12 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
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 will 
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 concerning 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 gram crops. 
