Nicotiana tabacum—flowers on self-fertilised plants of the 3rd generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as (by estimation): 110.
Anagallis collina—flowers on red variety crossed by a blue variety, and other flowers on the red variety self-fertilised yielded seeds as: 48.
Canna warscewiczi—crossed and self-fertilised flowers on the crossed and self-fertilised plants of three generations taken together yielded seeds as: 85.
As both these tables relate to the fertility of flowers fertilised by pollen from another plant and by their own pollen, they may be considered together. The difference between them consists in the self-fertilised flowers in Table 9/G, being produced by self-fertilised parents, and the crossed flowers by crossed parents, which in the later generations had become somewhat closely inter-related, and had been subjected all the time to nearly the same conditions. These two tables include fifty cases relating to thirty-two species. The flowers on many other species were crossed and self-fertilised, but as only a few were thus treated, the results cannot be trusted, as far as fertility is concerned, and are not here given. Some other cases have been rejected, as the plants were in an unhealthy condition. If we look to the figures in the two tables expressing the ratios between the mean relative fertility of the crossed and self-fertilised flowers, we see that in a majority of cases (i.e., in thirty-five out of fifty) flowers fertilised by pollen from a distinct plant yield more, sometimes many more, seeds than flowers fertilised with their own pollen; and they commonly set a larger proportion of capsules. The degree of infertility of the self-fertilised flowers differs extremely in the different species, and even, as we shall see in the section on self-sterile plants, in the individuals of the same species, as well as under slightly changed conditions of life. Their fertility ranges from zero to fertility equalling that of the crossed flowers; and of this fact no explanation can be offered. There are fifteen cases in the two tables in which the number of seeds per capsule produced by the self-fertilised flowers equals or even exceeds that yielded by the crossed flowers. Some few of these cases are, I believe, accidental; that is, would not recur on a second trial. This was apparently the case with the plants of the fifth generation of Ipomoea, and in one of the experiments with Dianthus. Nicotiana offers the most anomalous case of any, as the self-fertilised flowers on the parent-plants, and on their descendants of the second and third generations, produced more seeds than did the crossed flowers; but we shall recur to this case when we treat of highly self-fertile varieties.
It might have been expected that the difference in fertility between the crossed and self-fertilised flowers would have been more strongly marked in Table 9/G, in which the plants of one set were derived from self-fertilised parents, than in Table 9/F, in which flowers on the parent-plants were self-fertilised for the first time. But this is not the case, as far as my scanty materials allow of any judgment. There is therefore no evidence at present, that the fertility of plants goes on diminishing in successive self-fertilised generations, although there is some rather weak evidence that this does occur with respect to their height or growth. But we should bear in mind that in the later generations the crossed plants had become more or less closely inter-related, and had been subjected all the time to nearly uniform conditions.
It is remarkable that there is no close correspondence, either in the parent-plants or in the successive generations, between the relative number of seeds produced by the crossed and self-fertilised flowers, and the relative powers of growth of the seedlings raised from such seeds. Thus, the crossed and self-fertilised flowers on the parent-plants of Ipomoea, Gesneria, Salvia, Limnanthes, Lobelia fulgens, and Nolana produced a nearly equal number of seeds, yet the plants raised from the crossed seeds exceeded considerably in height those raised from the self-fertilised seeds. The crossed flowers of Linaria and Viscaria yielded far more seeds than the self-fertilised flowers; and although the plants raised from the former were taller than those from the latter, they were not so in any corresponding degree. With Nicotiana the flowers fertilised with their own pollen were more productive than those crossed with pollen from a slightly different variety; yet the plants raised from the latter seeds were much taller, heavier, and more hardy than those raised from the self-fertilised seeds. On the other hand, the crossed seedlings of Eschscholtzia were neither taller nor heavier than the self-fertilised, although the crossed flowers were far more productive than the self-fertilised. But the best evidence of a want of correspondence between the number of seeds produced by crossed and self-fertilised flowers, and the vigour of the offspring raised from them, is afforded by the plants of the Brazilian and European stocks of Eschscholtzia, and likewise by certain individual plants of Reseda odorata; for it might have been expected that the seedlings from plants, the flowers of which were excessively self-sterile, would have profited in a greater degree by a cross, than the seedlings from plants which were moderately or fully self-fertile, and therefore apparently had no need to be crossed. But no such result followed in either case: for instance, the crossed and self-fertilised offspring from a highly self-fertile plant of Reseda odorata were in average height to each other as 100 to 82; whereas the similar offspring from an excessively self-sterile plant were as 100 to 92 in average height.
With respect to the innate fertility of the plants of crossed and self-fertilised parentage, given in the previous Table 9/D—that is, the number of seeds produced by both lots when their flowers were fertilised in the same manner,—nearly the same remarks are applicable, in reference to the absence of any close correspondence between their fertility and powers of growth, as in the case of the plants in the Tables 9/F and 9/G, just considered. Thus the crossed and self-fertilised plants of Ipomoea, Papaver, Reseda odorata, and Limnanthes were almost equally fertile, yet the former exceeded considerably in height the self-fertilised plants. On the other hand, the crossed and self-fertilised plants of Mimulus and Primula differed to an extreme degree in innate fertility, but by no means to a corresponding degree in height or vigour.
In all the cases of self-fertilised flowers included in Tables 9/E, 9/F, and 9/G, these were fertilised with their own pollen; but there is another form of self-fertilisation, namely, by pollen from other flowers on the same plant; but this latter method made no difference in comparison with the former in the number of seeds produced, or only a slight difference. Neither with Digitalis nor Dianthus were more seeds produced by the one method than by the other, to any trustworthy degree. With Ipomoea rather more seeds, in the proportion of 100 to 91, were produced from a crossed between flowers on the same plant than from strictly self-fertilised flowers; but I have reason to suspect that the result was accidental. With Origanum vulgare, however, a cross between flowers on plants propagated by stolons from the same stock certainly increased slightly their fertility. This likewise occurred, as we shall see in the next section, with Eschscholtzia, perhaps with Corydalis cava and Oncidium; but not so with Bignonia, Abutilon, Tabernaemontana, Senecio, and apparently Reseda odorata.
SELF-STERILE PLANTS.
The cases here to be described might have been introduced in Table 9/F, which gives the relative fertility of flowers fertilised with their own pollen, and with that from a distinct plant, but it has been found more convenient to keep them for separate discussion. The present cases must not be confounded with those to be given in the next chapter relatively to flowers which are sterile when insects are excluded; for such sterility depends not merely on the flowers being incapable of fertilisation with their own pollen, but on mechanical causes, by which their pollen is prevented from reaching the stigma, or on the pollen and stigma of the same flower being matured at different periods.