But a conversion of this kind will not have occurred
unless cross-fertilisation was already assured, generally by the agency
of insects; but why the production of male and female flowers on
distinct plants should have been advantageous to the species,
cross-fertilisation having been previously assured, is far from obvious.
A plant might indeed produce twice as many seeds as were necessary to
keep up its numbers under new or changed conditions of life; and if it
did not vary by bearing fewer flowers, and did vary in the state of its
reproductive organs (as often occurs under cultivation), a wasteful
expenditure of seeds and pollen would be saved by the flowers becoming
diclinous.
A related point is worth notice. I remarked in my Origin of Species that
in Britain a much larger proportion of trees and bushes than of
herbaceous plants have their sexes separated; and so it is, according to
Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in
the 'London Catalogue of British Plants' that there are thirty-two
indigenous trees and bushes in Great Britain, classed under nine
families; but to err on the safe side, I have counted only six species
of willows. Of the thirty-two trees and bushes, nineteen, or more than
half, have their sexes separated; and this is an enormous proportion
compared with other British plants. New Zealand abounds with diclinous
plants and trees; and Dr.
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