The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom

by Charles Darwin

CONCLUDING REMARKS ON SELF-STERILE PLANTS.

In order to favour as far as possible the self-fertilisation of some of the foregoing plants, all the flowers on Reseda odorata and some of those on the Abutilon were fertilised with pollen from other flowers on the same plant, instead of with their own pollen, and in the case of the Senecio with pollen from other flowers on the same corymb; but this made no difference in the result. Fritz Muller tried both kinds of self-fertilisation in the case of Bignonia, Tabernaemontana and Abutilon, likewise with no difference in the result. With Eschscholtzia, however, he found that pollen from other flowers on the same plant was a little more effective than pollen from the same flower. So did Hildebrand in Germany; as thirteen out of fourteen flowers of Eschscholtzia thus fertilised set capsules, these containing on an average 9.5 seeds; whereas only fourteen flowers out of twenty-one fertilised with their own pollen set capsules, these containing on an average 9.0 seeds. (9/11. ‘Pringsheim’s Jahrbuch fur wiss. Botanik’ 7 page 467.) Hildebrand found a trace of a similar difference with Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. ‘Variation under Domestication’ chapter 17 2nd edition volume 2 pages 113-115.)

In considering the several cases above given of complete or almost complete self-sterility, we are first struck with their wide distribution throughout the vegetable kingdom. Their number is not at present large, for they can be discovered only by protecting plants from insects and then fertilising them with pollen from another plant of the same species and with their own pollen; and the latter must be proved to be in an efficient state by other trials. Unless all this be done, it is impossible to know whether their self-sterility may not be due to the male or female reproductive organs, or to both, having been affected by changed conditions of life. As in the course of my experiments I have found three new cases, and as Fritz Muller has observed indications of several others, it is probable that they will hereafter be proved to be far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal in the United States quoted in ‘Gardeners’ Chronicle’ 1868 page 1286, states that Lilium auratum, Impatiens pallida and fulva, and Forsythia viridissima, cannot be fertilised with their own pollen.)

As with plants of the same species and parentage, some individuals are self-sterile and others self-fertile, of which fact Reseda odorata offers the most striking instances, it is not at all surprising that species of the same genus differ in this same manner. Thus Verbascum phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis are quite self-fertile, as I know by trial. There is the same difference between some of the species of Papaver, Corydalis, and of other genera. Nevertheless, the tendency to self-sterility certainly runs to a certain extent in groups, as we see in the genus Passiflora, and with the Vandeae amongst Orchids.

Self-sterility differs much in degree in different plants. In those extraordinary cases in which pollen from the same flower acts on the stigma like a poison, it is almost certain that the plants would never yield a single self-fertilised seed. Other plants, like Corydalis cava, occasionally, though very rarely, produce a few self-fertilised seeds. A large number of species, as may be seen in Table 9/F, are less fertile with their own pollen than with that from another plant; and lastly, some species are perfectly self-fertile. Even with the individuals of the same species, as just remarked, some are utterly self-sterile, others moderately so, and some perfectly self-fertile. The cause, whatever it may be, which renders many plants more or less sterile with their own pollen, that is, when they are self-fertilised, must be different, at least to a certain extent, from that which determines the difference in height, vigour, and fertility of the seedlings raised from self-fertilised and crossed seeds; for we have already seen that the two classes of cases do not by any means run parallel. This want of parallelism would be intelligible, if it could be shown that self-sterility depended solely on the incapacity of the pollen-tubes to penetrate the stigma of the same flower deeply enough to reach the ovules; whilst the greater or less vigorous growth of the seedlings no doubt depends on the nature of the contents of the pollen-grains and ovules. Now it is certain that with some plants the stigmatic secretion does not properly excite the pollen-grains, so that the tubes are not properly developed, if the pollen is taken from the same flower. This is the case according to Fritz Muller with Eschscholtzia, for he found that the pollen-tubes did not penetrate the stigma deeply; and with the Orchidaceous genus Notylia they failed altogether to penetrate it. (9/14. ‘Botanische Zeitung’ 1868 pages 114, 115.)

With dimorphic and trimorphic species, an illegitimate union between plants of the same form presents the closest analogy with self-fertilisation, whilst a legitimate union closely resembles cross-fertilisation; and here again the lessened fertility or complete sterility of an illegitimate union depends, at least in part, on the incapacity for interaction between the pollen-grains and stigma. Thus with Linum grandiflorum, as I have elsewhere shown, not more than two or three out of hundreds of pollen-grains, either of the long-styled or short-styled form, when placed on the stigma of their own form, emit their tubes, and these do not penetrate deeply; nor does the stigma itself change colour, as occurs when it is legitimately fertilised. (9/15. ‘Journal of the Linnean Society Botany’ volume 7 1863 pages 73-75.)

On the other hand the difference in innate fertility, as well as in growth between plants raised from crossed and self-fertilised seeds, and the difference in fertility and growth between the legitimate and illegitimate offspring of dimorphic and trimorphic plants, must depend on some incompatibility between the sexual elements contained within the pollen-grains and ovules, as it is through their union that new organisms are developed.

If we now turn to the more immediate cause of self-sterility, we clearly see that in most cases it is determined by the conditions to which the plants have been subjected. Thus Eschscholtzia is completely self-sterile in the hot climate of Brazil, but is perfectly fertile there with the pollen of any other individual. The offspring of Brazilian plants became in England in a single generation partially self-fertile, and still more so in the second generation. Conversely, the offspring of English plants, after growing for two seasons in Brazil, became in the first generation quite self-sterile. Again, Abutilon darwinii, which is self-sterile in its native home of Brazil, became moderately self-fertile in a single generation in an English hothouse. Some other plants are self-sterile during the early part of the year, and later in the season become self-fertile. Passiflora alata lost its self-sterility when grafted on another species. With Reseda, however, in which some individuals of the same parentage are self-sterile and others are self-fertile, we are forced in our ignorance to speak of the cause as due to spontaneous variability; but we should remember that the progenitors of these plants, either on the male or female side, may have been exposed to somewhat different conditions. The power of the environment thus to affect so readily and in so peculiar a manner the reproductive organs, is a fact which has many important bearings; and I have therefore thought the foregoing details worth giving. For instance, the sterility of many animals and plants under changed conditions of life, such as confinement, evidently comes within the same general principle of the sexual system being easily affected by the environment. It has already been proved, that a cross between plants which have been self-fertilised or intercrossed during several generations, having been kept all the time under closely similar conditions, does not benefit the offspring; and on the other hand, that a cross between plants that have been subjected to different conditions benefits the offspring to an extraordinary degree. We may therefore conclude that some degree of differentiation in the sexual system is necessary for the full fertility of the parent-plants and for the full vigour of their offspring. It seems also probable that with those plants which are capable of complete self-fertilisation, the male and female elements and organs already differ to an extent sufficient to excite their mutual interaction; but that when such plants are taken to another country, and become in consequence self-sterile, their sexual elements and organs are so acted on as to be rendered too uniform for such interaction, like those of a self-fertilised plant long cultivated under the same conditions. Conversely, we may further infer that plants which are self-sterile in their native country, but become self-fertile under changed conditions, have their sexual elements so acted on, that they become sufficiently differentiated for mutual interaction.

We know that self-fertilised seedlings are inferior in many respects to those from a cross; and as with plants in a state of nature pollen from the same flower can hardly fail to be often left by insects or by the wind on the stigma, it seems at first sight highly probable that self-sterility has been gradually acquired through natural selection in order to prevent self-fertilisation. It is no valid objection to this belief that the structure of some flowers, and the dichogamous condition of many others, suffice to prevent the pollen reaching the stigma of the same flower; for we should remember that with most species many flowers expand at the same time, and that pollen from the same plant is equally injurious or nearly so as that from the same flower. Nevertheless, the belief that self-sterility is a quality which has been gradually acquired for the special purpose of preventing self-fertilisation must, I believe, be rejected. In the first place, there is no close correspondence in degree between the sterility of the parent-plants when self-fertilised, and the extent to which their offspring suffer in vigour by this process; and some such correspondence might have been expected if self-sterility had been acquired on account of the injury caused by self-fertilisation. The fact of individuals of the same parentage differing greatly in their degree of self-sterility is likewise opposed to such a belief; unless, indeed, we suppose that certain individuals have been rendered self-sterile to favour intercrossing, whilst other individuals have been rendered self-fertile to ensure the propagation of the species. The fact of self-sterile individuals appearing only occasionally, as in the case of Lobelia, does not countenance this latter view. But the strongest argument against the belief that self-sterility has been acquired to prevent self-fertilisation, is the immediate and powerful effect of changed conditions in either causing or in removing self-sterility. We are not therefore justified in admitting that this peculiar state of the reproductive system has been gradually acquired through natural selection; but we must look at it as an incidental result, dependent on the conditions to which the plants have been subjected, like the ordinary sterility caused in the case of animals by confinement, and in the case of plants by too much manure, heat, etc. I do not, however, wish to maintain that self-sterility may not sometimes be of service to a plant in preventing self-fertilisation; but there are so many other means by which this result might be prevented or rendered difficult, including as we shall see in the next chapter the prepotency of pollen from a distinct individual over a plant’s own pollen, that self-sterility seems an almost superfluous acquirement for this purpose.

Finally, the most interesting point in regard to self-sterile plants is the evidence which they afford of the advantage, or rather of the necessity, of some degree or kind of differentiation in the sexual elements, in order that they should unite and give birth to a new being. It was ascertained that the five plants of Reseda odorata which were selected by chance, could be perfectly fertilised by pollen taken from any one of them, but not by their own pollen; and a few additional trials were made with some other individuals, which I have not thought worth recording. So again, Hildebrand and Fritz Muller frequently speak of self-sterile plants being fertile with the pollen of any other individual; and if there had been any exceptions to the rule, these could hardly have escaped their observation and my own. We may therefore confidently assert that a self-sterile plant can be fertilised by the pollen of any one out of a thousand or ten thousand individuals of the same species, but not by its own. Now it is obviously impossible that the sexual organs and elements of every individual can have been specialised with respect to every other individual. But there is no difficulty in believing that the sexual elements of each differ slightly in the same diversified manner as do their external characters; and it has often been remarked that no two individuals are absolutely alike. Therefore we can hardly avoid the conclusion, that differences of an analogous and indefinite nature in the reproductive system are sufficient to excite the mutual action of the sexual elements, and that unless there be such differentiation fertility fails.