452 PLANT RESPONSE 



optimum point, for Crinum Lily and the hypocotyl ot Balsam 

 The balanced reading at 30 C. is taken as zero. The ad- 

 justment of the stop-cocks for regulation of outflow was 

 different in the two cases. 



(4) The method of excitatory response.— The method 

 which I am about to describe — and by which the relative 

 rates of growth at different temperatures are afforded in- 

 directly—is one of much theoretical importance, for it proves 

 what I have already suggested, that growth is a phenomenon 

 of excitatory response. This being so, it would follow that 

 the reason why growth is at its optimum at about 35 C. in 

 the case of most tropical plants, is that the excitability of 

 the tissue is greatest at that temperature. The different 

 excitabilities at different temperatures might further be 

 expected, this being true, to offer an independent indication 

 of the characteristic rate of growth of the tissue at those 

 temperatures. 



The excitability of the tissue can be tested, in the case 

 of radial organs, by its longtitudinal contractile response to 

 external stimulus, which, as we have seen, will be represented 

 in growing organs by a retardation of growth, proportionate 

 to the excitability. We must bear in mind, at this point, 

 certain differences between responsive effects in mature and 

 in growing organs. In the former, owing to the increase of 

 internal energy brought about by rise of temperature, the 

 tissue becomes over-turgid and the internal hydrostatic 

 pressure is greatly increased. The contractile action of 

 external stimulus is thus strongly resisted by the tissue, 

 which in this way antagonises the normal extent of response 

 (P- 338). Similarly, a closed india-rubber ball, fully distended 

 with water, will not yield to any great extent when struck. 

 But if we have, instead, a tube through which water is 

 running, the flexible pipe when struck will yield, and cause a 

 proportionate retardation or reversal of current behind. We 

 have a case somewhat analogous in a growing organ. For 

 here the tissue cannot be regarded as closed, since it is 

 constantly elongating. It therefore represents, not a static 



