[page 409]
mobility of the stems and leaves in this family, as shown by the large
number of climbing species which it contains. Next to the Leguminosae come
the Malvaceae, together with some closely allied families. But by far the
most important point in the list, is that we meet with sleeping plants in
28 families, in all the great divisions of the Phanerogamic series, and in
one Cryptogam. Now, although it is probable that with the Leguminosae the
tendency to sleep may have been inherited from one or a few progenitors,
and possibly so in the cohorts of the Malvales and Chenopodiales, yet it is
manifest that the tendency must have been acquired by the several genera in
the other families, quite independently of one another. Hence the question
naturally arises, how has this been possible? and the answer, we cannot
doubt is that leaves owe their nyctitropic movements to their habit of
circumnutating,--a habit common to all plants, and everywhere ready for any
beneficial development or modification.
It has been shown in the previous chapters that the leaves and cotyledons
of all plants are continually moving up and down, generally to a slight but
sometimes to a considerable extent, and that they describe either one or
several ellipses in the course of twenty-four hours; they are also so far
affected by the alternations of day and night that they generally, or at
least often, move periodically to a small extent; and here we have a basis
for the development of the greater nyctitropic movements.
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