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TEXTBOOK OF BOTANY 



Fig. 46. Agave in bloom. From U. S. Biological Survey. 



The pulvinus. At the base of the leaves and leaflets of certain plants, 

 such as beans, clovers, honey locust, and red-bud, there is a thickened 

 portion of the petiole, termed the ptilvimis. It is composed primarily of 

 parenchyma cells; the veins are more nearly central in it than they are 

 in the rest of the petiole ( Fig. 47 ) . Water passes into or out of the cells 

 more freely on one side of the pulvinus than on the other. This unequal 

 movement of water causes unequal enlargement or shrinkage on oppo- 

 site sides of the pulvinus, and a consequent movement of the attached 

 petiole and blade. A more detailed account of the movement of water 

 into and out of cells is included in Chapters XXII and XXIII. 



In a pulvinus of the sensitive plant (Mimosa pudica) such changes 

 in water content may be brought about on a warm day by simply touch- 

 ing the leaf. The observed effects are a hfting of the leaflets, a gradual 

 closure of the branch "petioles," and a downward movement of the main 



