CHIEF ENVIRONMENTAL CONDITIONS. 127 



ture that might prevent the occurrence of any of the simpler physical 

 and chemical changes essential to the make-up of this general activity. 



According to the kinetic theory of matter an alteration in the tem- 

 perature of any body is to be considered as a change in the rate of 

 motion of its component particles and, consequently, as a gain or loss 

 of kinetic energy by the latter. The degree to which this energy of 

 motion is possessed by the molecules, etc., of a mixture — that is, its 

 temperature — is to be regarded as the proximate condition determin- 

 ing the nature of the transformations that occur therein. Therefore 

 every temperature change must be regarded as affecting a more or 

 less marked alteration in the velocity of each of the many physical 

 and chemical processes that make up the life activity of a plant. 



It is possible, therefore, to proceed a step farther than we have 

 gone in our generahzation above. Not only is it true that the prime 

 essential temperature condition for general vitality is that a tempera- 

 ture must obtain under which all the necessary component processes 

 may occur, but the temperature must be such that these component 

 processes may go on with adequate velocities. They must not proceed 

 with too high nor with too low rates; otherwise death must occur. 

 Thus photosynthesis, for example, might occur in a plant at a certain 

 temperature, but the rapidity of the formation of carbohydrates might 

 at the same time not be great enough to make up for the loss entailed 

 by respiration, growth, etc., at that temperature. 



As has been mentioned in another place, however, it is quite possible 

 for an organism to survive a brief period of exposure to a condition to 

 which it would succumb with a more prolonged exposure. It is clear, 

 then, that the temperature limits usually given for plants are not 

 definite and quantitative measures of the limiting conditions for vital 

 activity unless they are taken in connection with the length of time 

 during which the organism is subjected to these temperatures. The 

 question of the duration of temperature conditions in connection ^-ith 

 the establishment of physiological limits has received attention from 

 Blackman,! from Miss Matthaei,^ and from Lehenbauer/ and is 

 worthy of still further study. 



From the foregoing paragraph we may formulate the foUomng 

 statement of a general and fundamental principle regarding the rela- 

 tion between temperature and vital activity. The temperature of 

 the living plant-body must not remain for more than a maximum time 

 period at any temperature which, if longer continued, would cause 

 any essential physical or chemical process of the general life acti\dty 

 to surpass the minimum or maximum limit of its velocity. 



1 Blackman, (1905)— Idem. The metabolism of the plant considered as a catalytic reaction, 

 Science, n.s.. 28: 628-636, 1908. 



2 Matthaei, Gabiielle L. C, Experimental researches on vegetable assimilation and respiration. 

 III. On the effect of temperature on carbon dioxide assimilation Phil. Trans. Roy. Soc. London, 



3.197:47-105.1904. t,u • i tj i. 



3 Lehenbauer, P. A., Growth of maize seedlings in relation to temperature, Physiol. Kes., i. 



247-288, 1914. 



