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 
