THE RELATION OF PLANTS TO WATER 139 



to a water column in the glass tube. It is quite probable that 

 the real cause of the forcible inflow of water into the parch- 

 ment tube, and of the osmotic pressure thus developed, may be 

 found in the attraction which the wall of the tube and the parti- 

 cles composing the solute have for the water molecules them- 

 selves. We are not here so much concerned, however, with the 

 physical explanation of osmosis as we are with its results, which 

 we need to study in order to understand the work of the plant 

 in absorbing and circulating water and foods. These results, as 

 we have indicated above, are, fiest, the tendency of solvents and 

 solutes to equalize through a separating membrane, and, second, 

 the development of a considerable osmotic pressure within a 

 closed membrane into which an excess is thus induced to flow. 



ABSORPTION BY ROOTS 



The absorption of soil water and soil salts by roots is governed 

 by processes very similar to those indicated above in the experi- 

 ment with a parchment tube. The root hair, which is the most 

 important absorbing portion of the root, is a tubular extension of 

 a single epidermal cell (Fig. 68). Like most plant cells it is 

 furnished with a delicate cell wall and a lining cytoplasmic sac 

 composed of living protoplasm. The center of the root-hair cell 

 is occupied by the water vacuole, containing a solution of organic 

 acids, salts absorbed from the soil, and in many instances sugar, 

 all dissolved in the water of the vacuole. The cell wall is per- 

 meable to mosfc substances, but the cytoplasmic sac resembles 

 closely th& parchment membrane of a parchment tube in being 

 more permeable to the water than to the solutes dissolved in it. 

 It differs from the parchment membrane in being composed of 

 living substance and in being thus able to control to a certain 

 extent its permeability to substances outside in the soil and also 

 within its water vacuole. For instance, in sugar beets the cells 

 of the root are able to retain from 14 to 18 per cent of sugar 

 within the water vacuoles of the root cells, while no sugar exists 

 in the soil water in which the roots are bathed. Beet roots at 

 the same time allow minute quantities of soil salts, amounting 



