KHYTHM 



363 



our attention here. Fig. 152 represents a longitudinal 

 section through one of them, which occurs at the base of 

 a leaf of Mimosa. The stalk of the leaf shows a swelling 

 at the point of union with the stem, the protuberance 

 being greatest on the under side. Here there is a cushion 



FIG. 152. PULVINUS OF Mimosa. 



a, 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 



