MECHANISM FOR LBAF MOTIONS 



99 



A good many plants of the pea family have leaflets which are 

 sensitive to the touch. The best-known species is the common 

 sensitive plant of the florists, 1 the leaflets of which close and 

 drop, like those of Fig. 108, and the leafstalks droop when the 

 plant is touched or jarred. Some of our common wild plants of 

 the same family 2 have leaves which promptly show irritability 



when touched, and one species is 

 locally known as "shame vine," 

 from this peculiarity. 



116. Structure of the parts 

 which cause leaf motions. In a 

 great number of cases the daily 

 movements of leaves are produced 

 by special organs at the bases of 

 the leafstalks. These 

 cushion-like organs, 

 called pulvini (Fig. 

 109), are composed 

 mainly of paren- 

 chymatous tissue, 

 which contains much 

 water. It is impossi- 

 ble 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 leafstalk 

 bend toward the opposite side. The promptness of these move- 

 ments is no doubt in considerable measure due to the fact that 

 in the pulvini, 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 

 1 Mimosa pudica. 2 Species of Cassia and Desmanthus. 



FIG. 109. 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. After Sachs 



