Studies in vSoiIv Physics. 113 



those determined by its environment. The former are again a 

 mechanism — the mechanism of root absorption — and a factor 

 depending upon the concentration of the solution in the root 

 cells. This concentration factor is here osmotic in nature and 

 depends upon the fact that water will pass through a semi- 

 permeable membrane, such as the cell wall, from a solution of 

 lower osmotic pressure to one of higher. If, then, the cell 

 solutions of the root liave a higher osmotic pressure (which 

 means, in most cases, higher concentration) than the soil solu- 

 tion, water passes from soil to root and, other things equal, the 

 rate of passage varies directly with the difference in concentra- 

 tion, though perhaps not in exact proportionality thereto. The 

 urging force which causes water to move into the plant at all 

 is this same difference in osmotic pressure, and the greater the 

 difference the greater the force. * The mechanism of 

 root absorption, (the surface exposed, the permeability 

 of the membrane, etc.,)is, like the mechanism of transpiration, 

 nearly constant for any given plant and, like it also, is of 

 practical importance only as setting an upper limit. 

 Not more than so much water can pass into a given plant even 

 under the best soil conditions, and if the transpiration exceed 

 this amount the plant will die though its roots be bathed in 

 unlimited water. 



The external factor in absorption is the capillary pressure X 

 of the water in the soil. For any given soil against any given 

 plant there is probably a certain content of soil water below 

 which that plant is unable to further extract moisture. The 

 pressure under which the water is held in the capillary spaces 

 and in the films about the soil grains is then greater than the 

 difference of osmotic pressures urging the w^ater into the root. 

 But this limiting water content even if it be a physical reality] | 

 is quite low and it is seldom that useful soils are so dry as this 

 throughout their whole mass. The importance of the phe- 



•It may seem that this theor>' of the action of osmotic forces on root absorption is incon- 

 sistent with the fact that dissolved food materials enter the plant through its roots. When 

 the cell solution is more concentrated than the soil solution nothing but water can enter, 

 and when it is less so nothing at all can enter. To the outline theor>', as above stated, 

 this objection is perfectly valid. But not all of the osmotic theory has been stated. The 

 considerations of partial and selective permeability and the effects of the presence of many 

 different salts in the solutions have not been discussed nor will they be. The theory as 

 stated applies accurately enough to the water stream, and with the more precise matters 

 of nutrition and total absorption we are not here concerned. 



JSee the previous paper of this series. Plant World, 14.60, 1911. As there pointed out 

 bygroscoptic forces are not practically important in ordinary moist soils. 

 II Thi» matter is more fully discussed below 



