EFFECT OF TEM7 J ERATURE I43 



which were extremely sensitive to cold, and thus, notwith- 

 standing the disadvantages incidental to the experiment, 

 I was able to obtain the very interesting records shown 

 below. The experiment was carried out as follows : 



The specimen was mounted in the special plant chamber. 

 By pioper manipulation of the stop-cocks it was possible 

 to send at will through the chamber, first, air at the 

 ordinary temperature — under which conditions the normal re- 

 sponses were taken — and, secondly, air which had been cooled 

 by ice, reducing the temperature of the chamber by several 

 degrees. The responses then 

 obtained showed the effect of cool- 

 ing. And, lastly, ordinary air was 

 re-introduced, and the responses 

 at this temperature again re- 

 corded. Fig. 77 shows that while Fig. 77. Effect of Cold on 



Longitudinal Response 



the amplitude of the normal , . _. , „ 



1 (<7) Normal response ; (n) after 



responses was eight divisions, cooling; (<) during gradual 



that of the responses at reduced [^ alion to normal tem P era - 



tempcrature was only three, and 



that, on the restoration of normal conditions, the responses 

 increased in a staircase manner tending to return to their 

 original value. 



Effect of rise of temperature : (a) on electrical re- 

 sponse. — So much for the results of cooling ; we have now 

 to study the effect of rise of temperature. And, first, I 

 shall refer to observations made by means of electric 

 responses. It, may be said that the optimum degree of 

 temperature for the excitability of the plant must be under- 

 stood to vary with different species. In several cases, how- 

 ever, I have found it to lie at about 22° C. But it must 

 be premised that this optimum of response refers to passive 

 tissues only, that is to say, to those in which there is no 

 growth. The optimum temperature for growth may be 

 different In the ordinary response of passive tissues, heat 

 has only to bring about a condition favourable to mobility. 

 In the case of growing tissues, however, something addi- 



