118 CELLS AND TISSUES 
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 surroundings 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 permeable 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 
