STIMULATION AND ITS RESULTS 383 
That these movements are essentially dependent on 
the power of the protoplasm to receive impressions from 
without, or in other words upon its possession of tone, can 
be seen from a study of the conditions under which they 
are performed. When the soil is too dry, or when from 
Fie. 155.—Putvinus oF Mimosa. 
a, b, the succulent parenchyma of its upper and lower sides; c, bud; 
d, parenchyma of stem; e, pith. 
any other cause the protoplasm in the cells is not supplied 
with water in sufficient quantity, they cease. When the 
temperature is too low they are interfered with. Violent 
disturbance of the protoplasm by shaking the plant will in 
some cases prevent their occurrence for one or two nights. 
The purpose of the movement is somewhat obscure; it 
frequently serves to protect the delicate leaves from excessive 
