ABSORPTION AND OSMOSIS 59 



itself and as mineral matter dissolved in that water; it acts as a 

 means of transfer within the plant for the various foods needed 

 in the different parts, much like the blood of animals; and this 

 absorbed water supports' many parts of the plant. This latter 

 statement will need some explanation. 



Turgescence. When a plant is deprived of water, its leaves 

 droop and we say it wilts. This is due to the fact that, normally, 

 each cell is expanded by the water within it and so is kept in posi- 

 tion. If the water be withdrawn, these cells will collapse like an 

 empty balloon, allowing the leaves and plant to droop. If water 

 be supplied before the protoplasm dies, however, the leaves and 

 plant will resume position. 



This stiffness of plants, due to presence of water, is called tur- 

 gescence and is very important in supporting the smaller plants 

 whose stems are not stiffened with wood fibers. Nearly all leaves 

 depend on this water pressure for their expansion. 



Osmosis. The water to supply these absolutely essential needs 

 comes from the soil, often apparently dry, but always containing 

 at least a little moisture which the plant must obtain if it is to live. 



This vastly important root function of absorption depends on 

 a physical process called osmosis which may be defined as the 

 mixing or diffusion of two liquids or gases of different densities, 

 through a non-porous membrane the greater flow being toward 

 the denser substance. Osmosis is one of the most important 

 biologic processes, and upon it depends not only absorption in 

 roots, but all forms of absorption in plant and animal, all digestive 

 processes, excretion, respiration, and assimilation. Wherever a 

 liquid or gas passes through any tissue, osmosis is the acting cause, 

 controlled sometimes by the living protoplasm that lines the cell. 



The essentials for osmosis are a dense liquid, a less dense liquid, 

 and the osmotic membrane. In the root the wall of the root hair 

 or epidermal cell acts as the membrane, the cell sap as the denser, 

 and the soil water as the less dense liquid. 



Root Hairs. It has been estimated that there may be a total 

 length of a mile in the roots of a corn plant, and alfalfa roots have 

 been found to extend twenty feet deep in dry soil. 



