On the
other hand, with dichogamous plants, early or late flowers on the same
individual may intercross; and a cross of this kind does hardly any or no good.
Whenever it is profitable to a species to produce a large number of seeds and
this obviously is a very common case, heterostyled will have an advantage over
dioecious plants, as all the individuals of the former, whilst only half of the
latter, that is the females, yield seeds. On the other hand, heterostyled plants
seem to have no advantage, as far as cross-fertilisation is concerned, over
those which are sterile with their own pollen. They lie indeed under a slight
disadvantage, for if two self-sterile plants grow near together and far removed
from all other plants of the same species, they will mutually and perfectly
fertilise one another, whilst this will not be the case with heterostyled
dimorphic plants, unless they chance to belong to opposite forms.
It may be added that species which are trimorphic have one slight advantage over
the dimorphic; for if only two individuals of a dimorphic species happen to grow
near together in an isolated spot, the chances are even that both will belong to
the same form, and in this case they will not produce the full number of
vigorous and fertile seedlings; all these, moreover, will tend strongly to
belong to the same form as their parents.
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