140 ENVIRONMENTAL CONDITIONS. 



If the inwardly diffusing substance be altered chemically upon reach- 

 ing the interior of the plant, and if this process of alteration be capable 

 of removing it as rapidly as it enters, it is clear that equilibrium be- 

 tween interior and exterior, or even any considerable solution con- 

 centration within the plant, can not be reached. Similarly, if some 

 poison be produced within the tissues, as organic acids in the case of 

 certain roots growing under low oxygen pressvu'e,^ and if the epidermal 

 tissues be adequately permeable to this substance, then the concen- 

 tration that is obtained within the cells must be determined by the 

 possible rate of removal of the poison from the immediate surround- 

 ings. A substance diffusing from roots may diffuse away through soil- 

 moisture films, it may be absorbed by the solid-liquid surfaces of the 

 soil, or it may be oxidized or otherwise transformed; but it is ob- 

 viously essential that the poison be removed from the soil solution in 

 the immediate neighborhood of the excreting roots; otherwise the rate 

 of outward passage must be lowered, and the consequent rise of the 

 internal concentration of this particular substance (supposing the 

 process of its formation to continue at the original rate) might soon 

 bring about a general upsetting of all the physiological processes so 

 that death might finally ensue. 



If plant activity depends upon the absolute and relative concentra- 

 tions of various substances within the body, then it is clear that 

 variations in the concentrations of these substances in the environ- 

 ment must be accompanied by more or less profound alterations in 

 the physiological processes. We thus arrive at the well-known 

 proposition that the concentration or diffusion tension of the various 

 substances in the environment is of prime importance in determining 

 how any plant may develop and, indeed, whether it may exist at all 

 in a given habitat. It is immaterial whether the environmental con- 

 centration of a substance at any time be the result of causes acting 

 wholly without the plant or of internal processes; the end-result must 

 be the same in either case. Thus, the lactic-acid organism of souring 

 milk is checked or killed by the accumulation of its own excretions 

 just as truly as though the acid content of the medium had arisen 

 solely from external causes. 



It thus emerges that the chemical relation, unlike those of water 

 and temperature but like that of light, must always be considered not 

 only with reference to intensity but also in regard to quality. Just 

 as there are many different wave-lengths of light that influence the 

 plant differently, so there are innumerable chemical compounds, all 

 differing qualitatively in their effect upon plants. Moreover, the 

 effect of each one of these compounds varies not only with its own 



1 Stoklasa, J., and A. Ernest, Die chemische Charakter der Wurzelausscheidung verschie- 

 denartiger Kulturpflaazen, Jahrb. wiss. Bot., 46: 52-102, 1908. 



