LEAF ADJUSTMENT 



51 



61. Compass Plants. — A very remarkable adjustment 

 is that of the rosinweed, or com- 

 pass plant {Silphium laciniatmn), 

 which grows in the prairies of Ala- 

 bama and westward, where it is 

 exposed to intense sunlight. The 

 leaves not only stand vertical, but 

 have a tendency to turn their edges 

 north and south so that the blades 

 are exposed only to the gentler 

 morning and evening rays. The 

 prickly lettuce manifests the same 

 habit. 



62. Leaves that go to Sleep. — 



The leaves of many plants change 



99 too 



99, 100. — A compass plant, 

 rosinweed {Silphium lacini- 

 atuni) : 99, seen from the 

 their position at night as if folding east ; loo, seen from the 



themselves for sleep. This habit is ^°"'^- 

 especially noticeable in certain members of the pea family 

 and also in the wood sorrel and the cultivated oxalis of 

 the gardens. The motions may be either spontaneous, as 

 in the telegraph plant {Desmodium gyrans), or in response 



to various external 

 agents, as light, heat, 

 irritation by contact 

 with other sub- 

 stances, etc. The po- 

 sitions assumed are 

 various and may 

 even differ in differ- 

 ent parts of the same 

 compound leaf ; in 

 the kidney bean {Phaseohts), for instance, the common 

 petiole turns up at night and the separate leaflets down. 



63. Experiments. — Place a healthy plant of oxalis, 

 spotted medick, or white Clover in a pot and keep it in your 

 room for observation. Notice the changes of position the 

 leaves undergo. Sketch one as it appears at night and in 



