118 CELLS AND TLSSUES 



one dissolved substance, both the cell sap and the solution 

 around the cell usually carry in solution a number of substances, 

 each of which in its osmotic influence is independent of the others, 

 although the osmotic influences of all are combined in determin- 

 ing the osmotic force of the solution. In the second place, the cell 

 membrane is a living membrane and, therefore, able to alter its 

 permeability, so that it may be permeable to certain substances 

 at one time but not at another. Another peculiar feature of pro- 

 toplasm is that substances are often allowed to pass in more 

 readily than out. Thus root hairs, which take in many sub- 

 stances from the soil, do not allow the sugars and many other 

 substances in their cell sap to pass out. If the cells of a red Beet 

 are laid in a strong salt or sugar solution, the water will pass out 

 but the coloring matter will be retained. Furthermore, when 

 some cells are placed in very dilute solutions of dyes as methylene 

 blue, the dye accumulates in the cell sap, which, therefore, be- 

 comes much more colored than the surrounding solution. In 

 this way various kinds of substances which are allowed to pass in 

 more readily than out may become more concentrated in the 

 cell sap than in the solution without. 



It is now seen that by osmosis cells obtain their water supply 

 which they pull from the soil, surrounding cells, conductive tracts, 

 or whatever suiToundings they may have that puts them in con- 

 tact with water. Furthermore, the more concentrated their cell 

 sap, the more forcibly and rapidly they can draw water from their 

 surroundings. Osmosis, although chiefly concerned with supply- 

 ing cells with water, assists some in supplying cells with dissolved 

 minerals, sugars, and other substances, which the cell membrane 

 permits to be carried in with the water. But in connection with 

 osmosis substances may pass into and out of cells by the same 

 principles which are active in ordinary diffusion. Thus if sub- 

 stances are less concentrated in the cell sap than without and the 

 membrane is permeable to them, they will diffuse to the cell sap, 

 more or less independently of the movement of water, although if 

 the water is moving in the same direction the substances will 

 move more rapidly. Likewise substances diffuse out of cells 

 when more concentrated within than without, provided the cell 

 membrane is permealile to them. 



Pressure Within the Cell. — In the case of the pig's bladder, 

 it is seen that the flow of water into the interior increases the 



