BHYTHM 



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 



FIG. 152. PULVINUS OP 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 



