112 PLANT FOODS AND PLANT GROWTH 



used even for its own growth. However, the process by which 

 these simple compounds are transformed into true food materials 

 is imperfectly understood. 



The amount of water in the soil varies with the amount of 

 rainfall and the texture of the soil. Coarse, sandy soils are poor 

 retainers of water as compared with the very fine clay soils. 

 Humus soils are excellent retainers of water, and this fact, 

 together with their richness in organic matter, makes them espe- 

 cially good for agricultural purposes. It is very evident that 

 water acts as a solvent for the available soil substances and salts, 

 and is then taken up by the root-hairs. We have already 

 studied the root-hairs, and know that they are long, delicate cells 

 containing very active living protoplasm and a watery sub- 

 stance called cell-sap. This cell-sap is water containing salts, 

 acids and sugars. The root-hairs are evidently well filled and 

 somewhat distended. When cells are distended in this man- 

 ner they are said to be turgescent, or in a state of turgor. All 

 the active cells of a normal, growing plant are turgid and it is 

 this condition which enables the soft parts of a plant to main- 

 tain their form and position. When cells lose their turgidity 

 the plants wilt. 



Osmosis. It is well known that when a substance dissolves 

 in a liquid it gradually diffuses until equally distributed. 

 If two solutions of equal density are separated by a plant or 

 animal membrane, the substances in solution will gradually dif- 

 fuse through the membrane until the two solutions are uniform 

 throughout. This is known as osmosis. (Fig. 80.) If two solu- 

 tions of unequal density are used there will be a flow from the 

 less dense to that of the greater density. When a plant is 

 growing under suitable conditions, the solution of greater dens- 

 ity is within the cells, especially the root-hairs ; and the solution 

 of lesser density surrounding them. The delicate root-hairs 

 ramify among the extremely fine particles of soil. The soil, 



