LEAVES, THEIR FORM AND MOVEMENT 87 



tage to the younger leaves angl to the growing point of the stem 

 since they are protected from drying out when in the delicate 

 stage. When the downward growth comes into play the leaves 

 are usually held in a horizontal position, and do not turn down- 

 ward during the day, because the stimulus of light (see preceding 

 paragraph) holds them in the most favorable light relation. But 

 at sunset the young leaves of many plants, turn downward, 

 because the stimulus of light is removed 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. Movements 

 of mature leaves, as in sensitive plants, are due to changes in 

 turgor (paragraph 65). 



139. 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, 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 under- 

 side near the point of attachment with the stem there is an 

 enlargement called a cushion (pulvinus) which controls the move- 

 ment by the contraction or collapse of its cells. There are 



