LEAVES AND THEIR USES 253 



of this family, as well as some of other families, including 

 various species of Oxalis, do respond to the stimulus of a 

 change from light to darkness by changing the positions 

 of their leaflets. The night positions of bean leaves have 

 been referred to, and those of clover leaves are very similar. 

 Upon the return of daylight the leaves and leaflets spread 

 out again in the position with which we are more familiar. 



264. Formation of Habits. It has just been said that 

 the night movements of leaflets and leaves are responses to 

 the stimulus of a change from light to darkness. Another 

 factor also may play a part in these movements. If a good- 

 sized bean plant or sensitive plant is placed in a dark room, 

 the leaves, as we should expect, take their night position. 

 But if the plant is kept in the dark room, we find that after 

 about twelve hours the leaves spread out into something 

 like the position that they ordinarily occupy in the daytime, 

 though they do not open so fully as they would if they were 

 exposed to bright daylight. At the end of another twelve 

 hours they fold ; after twelve hours more they open ; and 

 so on for some days. If the plant is exposed to continuous 

 light instead of continuous darkness, the same thing hap- 

 pens. These results mean that, having been exposed al- 

 ternately to light and darkness at intervals of about twelve 

 hours during its life, the plant has formed the habit of 

 changing the position of its leaves at those intervals, and 

 that now, even if there is no change from light to darkness 

 or vice versa, it goes on with its leaf movements in the 

 way that it has formed the habit of doing. 



A young plant which has been kept in continuous light all 

 its life has not formed the habit of changing the position of 

 its leaves each morning and evening. But as soon as it is 

 placed under ordinary conditions of light and darkness, its 

 leaflets begin to fold up each night and to spread out again 

 each morning, and in time the habit becomes fixed upon the 

 plant. The formation of a -habit such as this by a' plant is 



