STIMULATION AND ITS RESULTS 



388 



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 



FIG. 155. PULVINUS 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 



