374 



VEGETABLE PHYSIOLOGY 



accompanying rhythmic variation of temperature no doubt 

 in some cases also plays a part in the stimulation. 



If a plant which changes the position of its leaves 

 as described, is placed for a time under constant conditions 

 such as darkness, the periodic movement is soon very 

 much interfered with, even before the effect of darkness is 

 evident in the loss of tone. If the rhythmic stimulus 

 is not regularly applied the movement ultimately stops. 

 The cessation is not however abrupt, but with most plants 



FIG. 156. Nicotiana glauca. (After Darwin.) 



A, shoots with leaves expanded during the day ; B, the same in the 

 nocturnal position. 



the movements will continue for at least a day. The 

 rhythm of the nyctitropic movement is excited by the 

 stimulus, and is dependent for its permanency upon the 

 continuation of the stimulating changes. Plants which 

 are found in other countries to show this sensibility will, 

 when cultivated in England, perform the movements at 

 the normal hours, and not at times corresponding to the 

 occurrence of day and night in the countries from which 

 they come. Nor is it the mere alternation of day and 



