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Show 346 SELF-STERILE PLANTS. CHAP. IX. between the sterility of the parent-plants when . ·elffertilised, and the extent to which their offspring suffer in vigour by this process; and so1:1.e su~h c?r_respondence 1niaht have been expected 1f self-stenhty had been acq~ired on account of the jnjury caused by selffertjlisation. 'rhe fact of individuals of the same parentage diff{,ring greatly in their degr~e of selfsterility is likewise opposed to such a behef; unless, indeed, \Ye suppose that certain individuals have been rendered self-sterile to favour intercrossing, whilst other individuals have be n rendered selffertile to ensure the propagation of the species. The fact of self-sterile individuals appearing only occasionally, as in the case of Lobelia, docs not countenance this latter view. But the strongest argtnnent against the belief that self-sterjlity 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 throngh natural sel ction ; 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 n1uch n1anure, heat, &e. I do 11ot, however, wish to maintain that self-sterility may not sometimes be of service to a plant in preventing self-fertilisation; but there are so 1nany other means by which this result 1night be prevented or rendered diffieult, including as we shall see in the nrxt chapter the prepotency of pollen from a distinct individual ov r a plant's own poll n, that self-sterility seems an almost superfluous acquiren1ent for this purpose. . Finally, the most interesting p0int in regard to self- CHAP. IX. SELF-FERTILE VARIETIES. 347 sterile plants is the evidence which thev afford of the advantage, or rather of the necessity, of ~orne 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 whi?~ were selected by chance, could be perfectly fert1hsed by pollen taken from any one of them, but not by their own pollen ; and a few additional trials were made with so1ne other individuals, which I have not thought worth recording. So again, Hildebrand and Fritz lVIiiller frequently speak of self-sterile plants being fertile with the pollen of any other individual· ,nd if there had been any exceptions to the rule, thes~ 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 el~ments of every inclividnal .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 l'e!narked that no two individuals are absolutely ahke .. Therefore we can hardly avoid the eonclusion, ~hat differences of an analogous and indefinite nature In the reproductive systen1 are sufficient to excite the mutual action of the sexual elernents, and that unless there be such differentiation fertility fails. The. appearance of highly self:fm~tile Va1~ieties.-We have JUst seen that the degree to which flowers are capable of being fertilised with their own pollen differs much, both with the species of the same genus, and |