132 ENVIRONMENTAL CONDITIONS. 



Ag, 2.55), sometimes it is over 3 (NaOCaHs+CHgl, 3.34)2°, ^^^d it 

 may be still higher. 



Since the temperature of the plant follows so closely that of the 

 surroundings, it will be safe to consider these two temperatures as 

 identical for our present purpose. Of course, we shall expect to find 

 that the principle of Van't Hoff and Arrhenius may be applied to plant 

 phenomena only between certain limits of temperature. It is perfectly 

 clear that the generalization can hold only so long as all or nearly all 

 of the component or partial processes are progressing according to this 

 principle. In purely chemical processes there are always a minimum 

 and a maximum, beyond which this principle of Van't Hoff and 

 Arrhenius no longer expresses the relation of temperature to velocity. 



A somewhat extensive literature already exists regarding the applica- 

 tion of this principle to physiological phenomena. We may mention 

 the main results with plants. Clausen^ determined the velocity of 

 carbon-dioxide excretion from seedlings and buds at different tempera- 

 tures. He found that the rate somewhat more than doubled for each 

 temperature rise of 10° C, to an upper limit about 40° C. Miss 

 Matthaei^ studied the effect of temperature on the evolution of the 

 same gas from leaves in darkness, and also on its fixation by leaves in 

 light, showing that the Van't Hoff-Arrhenius principle holds here in a 

 very satisfactory manner. Blackman^ has presented a very good state- 

 ment of this entire problem, especially in regard to plants, and his con- 

 cluding sentences are worthy of quotation here. He writes : 



"To me it seems impossible to avoid regarding the fundamental processes of anabolism, 

 katabolism, and growth as slow chemical reactions catalytically accelerated by protoplasm 

 and inevitably accelerated by temperatiire. This soon follows if we once admit that the 

 atoms and molecules concerned possess the same essential properties during their brief 

 sojourn in the living nexus as they do before and after." 



On the whole, it seems allowable to conclude that the majority of 

 the elementary chemical processes of living things proceed according to 

 the general principle of Van't Hoff and Arrhenius, and that such 

 processes exhibit temperature coefficients, within the ordinary Hmits 

 of environmental temperature of from 2.0 to 2.5. When, however, 

 these elementary or component processes are combined into such a 

 complex resultant as growth, it does not necessarily follow that the 

 temperature coefficient of the complex process must be the same as 

 that of its components. Russell^ states that ''the effect of temperature 

 on the rate of growth of a plant is in no wise like its effect in accelerat- 

 ing chemical change," citing Bialoblocki^ to support this view. 



1 Clausen, H., Beitrage zur Kenntnis der Athmung der Gewachae und des pflanzlichen Stoff- 

 wechsels, Landw. Jahrb., 19: 893-930, 1890. 



2 Matthaei, 1904. 



3 Blackman, 1908. 



♦ Russell, E. J., Soil conditions and plant growth, London, 1917. _ 



6 Bialoblocki, J., Ueber den Einfluss der Bodenwarme auf die Entwicklung eimger Oultur- 

 pflanzen. Landw. Versuchsstat., 13: 424-472, 1870. 



