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.
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