IRRITABILITY. 575 



as a whole to exposure to light. We shall illustrate this 

 subject more fully when we have to consider the daily periodic 

 movements of certain floral and foliage leaves. 



We have now to enquire into the mechanism of this 

 response : by what means does the organ thus respond to the 

 stimulus of exposure to light ? One of the most striking 

 cases of the paratonic influence of light, an influence, be it 

 said, which does not only affect growing organs, is afforded 

 by the diminution in the tensions of the tissues induced by 

 exposure to light, allusion to which was made in a previous 

 lecture (p. 405). This diminution in the tissue-tensions is due, 

 as was pointed out, mainly to a loss of water by the paren- 

 chymatous cells. Without again discussing whether this loss 

 of water takes place in consequence of changes in the proper- 

 ties of the cell-wall, or of the cell-sap, or of the protoplasm, 

 we will simply assume that it is the protoplasm which is 

 directly concerned. Leaving out of the question the possibi- 

 lity that the expulsion of water from the cells is to some 

 extent effected by an active contraction of the protoplasm, 

 we will account for it in this way, that the permeability of the 

 protoplasm is increased so that water is now forced out by 

 filtration under the pressure of the elastic cell-wall. The 

 retardation of the growth of an organ when exposed to light 

 is due to a diminution in the turgidity of the growing cells 

 brought about in the manner just described. 



So far we have had the case of a multicellular organ 

 in view ; we will now briefly consider the case of a unicellular 

 organ. We have learned, for instance, that the growth of a 

 sporangiferous hypha of Phycomyces is retarded on exposure 

 to light (p. 394). In the absence of any direct information 

 as to the turgidity of the organ, we may assume, in view of 

 the facts now before us, that it is diminished. Now diminu- 

 tion of turgidity means loss of water. It is clear, inasmuch 

 as the whole plant consists of one continuous much-branched 

 hypha, that water cannot be driven from the sporangiferous 

 hypha into other parts of the plant. In this case it is a 

 question not of redistribution of water in the plant, but of a 

 loss of it. The loss is probably due, ultimately, to increased 



