MOVEMENT IN PLANTS. 25 



its nourishment. This pulvinus is the lower portion 

 of the petiole, or leaf-stalk ; and the movements of 

 the leaf depend upon its cells, which expand alter- 

 nately, first on one side, then on the other. Struc- 

 turally, the pulvinus consists of small cells arrested 

 in their development while still young. These, 



Fig. 1. After Darwin. Longitudinal section of a Pulvinus, magnified seventy- 

 five times, p p, petiole or leaf-stalk ; /, fibro-vascular bundle ; & 6, com- 

 mencement of blade of cotyledon. 



when turgescent with sap, swell up, thus increasing 

 suddenly the bulk of the structure composed of 

 them : and the turgescence leads to motion only it 

 is not followed by growth ; no nutrition of the plant 

 results from the turgescence. It must be noted that 

 the cells possessing this. -special function of pro- 

 ducing motion only, fulfilling no direct nutritive 

 purpose, are smaller than their neighbours, arrested 



