A pulvinus seems to be
formed (as formerly shown) by a group of cells ceasing to grow at a very
early age, and therefore does not differ essentially from the surrounding
tissues. The cotyledons of some species of Trifolium are provided with a
pulvinus, and others are destitute of one, and so it is with the leaves in
the genus Sida. We see also in this same genus gradations in the state of
the development of the pulvinus; and in Nicotiana we have what may probably
be considered as the commencing development of one. The nature of the
movement is closely similar, whether a pulvinus is absent or present, as is
evident from many of the diagrams given in this chapter. It deserves notice
that when a pulvinus is present, the ascending and descending lines hardly
ever coincide, so that ellipses are habitually described by the leaves thus
provided, whether they are young or so old as to have quite ceased growing.
This fact of ellipses being described, shows that the alternately increased
turgescence of the cells does not occur on exactly opposite sides of the
pulvinus, any more than the increased growth which causes the movements of
leaves not furnished with pulvini.
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