80 GROWTH AND WORK OF PLANTS 



and the tendency to downward growth, which was overcome 

 during the day, now produces its effect. On the following day, 

 however, the light stimulus again overcomes this downward 

 growth and lifts the leaf again. This drooping of leaves at 

 night is often called " sleep of plants." There is an advantage 

 to the plant in this drooping position of the leaf at night, since 

 radiation of heat is less than if the surface were exposed to 

 the zenith. 



135. Movement of leaves in response to touch. Some 

 plants are very sensitive to touch. Remarkable among these are 

 the " sensitive " plants. A good example of sensitive plants is 

 the Mimosa pudica so often grown in greenhouses. The leaves 

 are twice compound and the pinnules (secondary leaflets) are 

 in pairs. If one of these terminal leaflets be pinched with the 

 fingers or with a pair of forceps, the first pair of leaflets close, 

 or fold together above the mid-vein of the pinna. This is fol- 

 lowed by the second pair and so on, and all the pairs of leaflets on 

 this pinna closing in succession. When the last pair, the one at 

 the base, has closed, all the pinnae then move, closing in together, 

 and the pairs of leaflets on the other pinnae then commence to 

 close beginning at the basal pair and extending to the terminal 

 one. Soon also the entire leaf drops down from its point of at- 

 tachment on the stem. If the plant is jarred, all the leaves droop 

 and the leaflets close. At the base of each petiole on the underside 

 near the point of attachment with the stem there is an enlargement, 

 called a cushion (pulvinus) which controls the movement by the 

 contraction or collapse of its cells. There are similar cushions at 

 the base of the leaflets and pinnae. At night, or when placed in 

 darkness, the leaflets close up, and the leaves droop, opening up 

 again with the coming of light. During the day if the sunlight is 

 too strong, the leaves adjust themselves to the profile position, i.e., 

 with the edge towards the source of strongest light. When the 

 soil becomes too dry and the plant is in danger from loss of water, 

 some or all of the leaflets close and the leaf droops, regulating itself 

 according to the degree of dryness or drought, since in the closed 

 position the leaves lose water less rapidly. 



