ABSORPTION OF WATER 55 



50. Advantage of the Air Spaces.— Living roots, like 

 everything else alive, need fresh air for respiration. If 

 the spaces between the soil particles were filled with water, 

 the air would be driven out, and the root-hairs could not 

 respire. They would soon cease to function at all, and 

 ultimately the whole plant would die. Thus it is seen 

 that plants may have too much water, as well as too 

 little. Farmers' crops (notably corn) often suffer from 

 this cause, as well as from drought. When the soil 

 contains too much water the leaves will commonly turn 

 yellow and die. In order to understand how the soil- 

 water passes into root-hairs, it is necessary to understand 

 the physical actions of diffusion and osmosis. 



51. Diffusion of Gases.— If a bottle of musk, or other 

 perfume, is opened in one corner of a room, free from all 

 air currents, a person standing some distance away could, 

 in time, detect the odor. Now the only way we can smell 

 a substance is to have one or more particles of that 

 substance, in gaseous form, touch the olfactory surfaces 

 of the nose. Therefore, in the case of the musk, tiny 

 invisible particles must have left the surface of the sub- 

 stance, passed up through the neck of the bottle, out into 

 the room, and travelled (though without any air currents) 

 to the person detecting the odor. This illustrates diffusion 

 of gases. 



52. Diffusion of Liquids. — If a small quantity of sugar 

 could be deposited, through a glass tube, at the bottom 

 of a tall tumbler filled with water, the sugar would first 

 dissolve, and the water near the bottom would become 

 sweet. If we carefully avoided stirring the water, and if 

 all currents in the liquid were avoided, nevertheless, 

 within a short time the water on the surface would taste 



