126 ENVIRONMENTAL CONDITIONS. 



III. TEMPERATURE. 



1. TEMPERATURE REQUIREMENT WITHIN THE PLANT. 



One of the fundamental conditions that have to be fulfilled in order 

 that life processes may go forward is that the body of the organism 

 must possess a temperature lying between certain limits; the tem- 

 perature of the living cells must be neither too high nor too low. 

 If the temperature rises beyond the maximum temperature limit for 

 life, or if it falls below the corresponding minimum, death must 

 follow. 



In this consideration, which is one of the most clearly established 

 principles of physiological science, it is to be borne in mind that the 

 numerous processes, or material and energy transformations, that 

 make up life are partly chemical in their nature and partly physical. 

 All processes that result in an alteration in the kind of matter within 

 the plant are chemical. Here belong photosynthesis in green plants, 

 all the various kinds of chemosynthesis, and all processes of oxidation 

 and reduction, of hydration and dehydration in the chemical sense, 

 of polymerization and hydrolysis, etc. On the other hand, all proc- 

 esses that result merely in a change of state of the matter within the 

 plant-body are physical. These latter do not usually receive so 

 much attention at the hands of physiologists as do the others, and 

 they are probably not as well known, but they are certainly no less 

 important. As examples of such physical changes may be men- 

 tioned such processes as coagulation or precipitation of substances 

 out of solution or suspension, the various possible alterations in the 

 viscosity of liquids, and even the transformations that may occur 

 between the solid, liquid, and gaseous states of matter. It is fre- 

 quently true of physiological phenomena that the chemical and 

 physical processes are so closely related that it is impossible to relegate 

 a material change to either category alone. In this connection it 

 may be recalled how modern researches along the border-line between 

 physics and chemistry are tending more and more to erase this line 

 and to prove it to be quite an arbitrary demarcation. 



All of the innumerable processes, physical and chemical, that occur 

 in the living plant must be thought of as having their temperature 

 limits, just as has the grand summation of these processes. In so 

 far as physiological studies have gone in this connection, it appears 

 that each component process possesses temperature limits more or 

 less different from those of others, and also different from those of the 

 grand summation. Thus, with falling temperature growth in size is 

 checked when a certain minimum temperature is reached, at lower 

 minima cell-division and photosynthesis are also checked, and at a 

 still lower minimum respiration ceases and death ensues. It follows 

 from this that general plant activity can not proceed at any tempera- 



