58 THE VEGETATIVE FUNCTIONS OF PLANTS 
pass out, that the membrane will begin to bulge, becom- 
ing convex and in a state of tension. This condition is 
known as turgor, and the membrane is said to be turgid. 
The turgor is the result of the osmotic pressure of the solu- 
tion within. If osmosis is allowed to continue after this 
condition is realized, the osmotic pressure will rupture 
the membrane, and allow the contents of the egg to escape. 
It is partly the osmotic pressure of the substances in solu- 
tion in the cell-sap that keeps the lining layer of cytoplasm 
closely appressed to the cell-wall of the root-hair, and other 
cells. 
65. Application to Root-hairs. The application of the 
experiment Vth the egg to root-hairs in the soil is obvious. 
The cell-sap in the vacuole is the denser liquid, the soil- 
water (a very weak solution of various substances dis- 
solved from the soil) is the less dense, while the two 
limiting membranes of the layer of cytoplasm lining the 
inner surface of the cell- wall constitute two porous osmotic 
membranes. Under normal conditions an interchange 
(osmosis) begins between the cell-sap and the soil-solu- 
tion. The latter soaks 1 through the thin cell-wall, passes 
by osmosis through the outer limiting membrane, diffuses 
through the lining layer of cytoplasm until it reaches the 
inner limiting membrane, through which it passes by 
osmosis into the vacuole, and becomes a part of the cell- 
sap. This process is sometimes referred to as endosmosis 
(osmosis from without in). In reverse order, minute 
traces of various substances pass out, by exosmosis. 
Doubtless the dissolved substances that enter the cell 
from without are in part altered, chemically, as they 
1 It is not necessary, here, to attempt to explain this process of soaking 
(imbibition), in the terms of physics. 
