ABSORPTION OF WATER 59 
diffuse through the cytoplasm between the two limiting 
or surface membranes. 
56. Plasmolysis. If the closed sac, formed by the 
porous membrane, contains the less dense, instead of the 
more dense liquid, then the reverse of turgor will take 
place, and the sac will collapse. This may be easily 
demonstrated under the microscope by irrigating root- 
hairs, or other plant cells, with solutions more dense than 
the cell-sap. For example, the root-hairs shown in 
Fig. 41, mounted in water on a glass slide, under a cover- 
glass, were found, by microscopic examination, to be 
turgid. Then they were irrigated with a 5 per cent, 
solution of common table salt. This solution is denser 
than the cell-sap of the root-hairs, so that exosmosis was 
more rapid than endosmosis, and cell-sap was withdrawn 
from the vacuoles faster than liquid entered from without. 
The salt-solution having soaked through the cell-wall, 
passed with difficulty through the limiting membrane of 
the cytoplasm, and thus began to exert an osmotic pressure 
from without, which loosened the protoplasm (plasmoly- 
sis 1 }, and caused it to collapse. .When this results, the 
cell is then said to be plasmolyzed. 
57. Importance of Osmosis. No physical phenomenon 
is more important than osmosis. Upon it depends the 
life and death of every living thing. By it, not only do 
plants take in necessary substances from the soil, but all 
the food assimilated by man and the lower animals passes 
into their cells. It has been demonstrated that the mainte- 
nance of turgidity is necessary in order that cells may con- 
tinue to perform their normal functions. In a state of 
plasmolysis they cannot do so. This is illustrated in a 
1 From the Greek, plasma -f- luein, to loose, or set free. 
