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 
due to the fact 
that in the pul- 
vini (as in many 
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 
Fie. 114.— Compound Leaf of Bean with 
Pulvinus. (The pulvinus shows as an 
enlargement, in the figure about three- 
eighths inch long, at the base of the 
petiole.) 
