IRRITABILITY. 577 



movement of closing which follows on a decrease in the 

 intensity, is due to antagonistic effects induced simultaneously 

 in the cells of the two halves of the growing region by the 

 direct action on them of the stimulus : it is the leaf as a whole 

 which is stimulated, and its response to a sudden increase in 

 the intensity of the light is that it opens, and to a sudden 

 decrease, that it closes. 



The manner in which temperature affects growth may 

 next be considered. We have learned (p. 293) that there is 

 for every growing organ a minimum temperature at which 

 growth begins, an optimum at which it is most rapid, and a 

 maximum at which it is arrested. Inasmuch as this relation 

 to temperature has been established in the case of almost 

 every vital process of plants, there can be little doubt that it 

 is the protoplasm of the growing cells which is especially 

 concerned. Though there can be no doubt that at different 

 temperatures the actual process of growth, the building up 

 of the cell-structure, takes place with different degrees of 

 rapidity, yet the rate of growth at these temperatures must 

 be dependent in the first instance upon the turgidity of the 

 cells. Kraus has shewn that, at low temperatures, the tension 

 of the tissues, or in other words, the turgidity of the paren- 

 chymatous cells, is very slight. This we attribute to the 

 induction of a permeable condition of the protoplasm. If the 

 organ be exposed for a long period to a low temperature, no 

 variations in the tissue-tensions can be observed ; the motility 

 of the protoplasm is abolished. The same holds good appa- 

 rently, though the observations are not so decisive, of the 

 effect of excessively high temperatures. From these facts we 

 can readily infer how it is that at extreme temperatures 

 growth is arrested. As the temperature rises above the 

 minimum, the turgidity of the parenchymatous cells, as esti- 

 mated both by the tensions and by the actual amount of water 

 which they contain, increases, and, accordingly, growth is 

 resumed ; the protoplasm resumes its normal condition. An 

 optimum temperature for the turgidity of the cells has not 

 been determined, but it cannot be doubted that there is one, 

 inasmuch as it has been determined in the case of growth. 



V. 37 



