212 



ELEMENTARY STUDIES IN BOTANY 



serving the purpose of a compass. It was actually thought, 

 for a time, that the direction of these leaves is controlled in 

 some mysterious way by magnetic influence ; but it is evident 

 that the plane of a profile leaf, exposing its faces to the morn- 

 ing and evening sun, will lie in a general north and south 

 direction. In other Words, the leaves point north and south 

 because the sun rises in the east and sets in the west. It is 

 a significant fact that compass plants grown in the shade do 

 not develop profile leaves. It must not be 

 supposed that there is anything like accuracy in 

 the north and south direction of a profile leaf. 



FIG. 178. Leaf of sensitive plant in two conditions : A, fully expanded, with the four 

 main branches and numerous leaflets well spread ; B, after a shock of some kind, 

 the leaflets having been thrown together forward and upward, the four branches 

 having been moved toward each other, and the main petiole having dropped sharply 

 downward. After DUCHARTRE. 



It may be the prevailing general direction of the profile 

 leaves of the rosinweed referred to, but in the prickly 

 lettuce (Fig. 177), a very common weed of waste grounds 

 even in cities, and one of the most striking of the compass 

 plants, the profile position is frequently assumed without 

 any reference to the north or south direction of the apex. 



Still more specialized are the motile leaves, which have the 

 power of shifting their position according to the needs, direct- 

 ing their flat surfaces towards the light, or more or less 



