78 ELEMENTARY AGRICULTURAL CHEMISTRY 



Such substances are termed semi-permeablej and though perfect 

 semi -permeable membranes are not known a near approach to 

 them can be obtained. If such a membrane be arranged as a 

 closed cell connected with a manometer, it can be shown 

 that when filled with a solution and immersed in water the 

 water enters the cell, while practically none of the solution 

 leaves it. The consequence is that a pressure, amounting in 

 some cases to several atmospheres, is set up within the cell. 



%3?his pressure is known as osmotic pressure, and is found to 

 increase with the concentration and to become greater with a 

 rise of temperature. The cells of which a plant is composed 

 (or rather the protoplasm within them) are probably approxi- 

 mately semi-permeable. If they be surrounded by a solution 

 of less concentration than their contents they will receive more 

 liquid than they lose and the pressure within will increase, 

 while if surrounded by a solution of greater concentration than 

 their contents more liquid will pass out than passes in and 

 the cells will shrink. The cellulose walls of the cells are not 

 semi-permeable, but permit of free diffusion and are nearly 

 rigid. The shrinking of the protoplasm from the walls of 

 plant cells can be seen under the microscope when they are 

 immersed in salt solution of proper strength. The phenomena 

 is known as plasmolysis, and results in the death of the plant. 

 Most vegetable tissues contain while alive a large proportion 

 of water, but in spite of this they are rigid and firm, because 

 of the swollen and turgid state of their cells. The stems and 

 leaves of plants largely depend for their erectness and rigidity 

 upon the strain set up between the rigid cellulose walls of 

 their cells and the water-distended state of their protoplasmic 

 contents. If the distension relaxes — e.g., by evaporation — the 

 plant wilts or flags, becoming quite flaccid. 



Diffusion of dissolved matter through a membrane and the 

 setting up of osmotic pressure within the membrane are pro- 

 cesses which are opposed to each other, though they may to 

 some extent occur simultaneously. They probably do so occur 



in the case of a plant's roots. The protoplasm admits of d, 



