced contained on an average 4.86 seeds per capsule; and 100 of these seeds weighed 36.77 grains. Several other flowers were fertilised with their own pollen, and twenty-six capsules thus produced contained on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains. So that a cross of this kind appears to have increased slightly the number of seeds per capsule, in the ratio of 100 to 91; but these crossed seeds were lighter than the self-fertilised in the ratio of 86 to 100. I doubt, however, from other observations, whether these results are fully trustworthy. The two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of nine pots, and were treated in every respect like the plants in the previous experiments. The remaining seeds, some in a state of germination and some not so, were sown on the opposite sides of a large pot (Number 10); and the four tallest plants on each side of this pot were measured. The result is shown in Table 2/12.

TABLE 2/12. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 82 : 77 4/8. Pot 1 : 75 : 87. Pot 1 : 65 : 64. Pot 1 : 76 : 87 2/8.

Pot 2 : 78 4/8 : 84. Pot 2 : 43 : 86 4/8. Pot 2 : 65 4/8 : 90 4/8.

Pot 3 : 61 2/8 : 86. Pot 3 : 85 : 69 4/8. Pot 3 : 89 : 87 4/8.

Pot 4 : 83 : 80 4/8. Pot 4 : 73 4/8 : 88 4/8. Pot 4 : 67 : 84 4/8.

Pot 5 : 78 : 66 4/8. Pot 5 : 76 6/8 : 77 4/8. Pot 5 : 57 : 81 4/8.

Pot 6 : 70 4/8 : 80. Pot 6 : 79 : 82 4/8. Pot 6 : 79 6/8 : 55 4/8.

Pot 7 : 76 : 77. Pot 7 : 84 4/8 : 83 4/8. Pot 7 : 79 : 73 4/8.

Pot 8 : 73 : 76 4/8. Pot 8 : 67 : 82. Pot 8 : 83 : 80 4/8.

Pot 9 : 73 2/8 : 78 4/8. Pot 9 : 78 : 67 4/8.

Pot 10 : 34 : 82 4/8. Pot 10 : 82 : 36 6/8. Pot 10 : 84 6/8 : 69 4/8. Pot 10 : 71 : 75 2/8. Crowded plants.

Total : 2270.25 : 2399.75.

The average height of the thirty-one crossed plants is 73.23 inches, and that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to 106. Looking to each pair, it may be seen that only thirteen of the crossed plants, whilst eighteen of the self-fertilised plants exceed their opponents. A record was kept with respect to the plant which flowered first in each pot; and only two of the crossed flowered before one of the self-fertilised in the same pot; whilst eight of the self-fertilised flowered first. It thus appears that the crossed plants are slightly inferior in height and in earliness of flowering to the self-fertilised. But the inferiority in height is so small, namely as 100 to 106, that I should have felt very doubtful on this head, had I not cut down all the plants (except those in the crowded pot Number 10) close to the ground and weighed them. The twenty-seven crossed plants weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20 1/2 ounces; and this gives a ratio of 100 to 124.

A self-fertilised plant of the same parentage as those in Table 2/12 had been raised in a separate pot for a distinct purpose; and it proved partially sterile, the anthers containing very little pollen. Several flowers on this plant were crossed with the little pollen which could be obtained from the other flowers on the same plant; and other flowers were self-fertilised. From the seeds thus produced four crossed and four self-fertilised plants were raised, which were planted in the usual manner on the opposite sides of two pots. All these four crossed plants were inferior in height to their opponents; they averaged 78.18 inches, whilst the four self-fertilised plants averaged 84.8 inches; or as 100 to 108. (2/2. From one of these self-fertilised plants, spontaneously self-fertilised, I gathered twenty-four capsules, and they contained on an average only 3.2 seeds per capsule; so that this plant had apparently inherited some of the sterility of its parent.) This case, therefore, confirms the last. Taking all the evidence together, we must conclude that these strictly self-fertilised plants grew a little taller, were heavier, and generally flowered before those derived from a cross between two flowers on the same plant. These latter plants thus present a wonderful contrast with those derived from a cross between two distinct individuals.

1  2  3  4  5  6  7  8  9  10  11     Next Part
Home - Random Browse