72 WATER AND MINERAL NUTRIENTS 



in the bottle. The tube may be readily secured in this position 

 by passing it through a hole in the cork. In a short time, we 

 notice that the liquid in N begins to rise, and in an hour or 

 so it stands at F, say. The diffusion of water through this 

 membrane into the salt solution is known as osmosis. Under 

 these conditions, there is pressure in the tube and this pres- 

 sure is known as osmotic pressure. We may have osmosis 

 taking place from a weak solution to a stronger solution. 



150. The root-hairs secure water from the soil. The 

 above experiment enables us to understand how the count- 

 less little root-hairs act, each one like the tube N, if only 

 the whole surface of the tube were a bladder membrane, or 

 something acting similarly. The soil-water does not contain 

 much of the soil fertility; that is, it is a very weak solution. 

 The active little root-hair, on the other hand, is always 

 filled with cell-sap, a more concentrated solution; hence 

 soil-water must come in, and along with it come also small 

 quantities of dissolved food materials. Some of these ma- 

 terials may be fertilizers that have been applied to the land. 



151. This principle of absorption of water by osmosis 

 may now be demonstrated by another experiment. Fleshy 

 pieces of root or stem will absorb water from weak solutions 

 and become rigid; in strong solutions such fleshy parts will 

 give up their water and become flexible. Cut several slices 

 of potato tuber about one-eighth of an inch in thickness, 

 and let them remain in the air half an hour. Make up two 

 solutions of cane-sugar: (1) dissolve four ounces of sugar in 

 a quart of water; (2) dissolve one-half ounce of sugar in a 

 quart of water. Place pieces of the potato tuber in these 

 solutions. In half an hour those pieces in the weak solution 

 will be rigid or stiff (turgid); those in the strong solution 

 will be flexible (flaccid). The potato tuber is composed of 

 thousands of minute cells, each with a cell wall, protoplasm, 

 starch grains, and cell-sap. The cell-sap contains sugars 

 and various salts in solution. When the slice of tuber is 



