The above three spontaneously self-fertilised capsules produced by the two almost completely self-sterile plants, contained altogether five seeds; and from these I raised in the following year (1871) five plants, which were kept under separate nets. They grew to an extraordinarily large size, and on August 29th were examined. At first sight they appeared entirely destitute of capsules; but on carefully searching their many branches, two or three capsules were found on three of the plants, half-a-dozen on the fourth, and about eighteen on the fifth plant. But all these capsules were small, some being empty; the greater number contained only a single seed, and very rarely more than one. After this examination the nets were taken off, and the bees immediately carried pollen from one of these almost self-sterile plants to the other, for no other plants grew near. After a few weeks the ends of the branches on all five plants became covered with capsules, presenting a curious contrast with the lower and naked parts of the same long branches. These five plants therefore inherited almost exactly the same sexual constitution as their parents; and without doubt a self-sterile race of Mignonette could have been easily established.
Reseda lutea.
Plants of this species were raised from seeds gathered from a group of wild plants growing at no great distance from my garden. After casually observing that some of these plants were self-sterile, two plants taken by hazard were protected under separate nets. One of these soon became covered with spontaneously self-fertilised capsules, as numerous as those on the surrounding unprotected plants; so that it was evidently quite self-fertile. The other plant was partially self-sterile, producing very few capsules, many of which were of small size. When, however, this plant had grown tall, the uppermost branches became pressed against the net and grew crooked, and in this position the bees were able to suck the flowers through the meshes, and brought pollen to them from the neighbouring plants. These branches then became loaded with capsules; the other and lower branches remaining almost bare. The sexual constitution of this species is therefore similar to that of Reseda odorata.
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.