IRRITABILITY. 573 



extremes of heat and cold, and, in the case of organs contain- 

 ing chlorophyll, long-continued darkness, and that in the case 

 of Mimosa pudica, the first effect upon the leaves is to deprive 

 them of their irritability to stimulation ; then the daily 

 periodic movements cease, and finally the spontaneous move- 

 ments. The position which the leaves assume when all move- 

 ment is at an end, indicates that the pulvini are in a state of 

 imperfect turgidity, though unfortunately we have no deter- 

 minations by Brticke's method of the rigidity of the pulvini in 

 the immotile condition. In accordance with our views as to 

 the intimate mechanism of the movements of these organs, we 

 attribute the immotile condition of the pulvini to a destruction 

 of the motility of the protoplasm. 



We now leave the mature motile organs, and pass to the 

 consideration of the mechanism of the movements of growing 

 organs, of the " auxotonic " movements, to use de Vries' term. 

 The mechanism of growth we have learned to be this (p. 334), 

 that the cell is expanded by the hydrostatic pressure of the 

 cell-sap, and that this expansion is rendered permanent by 

 deposition of substance. In view of the facts with which we 

 have already become acquainted, there can be no doubt as 

 to the importance of the absorption of water in bringing 

 about the expansion of the cell. But it is a question whether 

 or not this simply mechanical explanation of the expansion 

 of growing cells is really adequate ; for it is quite possible 

 that the protoplasm takes an active part in producing it. The 

 importance of this consideration will become apparent in the 

 following discussions. 



We will begin by considering the influence of light upon 

 growing organs. We have learned (p. 380) that this influence 

 is two-fold. In the case of most leaves, growth is altogether 

 arrested by prolonged exposure to darkness, and the power of 

 growth is restored on exposure to light ; this we term the 

 phototonic influence of light. In the case of all growing 

 organs, growth is more rapid in darkness than in light ; this 

 effect of light in retarding growth we term the paratonic 

 influence of light. 



What, now, is the nature of the phototonic influence of 



