38 CHAP. IV. VARIATION OF TEMPERATURE 



air, which is a bad conductor of heat. The plant could not, 

 therefore, quickly assume the temperature of the outside 

 regulator. In the method of continuous observation which 

 was contemplated, it was essential that the temperature 

 of the plant itself should undergo continuous and rapid 

 change. 



An additional difficulty arose from the radiation of rays 

 of heat from the sides of the thermal regulator, which 

 induces retardation of growth. The enhancement of the 

 rate of growth by rise of temperature is thus antagonised 

 by the effect of radiation. The difficulties arising from the 

 non-conductivity of the air and from thermal radiation 

 were removed by filling the inner plant-chamber with water, 

 which is a better medium for transport of heat. Further 

 experiments showed that the plant quickly assumed the 

 temperature of the water in which it w^as immersed, provided 

 that the thermal rise was at a uniform rate of, say, i° C. 

 per minute. Heat being conveyed by the water, the dis- 

 turbing effect of radiation was eliminated. 



The elongation recorded by the Crescograph during rise 

 of temperature may be due (i) to physical expansion, (2) to 

 expansion resulting from absorption of water by the plant, 

 and (3) to an enhanced rate of growth. The relative values 

 of the first two factors in comparison with growth-elongation 

 were found by carrying out parallel experiments with two 

 different specimens, in one of which growth had already 

 been completed whilst in the other it was still in full activity. 

 Records of the two specimens were taken under the same 

 magnification for a rise of 20° C. In order to keep the 

 record of the actively growing organ within the plate, the 

 magnification had to be reduced to 250 times. The joint 

 effects of thermal expansion and absorption of water in 

 the non-growing plant gave an elongation of only i mm. ; 

 whereas, under similar circumstances, the elongation of the 

 actively growing plant was more than 100 mm. Compared 

 with the physiological reaction, the physical effects were 

 quite negligible. 



