Scarcely any result from my experiments has surprised me so much
as this of the prepotency of pollen from a distinct individual over each
plant's own pollen, as proved by the greater constitutional vigour of
the crossed seedlings. The evidence of prepotency is here deduced from
the comparative growth of the two lots of seedlings; but we have similar
evidence in many cases from the much greater fertility of the
non-castrated flowers on the mother-plant, when these received at the
same time their own pollen and that from a distinct plant, in comparison
with the flowers which received only their own pollen.
From the various facts now given on the spontaneous intercrossing of
varieties growing near together, and on the effects of cross-fertilising
flowers which are self-fertile and have not been castrated, we may
conclude that pollen brought by insects or by the wind from a distinct
plant will generally prevent the action of pollen from the same flower,
even though it may have been applied some time before; and thus the
intercrossing of plants in a state of nature will be greatly favoured or
ensured.
The case of a great tree covered with innumerable hermaphrodite flowers
seems at first sight strongly opposed to the belief in the frequency of
intercrosses between distinct individuals. The flowers which grow on the
opposite sides of such a tree will have been exposed to somewhat
different conditions, and a cross between them may perhaps be in some
degree beneficial; but it is not probable that it would be nearly so
beneficial as a cross between flowers on distinct trees, as we may infer
from the inefficiency of pollen taken from plants which have been
propagated from the same stock, though growing on separate roots.
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