42 PLANT GROWTH 



air, or the diffusion of salt through meat. Diffusion from the 

 soil into the plant will be discussed later. 



Osmosis is diffusion of molecules through a membrane. 

 This is of great importance to an understanding of the 

 plant's processes of absorption and excretion. The cell mem- 

 brane of protoplasm acts as the membrane which is perme- 

 able to water, salts, and some organic molecules, but it is not 

 permeable to sugars and proteins, or at least only very 

 slightly. A membrane of a root hair filled with cell sap, in 

 contact with the soil water, is a good example. 



The cell sap consists of a sugar, soluble proteins, mineral 

 salts, and plant acids in a water solution. Since so many 

 molecules are dissolved in the water, the solution is concen- 

 trated, or the amount of water is less than in a solution with 

 fewer dissolved molecules. Various cells of the plant differ 

 in the concentration of their cell sap. It is usually lowest in 

 the water-conducting cells. It may vary from less than 1 per 

 cent to 20 per cent but is usually close to the lower figure. 

 The concentration becomes greater in dry soil and less in 

 more moist soil. 



The water in the soil, or the soil solution, has mineral 

 salts and products of decay, but seldom sugar, and the total 

 concentration must always be less than the concentration of 

 the cell sap of the root hair. When this is true, the water 

 diffuses from more water in the soil solution toward less 

 water into the root hair. If excessive amounts of salts, as 

 fertilizers, are applied to the soil, it may increase the concen- 

 tration of the soil solution above that of the cell, causing the 

 water to go from the root hair to the soil. This would cause 

 the plant to wilt and die. One method of killing weeds is to 

 apply salt to the soil in sufficient quantity to prevent the 

 diffusion of water into the root hairs. In large areas of soil 

 there is little danger of killing plants with fertilizing salts, 

 but in flower pots only small amounts may be used. 



The cellulose walls of the cells (Fig. 6, A) are in the form 

 of very small needle-like crystals separated by exceedingly 



