376 VEGETABLE PHYSIOLOGY 



panying or preceding growth. This naturally follows from 

 the fact that the growth of leaves is not as a rule very 

 prolonged. 



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 



FIG. 157. PULVINUS OF Mimosa. 



a, &, the succulent parenchyma of its upper and lower sides ; c, bud ; 

 d, parenchyma of Bachis ; e, pith. 



be seen from a study of the conditions under which they 

 are performed. When the soil is too dry, or when from 

 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 



