RISE OF TEMPERATURE 389 



gives the changes induced in Thunbergia by a cycHc varia- 

 tion of temperature, first of thermometric fall to the 

 critical point, and then of rise to normal temperature. 



Effect of rise of temperature. — The torsional activity 

 increased with rise of temperature till an optimum was 

 reached. Two different t^^pes of effect were observed, as 

 described below. 



Experiment 208. — The first type is exemplified by 

 Porana. The activity increased till the optimum tempera- 

 ture was reached, which is between 32° and 33° C. ; above 

 this point it underwent a decline. It may be stated here 

 that my other investigations show that the optimum tem- 

 perature for growth, for photosynthesis, for ascent of sap, 

 for transpiration, and for maximum sensibility of Mimosa, 

 is also about 33° C. This characteristic, it should be 

 remembered, relates to plants in the tropics. 



Table XLIV. — Showing Variatiox of Torsional Activity 

 UNDER Rise of Temperature (Porana). 



Experiment 209. — Thunbergia represents the second 

 type in which there are two optima instead of one. The 

 rate of torsion increased up to 32° C, which is the first 

 optimum. There was then a continuous decline till 40° C, 

 after which the rate showed a sudden increase, reaching 

 its maximum at 47° C, which is the second optimum. This 

 double optimum will be noted in the record given in fig. 219, 

 where the violent contraction at 60° C. is the death-spasm 

 which will now be explained. 



