CHAPTER XII. 



excf:ss of heat. 



Death from Heat. 



Supported by numerous psychological works^ we have arrived at the 

 conclusion that, in judging injuries produced by excess of heat, the same 

 points of view make themselves felt as in judging those due to lack of 

 heat. In our cultivated plants we are confronted by constantly changing 

 organizations. Not only has each species its special requirements as to the 

 amount of heat which it can endure, but, even within a wide range of heat, 

 the different individuals, in each species, and indeed their different develop- 

 mental stages, behave quite differently. The individual susceptibility to a 

 degree of heat, exceeding tlie optimum amount, varies according to the 

 habitat, the supply of water and nutritive substances and the action of the 

 other vegetative factors so that definite figures as to admissable temperature 

 values can only have a limited validity. 



We see, from this, that, in our plantations, the plants can accustom 

 themselves to higher amounts of heat up to a certain degree. Their struc- 

 ture becomes different, their development more rapid, but their life pro- 

 cesses, as a whole, still take place within the latitude of health. In regard 

 to the different susceptibility of the different organs, according to their 

 momentary developmental stage, we favor the theory that the part of the 

 plant is the more resistant to an excess of heat, the richer the tissues are 

 in cyptoplasm and the relatively poorer in water. Death from heat, like 

 death from frost, is produced by the irreparable destruction of the mole- 

 cular structure of the cytoplasmic body. We do not know in what way 

 this takes place, nor how far a coagulation of certain protein bodies co-oper- 

 ates in it. The more porous the cytoplasmic body is within its specific 

 composition, due to the in-layering of abundant water, the more easily such 

 a destruction takes place. On this account we find that organs, rich in 

 water, die more quickly from excess of heat. Death from heat is often 

 preceded by a "heat rigor," from which the plants can recover^ when the 

 super-maximal temperature abates, and can begin their growth again. The 



1 Pfeffer, W., Pflanzenphysiologie, 2d ed., Vol. II, Leipzig 1904. 



