12 BULLETIN 355, U. S. DEPARTMENT OF AGEICULTUKE. 



grains. Root absorption of liquids takes place by a physical action 

 called osmosis. If a bladder be filled with a solution like the white of 

 an egg, the opening tightly tied with string, and the bladder put in 

 a dish of salt dissolved in water, there is set up a movement of the salt 

 solution through the walls of the bladder to the inside which soon 

 distends the bladder to a considerable extent. The movement of 

 liquid in this case is mainly inward, as colloidal solutions like the white 

 of an egg pass but slowly through porous membranes. This move- 

 ment will continue until the tension force from the stretch of the 

 bladder walls equals the force which causes the water to move inward. 

 The cause of the movement of the water through the bladder is 

 called osmotic pressure. The illustration helps one to understand 

 the movement of soil solution into the roots of growing plants. The 

 walls of the cells composing the roots, like a bladder, are permeable 

 to dissolved salts only, and the dilute salt solutions of the soil pass 

 by osmosis through the cell walls into the denser solutions of the cell 

 sap. When all the root cells become sufficiently turgid (distended) 

 the plant-food solution is forced into the minute vessels and channels 

 of the stem structure and upward to be utilized for growth. 



Three conditions are necessary for the osmotic absorption of water 

 by plant roots. These are: (1) A favorable temperature of the sur- 

 rounding soil; (2) a supply of fresh air; and (3) a suitable quantity 

 of water. Some plants are able to absorb water at temperatures as 

 low as the freezing point, but this is not common. It has often been 

 observed that the growth of potted plants is hindered by lowering the 

 temperature of the soil by the use of cold water. A proper supply of 

 water in the soil is indispensable for root absorption, but an excess of 

 water shuts out the air from the soil and causes carbon dioxid poison- 

 ing and death of the root hairs, due to improper respiration or breath- 

 ing in their cells. Soils are also made cold by much evaporation due 

 to excess of water. The matter of air and water supply in soils wiU 

 be considered at length in Lessons IV and V. 



How elements of soil and air function in plants (Ref. No. 1, p. 37). — 

 By supplying varying quantities of available mineral plant foods to 

 growing plants with a suitable supply of moisture in the soil some 

 conclusions have been reached concernmg the functions of the 

 essential elements. When a liberal supply of materials giving up 

 nitrogen has been used, plants have produced rank, green foliage, 

 often to the detriment of seed production. Therefore, when leaves 

 and stems furnish the food part of plants, as with cabbage and 

 celery, the soil growing these crops should be well supplied with 

 available nitrogen. Seeds and grain contain relatively large quanti- 

 ties of the element phosphorus in combination. A good supply of 

 available phosphorus-bearing materials hastens the maturing of 

 plants and is particularly essential in the seed and grain crops. 



