Factors in Evolution 37 



other low forms growing in hot springs like those 

 of Yellowstone Park and elsewhere are said to en- 

 dure a temperature of 85 C. or even more. Some 

 bacteria, also, thrive in similar high temperatures 

 which would be immediately fatal to most plants. 

 These heat-loving organisms must have protoplasm 

 of somewhat different constitution from that of 

 most plants, as a temperature much lower than that 

 at which they thrive causes the coagulation of the 

 albuminous contents of the cells of most plants, and 

 this means the death of the protoplasm. 



Among the higher plants, many of the desert 

 forms, under the fierce rays of an unclouded 

 tropical sun, must be exposed to temperatures 

 which would quickly destroy the protoplasm of the 

 living cells were these not amply protected 

 so that they are not fully exposed to the un- 

 tempered heat of the sun's rays. Naturally 

 plants of warm regions differ much from those 

 of colder countries in the range of temperature 

 suitable for their growth. Thus while the com- 

 mon white mustard of Northern Europe will germ- 

 inate at a temperature near the freezing point, In- 

 dian corn, which is of tropical origin, requires 9 C. 

 before its seeds will sprout. The optimum tem- 

 perature for the mustard is 27, the maximum tem- 

 perature which it will endure being 37. For corn 

 the optimum and maximum temperatures are re- 

 spectively 34 and 46, but the latter figure refers 

 to the endurance of the plant when placed in water 



