CHIEF ENVIRONMENTAL CONDITIONS. 123 



must be a function of the internal water-conditions of the plant-body 

 and of the difference between the rate of supply to the roots and the 

 rate of transpiration. It will be seen at once that a statement of the 

 residual water-content for any soil and plant is meaningless without 

 a statement of the transpiration-rate at the time the determination 

 was made, or, at least, the statement of some measure of the condi 

 tions that determine transpiration. It is apparently quite possible 

 to define these conditions with some precision by means of measure- 

 ments of the evaporating power of the air. 



Aside from actual dryness of the soil, another condition produces 

 the same effect upon the plant. This is included in what has been 

 termed by Schimper physiological dryness. This condition exists where 

 a plant apparently suffers from drought and yet is rooted in a 

 moist or even wet soil. The optimally moist soil of the experiment 

 with morning-glory described on page 119, might be said to be 

 physiologically dry for that plant under those conditions of transpira- 

 tion. The term more commonly connotes those cases where the 

 plant suffers from lack of water, due either to some pathological 

 condition of the roots or conducting organs or to a too high physical 

 concentration (osmotic pressure) of the soil solution. In either case 

 the symptoms are those produced by a dry soil, but the actual amount 

 of moisture present in the soil may still be relatively very high, or the 

 soil may even be completely saturated. 



The best known cases of adverse osmotic conditions in the soil 

 solution are those of the so-called "alkali" soils, where the salt-con- 

 tent is usually high, although the component salts are not highly 

 toxic. In such soils ordinary plants suffer from lack of water, ap- 

 parently not because water movement into the roots is checked 

 through increased external capillary resistance, as in a soil that is 

 actually nearly dry, nor because of toxic effects, but merely because 

 of the high osmotic pressure of the soil solution itself, which may result 

 in plasmolysis of the superficial root-cells and the consequent derange- 

 ment of the absorbing mechanism. Plants that are characterized 

 by an unusually high osmotic pressure in their absorbing organs seem 

 to succeed in such soils.^ 



Those instances of physiological dryness which are produced by 

 injury or by a pathological condition of the absorbing or conducting 

 system need not here be treated in detail; it needs only to be re- 

 marked that any condition leading to an inadequate power of absorp- 

 tion or conduction may bring about a correspondingly inadequate 

 water-supply and, in general, may result in the same symptoms as 

 those produced by soils of low moisture-content or of low power of 

 water delivery. 



1 See in this regard Fitting, 1911, and the remarks on this paper by Livingston, 1911. 



