OSMOTIC PRESSURE 53 



beets, and the sugar in the roots of sugar beets, are re- 

 tained within the cells ; they do not leak out into the soil. 

 On the other hand, water is taken up by the roots. Hence 

 we see that cell-membranes may be permeable to water, 

 but not to all substances even though they may be dis- 

 solved in water. Cell-membranes are therefore spoken 

 of as partially permeable or semi-permeable membranes. 



Osmotic Pressure. — Molecules of water move with 

 increasing freedom as water is transformed from the solid 

 to the liquid or to the gaseous condition. Similarly, 

 the molecules of salt, for example, move with increasing 

 speed and amplitude as the salt dissolves in water. In 

 consequence the movements of the molecules o'f sub- 

 stances beating upon the walls of the enclosing or dis- 

 solving medium will exert pressure. This pressure is 

 known as osmotic pressure, and it will obviously be pro- 

 portional to the concentration, temperature, and other 

 conditions prevailing. If equal proportions of the same 

 substance are in solution on opposite sides of a mem- 

 brane, or in different parts of a solution, the osmotic 

 pressure will be equal; but if the proportions are un- 

 equal the osmotic pressure in the more concentrated part 

 of the solution will be greater; and if the enclosing mem- 

 brane is permeable, the movements through it will result 

 in equalizing the pressure by the entrance of more of the 

 dissolved substance into the less concentrated and more 

 of the water into the more concentrated parts. Osmotic 

 pressure, therefore, determines the direction and rate of 

 movement of dissolved substances and of the solvent 

 itself if the intervening membranes are permeable. 



Living Cells are Osmotic Systems, taking in and 

 giving off in solution in water solids, liquids, and gases. 

 Intake we think of as occurring principally in roots and 

 leaves in land plants, but the outgo is not less certain. 

 Roots absorb water, oxygen, and mineral salts from the 

 soil; leaves absorb COo and O2 from the air. 



The Structure of Roots. — The root of the ordinary 



