IRRITABILITY. 391 



parenchymatous cells, Jhen, of etiolated internodes -do not 

 merely expand, but actually grow. T/he excess of water 

 in etiolated as compared with normal internodes is doubtless 

 attributable to the fact, to which attention has been already 

 drawn (p, 268), that the cell-sap of the former is richer in 

 organic acids than that of the latter. In view of the osmotic 

 activity of these substances (p. 41), it may be inferred that the 

 turgidity of the cells is greater in etiolated than in normal 

 internodes. From observations made in his laboratory, by 

 means of the method of plasmolysis, on normal and etiolated 

 internodes, which shew that the amount of shortening is about 

 the same in both, Pfeffer concludes that the turgidity is the 

 same in both. It must, however, be borne in mind that the 

 shortening exhibited by an organ on plasmolysis is simply 

 the expression of that amount of elongation of the cells due 

 to the hydrostatic pressure of their contents, which has not 

 yet been rendered permanent by actual growth. It is quite 

 possible to imagine that the elongation of highly turgid cells 

 might be so rapidly followed up and rendered permanent by 

 growth that an internode would shorten scarcely at all on 

 plasmolysis. What Pfeffer's observations tend to prove is 

 that the elongation of the cells in etiolated internodes is 

 rendered permanent by growth more rapidly than in normal 

 internodes. The final conclusion to be drawn is this, thaC 

 since there is reason to believe that the turgidity of the cells 

 of etiolated internodes is greater than that of normal inter- 

 nodes, and since the shortening of the former on plasmolysis 

 is not greater than that of the latter, the increase in surface 

 of the cell-walls by actual growth is more active in the 

 former than in the latter. 



It is, then, to the active growth of their parenchymatous 

 cells that the excessive elongation of etiolated internodes 

 is to be ascribed. We will now enquire into the causes of 

 this more active growth in darkness. Various explanations 

 have been offered. It has been suggested, namely, that 

 under these conditions the normal correlation of nutrition 

 between the different organs may be interfered with. For 

 example, Famintzin observed, in the case of Cress-seedlings, 



