RHYTHM 355 
our attention here. Fig. 152 represents a longitudinal 
section through one of them, which occurs at the base of 
a leaflet of Mimosa. The stalk of the leaflet shows a 
swelling at the point of union with the rachis, the pro- 
tuberance being greatest on the under side. Here there 
is a cushion of cells which are capable of containing a 
Fic, 152.—Punvinus or Mimosa. 
a, b, the succulent parenchyma of its upper and lower sides; c, bud; 
d, parenchyma of rachis ; e, pith. : 
relatively considerable 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 
