FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 121 



At least a suggestion of this osmotic relation is contained in a state- 

 ment by Livingston (///) as early as 1902, and a much clearer treat- 

 ment of the subject has gradually evolved in numerous other papers. 

 Livingston says: 



A strong solution will extract water from the organism, a weak one will allow 

 it to be absorbed. * * * Change in the water content of the protoplasm 

 may be directly effective by causing a change in its physical properties. For 

 instance, if water is extracted, the viscosity of the protoplasm must be increased. 

 The change in water content may result in a change in the chemical activity of 

 the protoplasmic solution, since chemical activity, in general, depends upon the 

 concentration of the solution involved. How it comes about is not known, but 

 a review of the literature of experiments upon animals and plants shows that 

 growth is very much retarded by an external solution which extracts water. 

 Especially is the elongation of cells retarded. 



Clements (-9) describes this relation in discussing the plant (par. 60) 

 but ignores it in the method of treating soil moisture control (par. 9) . 



The investigations of Bouyoucos (S) and Hoagland (12), especially, 

 in more recent years, have created the conception of the soil solution 

 as one exerting a definite osmotic pressure. This conception has been 

 attained through the study of freezing points, and their correlation 

 with concentration of the soil extract. In the " Research Methods 

 in the Study of Forest Environment" evidence was introduced to 

 show that the soil water exerts a definite vapor pressure. 



With this light on the subject, it must be evident that the total 

 quantity of water present in the soil has very little bearing on either 

 growth or survival. The ability of the plant to obtain water in 

 sufficient quantity to maintain life, or in sufficient quantity to main- 

 tain conditions favorable for metabolism and growth, must at all 

 times depend primarily on the differential between the osmotic 

 pressure exerted by the cell contents, and that which, for the sake of 

 distinction, we may term the " antiosmotic pressure" of the soil water. 



It is of course to be kept in mind that the osmotic pressure of the 

 plant cells is not stable. If atmospheric conditions create a heavy 

 loss by transpiration, and thereby create a great need for water in 

 the plant, it follows that the ability of the plant to extract the water 

 from the soil is automatically increased, by reason of increased 

 osmotic pressure in the cells. It does not follow that this increase 

 in pressure will immediately insure all that is needed. Hence, if 

 the demand is very great or arises too suddenly, there may be wilting; 

 and, if it is too long continued, there may be permanent injury. 



Unfortunately the treatment of the soil solution as one exhibiting 

 or following the laws of osmosis is not entirely simple, because, 

 although its properties are primarily determined by the solutes 

 present, they are also affected at all stages by the solid particles 

 and colloidal masses of the soil, whose affinity for liquid water has 

 the same effect on the activity of the water molecules as has the 

 affinity of solids in solution. The critical point, however, is this, 

 that while there is still some water in the soil— sometimes 10 per cent 

 or more — it may be so completely and effectively held in thin films 

 or small aggregates by the soil solids, that the water entirely loses 

 its properties as a liquid. The amount of water so held, which is 

 completely nonavailable for plants, is very important, for, at least 

 broadly, this amount is indicative of the aggregate of forces which 

 may withhold the water from plants when the amount is larger. 



