*
No one supposes that the rapid movements of the lateral leaflets of 'D.
gyrans' are of any use to the plant; and why they should behave in this
manner is quite unknown. We imagined that their power of movement might
stand in some relation with their rudimentary condition, and therefore
observed the almost rudimentary leaflets of Mimosa albida vel sensitiva (of
which a drawing will hereafter be given, Fig. 159); but they exhibited no
extraordinary movements, and at night they went to sleep like the
full-sized leaflets. There is, however, this remarkable difference in the
two cases; in Desmodium the pulvinus of the rudimentary leaflets has not
been reduced in length, in correspondence with the reduction of the blade,
to the same extent as has occurred in the Mimosa; and it is on the length
and degree of curvature of the pulvinus that the amount of movement of the
blade depends. Thus the average length of the pulvinus in the large
terminal leaflets of Desmodium is 3 mm., whilst that of the rudimentary
leaflets is 2.86 mm.; so that they differ only a little in length.
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