574 LECTURE XXI. 



light? We have no reason to suppose that the arrest of 

 the growth of the cells of a young leaf when kept in darkness 

 is due to any diminution in the quantity, or to a change in the 

 properties, of the osmotically active substances present in the 

 cell-sap; on the contrary, all that we know on the matter 

 goes to prove that these substances are more abundantly 

 present in darkness than in light. Nor is there any ground 

 for assuming that long-continued darkness induces an increase 

 in the rigidity of the cell-wall. We must assume that it is 

 the protoplasm of the cells which is affected, and when we re- 

 call the behaviour of mature motile organs under the same 

 conditions, and the cessation of the movement of Bacterium 

 photometricum in darkness (p. 298), we see ^that this assump- 

 tion is not groundless. What exactly the nature of the effect 

 produced on the protoplasm is, we cannot decide, as we have 

 no information as to the state of turgidity or flaccidity of the 

 cells in question. All that we can say is that the arrest of 

 growth induced by long-continued darkness is to be ascribed 

 to the abolition of the motility of the protoplasm of the cells. 

 The phototonic influence of light is, then, of this nature, that 

 it restores the motility of the protoplasm ; and it probably 

 does so by rendering possible the performance of certain 

 nutritive processes. 



Now as to the paratonic influence of light. In dealing 

 with this subject we must have this view clearly in our minds, 

 that the paratonic effect of light on plant-organs is not to be 

 attributed to the action of light upon the individual cells of 

 the organ, but to its action on the organ as a whole ; the 

 change induced in the individual cells is the expression of the 

 response of the organ as a whole to the action of light. Thus, 

 we may make the general statement that the effect of exposure 

 to light is to diminish the turgidity of turgid organs. This 

 loss of turgidity by the organ as a whole is of course due to a 

 loss of turgidity by its constituent cells. But in endeavouring 

 to account for this fact we must not say that the organ has 

 become less turgid because exposure to light has diminished 

 the turgidity of its cells, but rather this, that the loss of 

 turgidity by its constituent cells is the response of the organ 



