Many of them
are generally dormant and await a period of activity. For some of them
this period comes regularly, while in others the awakening depends upon
external influences, and consequently occurs very irregularly. Those of
the first group correspond to the differences in age; the second
constitute the responses of the plant to stimuli including
wound-injuries.
Some illustrative examples may be quoted in order to give a precise idea
of this general conception of dormant or latent characters. Seed leaves
are only developed in the seed and the seedling; afterwards, during the
entire lifetime of the plant, the faculty of producing them is not made
use of. Every new generation of seeds however, bears the same kind of
seed leaves, and hence it is manifest that it is the same quality, which
shows itself from time to time.
The primary leaves, following the seed-leaves, are different in many
species, from the later ones, and the difference is extremely pronounced
in some cases of reduction. Often, when leaves are lacking in the adult
plant, being replaced by flattened stalks as in the case of the acacias,
or by thorns, or green stems and twigs as in the prickly broom or _Ulex
europaeus_, the first leaves of the young plant may be more highly
differentiated, being pinnate in the first case and bearing three
leaflets in the second instance.
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