RHYTHM 363 
our attention here. Fig. 152 represents a longitudinal 
section through one of them, which occurs at the base of 
a leat of Mimosa. The stalk of the leaf shows a swelling 
at the poimt of union with the stem, the protuberance 
being greatest on the under side. Here there ig a cushion 
Fig. 162.—Punvinus or Mimosa. 
u, b, the succulent parenchyma of its upper and lower sides; c, bud; 
d, parenchyma of stem; e, pith. 
of cells which are capable of containing a relatively con- 
siderable quantity of water. When turgid they swell out 
and force the leaf into an erect, or almost erect, position. 
When they part with their water and become flaccid, the 
stalk of the leaf loses its support and the weight of the blade 
causes it to fall downwards. This is rendered more easy 
by the fact that the vascular strand or bundle which passes 
from the stele of the stem through the petiole is somewhat 
