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. 



55. Application to Root-hairs. — The application of the 

 experiment with 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^ 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 



^ It is not necessary, here, to attempt to explain this process of soaking 

 {imbibition), in the terms of physics. 



