Seeds often lie dormant for several years in the ground, and germinate
when brought near the surface by any means, as by burrowing animals.
They would probably be affected by the mere circumstance of having long
lain dormant; for gardeners believe that the production of double
flowers and of fruit is thus influenced. Seeds, moreover, which were
matured during different seasons, will have been subjected during the
whole course of their development to different degrees of heat and
moisture.
It was shown in the last chapter that pollen is often carried by insects
to a considerable distance from plant to plant. Therefore one of the
parents or ancestors of our two plants of Digitalis may have been
crossed by a distant plant growing under somewhat different conditions.
Plants thus crossed often produce an unusually large number of seeds; a
striking instance of this fact is afforded by the Bignonia, previously
mentioned, which was fertilised by Fritz Muller with pollen from some
adjoining plants and set hardly any seed, but when fertilised with
pollen from a distant plant, was highly fertile. Seedlings from a cross
of this kind grow with great vigour, and transmit their vigour to their
descendants. These, therefore, in the struggle for life, will generally
beat and exterminate the seedlings from plants which have long grown
near together under the same conditions, and will thus tend to spread.
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