CHIEF ENVIRONMENTAL CONDITIONS. 139 



of the cell are always tending toward chemical and physical equilib- 

 rium. Such equilibrium is, of course, never really attained, even in 

 the case of the dormant phases of plants, like seeds and spores. Thus, 

 as long as life exists there is always in progress a more or less pro- 

 nounced interchange of materials between the organism and its 

 surroundings. Oxygen, for example, disappears in the process of 

 normal respiration and carbon dioxid is produced. If no material 

 exchange were possible, if the system of the plant were not continuous 

 with that of the universe about it, then this process must shortly come 

 to a standstill; an equilibrium between the internal diffusion tensions 

 of oxygen and carbon dioxid must be reached, and no further oxidation 

 might occur. Under existing conditions, however, a fall in the dif- 

 fusion tension of oxygen within the plant-body immediately creates 

 a diffusion gradient between the interior and exterior, and the gas 

 finds its way in from the outside. Conversely, the mere occurrence of 

 the process of respiration sets up an outward diffusion of carbon dioxid. 



It appears probable that, other conditions remaining constant, 

 every substance might prove to have its maximum concentration, or 

 its maximum and minimum, below or between which life is possible. 

 We must expect, however, that the concentrations of other substances, 

 as well as light and temperature conditions, will be found to alter these 

 limits for any given substance. It hardly needs to be mentioned 

 here that variations in the concentrations of non-aqueous materials 

 accompany alterations in water-content. 



2. MATERIAL EXCHANGES BETWEEN THE PLANT AND ITS SURROUNDINGS. 



From the preceding paragraph it is to be inferred that the importance 

 of the chemical environment, in determining the nature of plant 

 growth, etc., and in limiting the kinds of plants that can exist in any 

 given habitat, is definitely dependent upon the generalization that 

 diffusion tends always to bring the plant-body and the surrounding 

 media into concentration equilibrium. Two groups of conditions 

 militate more or less against the attainment of this equilibrium: 

 (1) The degree of permeabihty of the plant periphery to the diffusing 

 materials, and (2) the rate of their transformation within the plant 

 or of their removal from or supply to the immediate environment. 

 Thus, if the air about a plant should contain ether-vapor, for example, 

 the diffusion gradient would insure an inward diffusion of the ether 

 until the vapor-pressure of the ether solution within the plant-body 

 just equaled its partial pressure in the surroundings. If the plant 

 epidermis were readily permeable to the poison it is clear that death 

 must soon ensue, and a lower permeability could only postpone, but 

 could not prevent, this result. It is probable that no substance 

 exists to wtdch the plant periphery is absolutely impermeable, though 

 there are many that penetrate only very slowly. 



