The Processes of Roots 



193 



The soil as a water-delivering mechanism. Soil is composed of 

 various-sized solid particles massed together with small spaces 

 between them. A root in its develop- 



t/ill other tissues 



ment pushes in among the particles and ( (diffusion) 



pushes some aside. The very small root '^"^iZti^aZj"'"'^ 

 hairs grow outward between the soil par- t f^nass movemene— 



-^ I dissolved in xuater) 



tides, and press against them on all sides. i/Water-condx-xHng tissues 

 Due to rains, water enters the soil and ^"'i (diffusion) 



spreads rapidly downward between the Corticai ceiis 

 soil particles. Some of the minerals ^oot hairs 

 among the soil particles dissolve, and ( ('^'^'^^'°'^) 



the result is a dilute solution, usually _ "* *^ ". "" 



Fig. 114. Diagram to show the 



called soil water." The water sinks, processes involved in the move- 

 partly because of its weight ; that is, it "^^^^^ of mineral salts into the 

 ,, , , , . ^ . - tissues of a plant. 



IS pulled down by gravity. It is also 



pulled in all directions by capillarity, just as water is pulled into 



small tubes, or as it is pulled into blotting paper. 



If a flower pot having a perforated bottom is filled with soil 

 and placed in a pail of water, the air in the spaces between the soil 

 particles will be gradually driven out and water will take its 

 place. The soil is then saturated with water. When the pot 

 is lifted out of the pail, a part of the water drains out and it will 

 continue to drip for some hours. This is the water that is pulled 

 down by gravity. The water that remains is held by capillarity 

 and by the attraction of the soil particles. As water percolates 

 out of the mass of soil, air is again drawn into a part of the soil 

 spaces, but films of water surround every soil particle. 



If a small plant is growing in the soil, its roots and root hairs 



If cut in the air, air bubbles get into the ^water-conducting tubes and prevent the 

 subsequent movement of water into them. Air bubbles already in stems that have 

 been cut in the air may sometimes be removed by placing the lower ends of the 

 stems in warm water and cutting off an inch or two. After standing in water for a 

 day, the tracheae may become clogged with bacteria and the rise of water prevented. 

 Florists avoid this possibility by cutting off an inch or two from the stems of cut 

 flowers each day. 



