112 SLEEP-MOVEMENTS [CH. 



due principally to internal causes, though they may be 

 modified and directed by the directive action of gravita- 

 tion and light as long as growth is still going on. The 

 principal feature for us to notice here is that all these 

 movements are adapted to bring the leaf into the best 

 position for illumination by the sun's rays. 



When the growth of the leaf is completed, it is never- 

 theless noticed that the position of the lamina is not 

 absolutely fixed, but that it describes periodic alterations 

 of position about a mean. Some of the most important 

 of these variations are again traced to the action of light 

 varying in intensity from hour to hour, and since the 

 nocturnal variations are most pronounced, these sleep- 

 movements are often called nyctitropic. 



As the intensity of the light increases, the leaf raises 

 or lowers the inclination of the plane of its lamina, and 

 performs the reverse movement as the intensity of the 

 light diminishes. Leaves have therefore a diurnal and a 

 nocturnal position, and it is found that the rays of light 

 which induce the diurnal position are the more refrangible 

 ones : in red light the lamina assumes the sleeping 

 position. 



These nyctitropic movements of fully-developed leaves, 

 induced by variations in the intensity of the light, are 

 again, at least for the present, distinguished from the 

 spontaneous periodic movements occurring in many leaves: 

 the last remark is the more necessary, because the two 

 classes of movements often conflict, rendering it difficult 

 to trace them to their proximal causes. 



It is owing to the nyctitropic movements of the leaves, 

 and the different nocturnal positions of the more erect or 

 more pendent laminae, that plants like Clover, Robinia 

 and other Leguminosas, Oxalis, Marsilia, Stellaria, Malva, 

 Impatiens, and even some Monocotyledons (Maranta, Colo- 



