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It consists of a mass of small cells
usually of a pale colour from the absence of chlorophyll, and with its
outline more or less convex, as shown in the annexed figure. In the case of
Oxalis sensitiva two-thirds of the petiole, and in that of Mimosa pudica,
apparently the whole of the short sub-petioles of the leaflets have been
converted into pulvini. With pulvinated leaves (i.e. those provided with a
pulvinus) their periodical movements depend, according to Pfeffer,* on the
cells of the pulvinus alternately expanding more quickly on one side than
on the other; whereas the similar movements of leaves not provided with
pulvini, depend on their growth being alternately more rapid on one side
than on the other.** As long as a leaf provided with a pulvinus is young
and continues to grow, its movement depends on both these causes
combined;*** and if the view now held by many botanists be sound, namely,
that growth is always preceded by the expansion of the growing cells, then
the difference between the movements induced by the aid of pulvini and
Fig. 63.


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