PLANT LIFE. OSMOSIS. FERTILIZERS 409 



the solution, that is, upon the extent to which the concentration 

 of water molecules has been reduced (compare, again, vapor pres- 

 sure depression of solutions, p. 117). 



A solution containing 1 g. molecular weight of an inert solute 

 dissolved in 1000 g. water should give at 20, with a perfectly 

 semi-permeable membrane, an osmotic pressure of 23.6 atmos- 

 pheres. 



Osmotic Pressure in Plant Life. Osmotic pressure was 

 first studied by Pfeffer, a botanist, in 1877. De Vries (1878) used 

 plant cells for the same purpose. The cell content included a liq- 

 uid containing various salts in solution, and a protoplasmic layer 

 lining, but not firmly attached to, the cell wall. This protoplas- 

 mic layer behaved like an imperfect semi-permeable membrane. 

 When such cells were immersed in a concentrated solution of 

 any substance, the water passed from the interior of the cell to 

 the solution, and by means of a microscope a shrinkage of the 

 protoplasmic layer away from the cell wall could be observed. 

 Conversely, when such cells were placed in pure water, or a solu- 

 tion of a very dilute nature, water passed from the outside into 

 the interior, and the protoplasmic layer was distended so as to 

 cause the cell to become turgid. 



Osmotic pressure is, therefore, a subject of great interest in 

 connection with the physiology of plants. It aids in explain- 

 ing why a withered flower, containing a solution in its cells, re- 

 vives when placed in pure water. The latter enters through the 

 walls of the cells, and the pressure thus produced distends the 

 structure and stiffens it. Similarly; the wilting of plants, when 

 too high a concentration of salts as fertilizers is added to the soil, 

 is explained. In the animal body also, osmosis plays a large part. 



Fertilizers. Many soils are either naturally deficient in one 

 or more of the necessary plant foods, or the supply may have 

 been exhausted by repeated cropping. Every crop removes 



