SLEEP OF PLANTS. 249 



plicated movement, which it is rather difficult to 

 comprehend at once from a description, is another 

 example of the prodigality of means by which in 

 nature the same end is accomplished. 



The common chickweed {Stcllaria media) has the 

 leaves in pairs, opposite to each other, and as night 

 approaches these leaves rise upwards with their faces 

 towards each other, the uppermost pair but one 

 closing over the terminal pair, and thus the growing 

 point is protected. 



The cultivated nasturtium {Tropaoluvi) has a very 

 simple nocturnal motion, which is not observed unless 

 the plants have been well illuminated during the day. 

 These leaves have naturalh' a tendency to turn to 

 the sun, so that when growing in full light the blade 

 of the leaf is sloped. At night, however, they become 

 vertical by the bending of the footstalk at about an 

 inch below the blade of the leaf. In the morning 

 they resume again their diurnal position. Were it 

 not the fact that these leaves maintain a vertical 

 position during the night, and assume a more 

 horizontal one in the early morning, it might have 

 been thought that the movements in these leaves 

 were only the result of heliotropism, or turning to the 

 sun. A series of experiments has, however, demon- 

 strated that this is not the case, and that the plant 

 has its true nocturnal motion. 



Several species of lupins are under cultivation. 



