228 



THE EFFECT OF TEMPERATURE 



at 10. At 31 to 34.9, however, the rate was only 9.0 

 times greater. Better instances of the more and more 

 unfavourable influence of increasing high temperature 

 are found amongst plants, as in them the optimum 

 temperature is much further removed from the " maxi- 

 mum ' ' temperature (the highest temperature at which 

 growth can take place at all) than it is in animals. 

 The following table shows the increments in the length 

 of the hypocotyls of various plants in a period of 48 

 hours, as determined by Koppen and by De Vries:* 



The optimum temperature was about 27 for every 

 plant but one, viz., Zea mais, and in this case it was 

 33.5. The rate of growth at the optimum was, in the 

 various plants, respectively 5.9, 10.8, 63.2, 13.7, 12.2, 

 and 29.9 times greater than at the lowest temperature 

 at which it was observed, and respectively 4.3, 6.2, 3.4, 



* Quoted from Vines' " Physiology of Plants," p. 293. 



