THE RELATION OF THE PLANT TO WATER 163 



plant Uie root hair is the extension of a single epidermal cell. 

 It contains protoplasm and cell sap which ai'e denser solutions 

 than the surrounding soil water. The outer surface of the 

 cytoplasm is the plasmatic membrane. Therefore, there is a 

 movement of the water by osmosis; from the soil, through the 

 plasmatic membrane, into the cell. This will naturally 'tend to 

 reduce the density of the cell contents and to establish an equi- 

 librium between the cell sap and the soil water. But it will make 

 the contents of the roo't-hair less dense than the contents of the 

 adjoining cells in the root and destroy the equilibrium. There- 

 fore, there will Be an osmotic movement from the rootrhair to the 

 adjoining cells and in fact between practically all the inner 

 living cells of the plant. However, the movements from cell to 

 cell will be due not only to osmosis but also to diffusion and 

 imbibition and possibly other physical laws. 



The Cell Membranes. — The cell wall is a membrane through 

 which water and dissolved materials pass very readily. It ap- 

 pears to have very little, if any, selective power and may be 

 considered as permeable to most and probably to all liquid 

 substances with which it ordinkrily comes in contact in nature. 

 Of course, this statement is not triie of the epidermal cells which 

 have been cutinized or suberized, but it does apply to practically 

 all other cells in the plant body. 



The cytoplasmic layer which lies next to the cell wall and 

 to which we have previously referred as the plasmatic mem- 

 brane does possess selective power. In fact, the life and growth 

 of the plant depends on this selective power of the protoplasm. 

 Its power and behavior may be summarized somewhat as fol- 

 lows: (a) it permits the entrance of necessary food-making ma- 

 terials; (h) it permits the entrance of some substances which 



