IRRITABILITY. 571 



sensitive, and its response to these variations is that it per- 

 forms movements of opening or closing according to cir- 

 cumstances, the active mechanism of these movements being, 

 as we have seen, variations in the turgidity of the cells of the 

 upper half of the pulvinus. When the variations in the 

 intensity of light occur periodically, as they do under normal 

 conditions, the response of the leaflet becomes rhythmical ; 

 the movements of opening and closing are performed at 

 definite intervals. And so deeply does this rhythm become 

 impressed upon the leaflet, that it will continue, as we have 

 seen, to perform its daily periodic movements for some days 

 when kept in darkness. 



What is true of the leaflet is true, in the main, of the 

 primary petiole. Because the cells of the upper half of the 

 pulvinus gain in turgidity during the day and lose during the 

 night, and the cells of the lower half lose during the day and 

 gain during the night, we are not justified in assuming that 

 the protoplasm of the cells of the upper half reacts differently 

 from the protoplasm of the cells of the lower half to variations 

 in the intensity of light, as we must assume if we regard the 

 protoplasm as being directly affected by the variations. Such 

 an assumption is altogether inadmissible. It is the leaf as a 

 whole which is affected, and its response to the variations in 

 the intensity of light is such that it sinks downwards during 

 the day and rises during the night, the downward movement 

 being effected by the concomitant increase of turgidity in the 

 cells of the upper half of the pulvinus and decrease in those of 

 the lower half, the upward movement by the concomitant de- 

 crease in the turgidity in the cells of the upper half and 

 increase in those of the lower. 



It is, from the nature of the case, impossible to ascertain 

 by direct investigation what, precisely, the mechanism of the 

 spontaneous movements of motile organs may be, but we may 

 reasonably infer that it essentially consists, like that of induced 

 movements, in variations in the turgidity in the cells of one or 

 both halves of the pulvinus, and that these variations are 

 likewise dependent upon molecular changes in the proto- 

 plasm. 



