134 ENVIRONMENTAL CONDITIONS. 



for complex life processes. The processes upon which growth imme- 

 diately depends are wholly or mainly physical, as has been stated above, 

 and these in turn depend upon chemical phenomena. Thus, the 

 formation of cell walls is surely a physical process (precipitation, 

 coagulation, etc.), but it is conditioned by such chemical processes 

 as the formation of cellulose from water-soluble carbohydrates. Under 

 such conditions it is reasonable to expect such physical processes to 

 exhibit chemical temperature coefficients, this being, again, an in- 

 stance of the operation of the law of the minimum. The true physical 

 temperature coefficient of cell-wall formation may never be evidenced, 

 since the rate of chemical formation of wall constituents at the 

 periphery of the protoplasmic mass may never be sufficient to allow 

 their solidification at the maximum rate for any given temperature. 

 Nevertheless, some physical phenomena that have to do with vital 

 processes show, independently of chemical phenomena, temperature 

 coefficients that rather closely approach a value of 2.0. The authors 

 last mentioned call attention to the fact that such is the case with the 

 vapor-tension of water between 4° and 34° C. 



It appears that we have here a general principle that seems to hold 

 with rather satisfactory approximation for a number of different 

 physiological processes in different organisms, for a considerable range 

 of temperatures such as is frequently met with in nature. This 

 problem of the temperature coefficient for physiological processes is 

 by far the most important temperature question now awaiting in- 

 vestigation. Its solution for a large number of plant-forms and for 

 a large number of developmental phases should do much for climatic 

 plant geography and for agriculture.^ We shall return to this matter 

 in another place. 



Aside from the simple matter of amount and duration of the tem- 

 perature of the environment, it is rather widely held that alterations 

 in temperature, if these are of great magnitude and if they occur 

 rapidly and frequently, are in themselves a potent cause for a change, 

 often a retardation, in the rate of plant growth. Frequent changes 

 of temperature seem, per se^ to act as a stimulus upon some plants and to 

 bring about a different form of development from that which might 

 occur under more stationary conditions of temperature. 



Nevertheless, Price (1911) has tested this last proposition in the 

 case of the flower-buds of peach and plum and finds that "a sudden 

 drop of temperature to some point below 50° F. results in the cessation 

 of all development, but that normal development is resumed immedi- 

 ately when favorable temperature conditions are restored, i.e., that the 

 retardation of development by cold is altogether temporary and directly 

 proportional to the time during which the low temperature prevails." 

 It is obvious that this matter is in need of a throughgoing investigation. 



^Certain general aspects of the temperature relations of organisms are well brought out io 

 the following: Fawcett, H. S., The temperature relations of growth in certain parasitic fungi. 

 Univ. Calif. Pub. Agric. Sci. 4: 183-232, 1921. 



