With respect to leaves, the evidence of their modification
is ample. In young plants of Bignonia the lower leaves often remain
quite unchanged, whilst the upper ones have their terminal leaflets
converted into perfect tendrils; in Eccremocarpus I have seen a
single lateral branch of a tendril replaced by a perfect leaflet; in
Vicia sativa, on the other hand, leaflets are sometimes replaced by
tendril-branches; and many other such cases could be given. But he
who believes in the slow modification of species will not be content
simply to ascertain the homological nature of different kinds of
tendrils; he will wish to learn, as far as is possible, by what
actual steps leaves, flower-peduncles, &c., have had their functions
wholly changed, and have come to serve merely as prehensile organs.
In the whole group of leaf-climbers abundant evidence has been given
that an organ, still subserving the functions of a leaf, may become
sensitive to a touch, and thus grasp an adjoining object. With
several leaf-climbers the true leaves spontaneously revolve; and
their petioles, after clasping a support grow thicker and stronger.
We thus see that leaves may acquire all the leading and
characteristic qualities of tendrils, namely, sensitiveness,
spontaneous movement, and subsequently increased strength.
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