146 



FOUNDATIONS OF BOTANY 



(Sect. 106), which contains much water. It is impossible 

 fully to explain in simple language the way in which the 

 cells of the pulvini act, but in a general way it may be 

 said that changes in the light to which the plant is exposed 

 cause rather prompt changes in the amount of water in 



the cells in one portion or 

 other of the pulvinus. If the 

 cells on one side are filled 

 fuller of water than usual, 

 that side of the pulvinus will 

 be expanded and make the 

 leaf-stalk bend toward the 

 opposite side. The prompt- 

 ness of these 

 movements is no 

 doubt in consid- 

 erable measure 



FIG. 114. Compound Leaf of Bean with '^'J due to the fact 



Pulvinus. (The pulvinus shows as an || | . . , .. 



enlargement, in the figure about three- that in the pul- 



eighths inch long, at the base of the yini /^ in many 



petiole.) J 



other parts of 



plants) the protoplasm of adjacent cells is connected. 

 Delicate threads of protoplasm extend through the cell- 

 walls, making the whole tissue a living web, so that any 

 suitable stimulus or excitant which acts on one part of 

 the organ will soon affect the whole organ. 



155. Vertically Placed Leaves. Very many leaves, like 

 those of the iris (Fig. 44), always keep their principal sur- 

 faces nearly vertical, thus receiving the morning and even- 

 ing sun upon their faces, and the noonday sun (which is 

 so intense as to injure them when received full on the 



