462 



EFFECT OF HEAT 



[Ch. XVUI 



stance diminislies slightly towards the optimum temperature for 

 growth. We conclude, consequently, that in the acceleration 

 of growth by heat all three processes are accelerated, but the im- 



bibitory process 

 than the 



MEAN TEMPERATURE 

 56°FAHR. 53^EAHR. 



Fig. 132. — A chart sJiowing the correlation between the 

 stage of development of the frog on successive days 

 and the temperature at which it has developed. (From 



HiGGENEOTTOM, '50.) 



more 

 others. 



Apparently con- 

 tradictory are the 

 results gained by 



COPELAND ('96), <)^ 



who found that 

 the cells in the 

 foliage of a moss 

 which was trans- 

 ferred from a cold 

 room at 2° to a 

 room at 18° to 20° 

 lost, in from one 

 to two weeks, a 

 degree of turges- 

 cence measured 

 by a 1 to 3% so- 

 lution of potassic 

 nitrate (p. 72). 

 When returned to 

 the cold room the 

 cells gained an 

 increased turges- 

 cence. The nearer 

 the temperature 



lay to the optimum, the more rapid therefore the plant growth, 

 the lower was the turgescence. This seeming paradox can be 

 explained upon the hypothesis that just because the whole tissue 

 is expanding, just because the superficial increase of the organic 

 walls keeps pace, or more than keeps pace, with that of the 

 included water, there is less osmotic pressure in the rapidly 

 growing plants than in the slow-growing ones where the plasma 

 walls are not expanding as fast as the imbibitory process would 

 demand. 



