TEMPERATURE 95 



acids are present in the free form but some of them 

 appear mostly as the acid salts of various metals. 



143. Temperature. The relation of the plant to 

 temperature will be discussed here as it is chiefly a ques- 

 tion of the effect of temperature upon the nutritive 

 functions. Five cardinal points for temperature can be 

 distinguished for these different processes. They are: 

 death point from cold, death point from heat (points 

 which are the same whatever the process and mentioned 

 here simply because when reached the process cannot 

 be resumed when normal temperatures are again re- 

 gained), minimum, optimum and maximum. The last 

 three are quite different for different life processes. 

 Thus the optimum and maximum for respiration are 

 usually much higher than for photosynthesis, in fact 

 they often lie close to the death point from heat. Be- 

 tween the death point from cold and the minimum for 

 various processes may be a small range or sometimes 

 a great range of temperature. Usually the minimum 

 point is a little above or not much below 0° C. The 

 maximum temperature for the various functions lies 

 usually between 36° and 43° C. and the death point be- 

 tween 50° and 55° C, but in a few plants of hot springs 

 as well as some bacteria causing the heating of manure, 

 etc., the optimum temperature may be about 60° and 

 the death point even as high as 75° to 85° C. 



144. The death of plants by heat appears to be due 

 to the coagulation of some of the protein constituents of 

 the protoplasm. Since this coagulation cannot occur 

 unless a certain amount of water is present we find that 

 some nearly water-free structures are able to endure 

 rather high temperatures. Thus the spores of some 

 bacteria can be boiled for several hours before they are 

 killed and some seeds can endure a dry heat exceeding 



