But in
diameter they differ much, that of the pulvinus of the little leaflets
being only 0.3 mm. to 0.4 mm.; whilst that of the terminal leaflets is 1.33
mm. If we now turn to the Mimosa, we find that the average length of the
pulvinus of the almost rudimentary leaflets is only 0.466 mm., or rather
more than a quarter of the length of the pulvinus of the full-sized
leaflets, namely, 1.66 mm. In this small reduction in length of the
pulvinus of the rudimentary leaflets of Desmodium, we apparently have the
proximate cause of their great and rapid circumnutating movement, in
contrast with that of the almost rudimentary leaflets of the Mimosa. The
small size and weight of the blade, and the little resistance opposed by
the air to its movement, no doubt also come into play; for we have seen
that these leaflets if immersed in water, when the resistance would be much
greater, were prevented from jerking forwards. Why, during the reduction of
the lateral leaflets of Desmodium, or during their reappearance--if they
owe their origin to reversion--the pulvinus should have been so much less
affected than the blade, whilst with the
* Desmodium vespertilionis is closely allied to D.
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