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With some species of Cassia, on the other hand, it was obvious without any
measurement that the pulvinated cotyledons continued to increase greatly in
length during some weeks; so that here the expansion of the cells of the
pulvini and the growth of the petiole were probably combined in causing
their prolonged periodic movements. It was equally evident that the
cotyledons of many plants, not provided with pulvini, increased rapidly in
length; and their periodic movements no doubt were exclusively due to
growth.
In accordance with the view that the periodic movements of all cotyledons
depend primarily on the expansion of the cells, whether or not followed by
growth, we can understand the fact that there is but little difference in
the kind or form of movement in the two sets of cases. This may be seen by
com-
[page 115]
paring the diagrams given in the last chapter. Thus the movements of the
cotyledons of Brassica oleracea and of Ipomoea caerulea, which are not
provided with pulvini, are as complex as those of Oxalis and Cassia which
are thus provided.


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