STIMULATION AND ITS RESULTS 373 
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 
the movements will continue for at least a day. The 
rhythm of the nyctitropic movement is excited by the 
Fic. 154.—Nicotiana glauca. (After Darwin.) 
A, shoots with leaves expanded during the day; 3B, the same in the 
nocturnal position. 
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 
night which they appreciate; it is rather the difference 
between the illumination they receive during the two 
