i84 



Journal of Agriculture , Victoria. [lo March, 191 2. 



in shape, and are invested with a film of water. This film of moisture 

 surrounding each soil particle is really a very dilute solution containing 

 various substances dissolved from the mineral constituents of the soil. It 

 is this film surrounding each soil particle which is the source of the plant 

 food obtained frorh the soil. The mode of entry of this dilute solution to 

 the epidermal cells of the root may be illustrated by a very simple ex- 

 periment. 



If a lamp chimney covered over one end with a piece of bladder or of 

 parchment be partially filled with strong brine, and then placed m a 

 vessel of water, the two liquids will be separated merely by the thin mem- 

 brane. Under these circumstances, each of the liquids will diffuse through 

 the membrane and mix with the liquid on the other side. The attraction 

 for water inwards will be greater the stronger the brine. The movement 

 will continue until the liquids on both sides of the diaphragm have the 

 same composition. 



This process is 

 called osmosis, and each 

 cell in the epidermal 

 layer of the root is a 

 small osmotic appara- 

 tus. The cell sap corre- 

 sponds tO' the salt solu- 

 tion, the cell wall and 

 protoplasm to the dia- 

 phragm or the bladder 

 separating the liquids, 

 and the soil solution 

 on the surface of the 

 soil particles corre- 

 sponds to water in the 

 vessel. But there is 

 one important differ- 

 ence to be borne in 

 mind. While the pro- 

 toplasm of the cell 

 readily allows the soil 

 water bearing in solu- 

 tion plant food to dif- 

 fuse inwards and mix 

 with the cell sap it refuses to allow the bulk of the dissolved sub- 

 stances in the cell sap to diffuse outwards. The osmosis of plant nutrition 

 is therefore a controlled osmosis, the control being exercised by the living 

 protoplasm of the cell. 



The density or concentration of the cell sap must on fertile soil be 

 always greater than that of the dilute solution investing the soil particles, 

 so that the solutions outside may be carried inside the cell and tend to 

 make it tight with water or turgid. The solutions which penetrate the 

 epidermal cells by osmosis, pass from cell to cell in the root tissue by the 

 same process. In the lower types of plants, such as the algae, which are 

 composed entirely of simple cellular tissue, the movement of water within, 

 the plant may be accounted for by osmosis alone 



II. EPIDERMIS OF ROOT OF FEDERATION 

 WHEAT (X 270). 



