ROOT-HAIRS 



49 



B 





&:^ 



with the more concentrated fluid. The extent and the 

 rapidity of this osmotic action depends (i) on the concentration 

 of the stronger fluid, (ii) on the nature of the substances it 

 holds in sohition. Each root-hair is a microscopic tubular 

 outgrowth from a single cell of the outermost layer — the 

 epidermis — of the root. Inside it contains 

 the cell sap, which is more concentrated 

 than the water of the soil. The more concen- 

 trated contents of the root-hair are separated 

 from the weaker water of the soil by a proto- 

 plasmic membrane and by the cellulose cell- 

 wall, and the less concentrated fluid of the 

 soil passes through the membrane into the 

 cavity containing the more concentrated cell 

 sap. These root-hairs greatly increase the 

 surface area of the root which is capable of 

 absorbing moisture. They grow out between 

 the particles of the soil and are surrounded 

 by the water, which in its turn surrounds the 

 constituent particles. When the water con- 

 tains a large amount of salts, as it does in 

 some marshes, the osmosis is slowed down or 

 even arrested; hence ordinary plants do not 

 grow well in soils with much salt in them. 

 The root-hairs seldom absorb the whole of 

 the available water, for there is always a 

 certain surface tension which favours the re- 

 tention of water between the particles of the 

 soil. A soil is rarely if ever wholly without 

 moisture. In water-plants a certain amount MustardT J, with 

 of water is taken up by the leaves and stems, soil adhering to the 

 Having once got the water with the dis- root-hairs; 5 ^^'ith 

 , , « ,„ '^i 1 , X • . J.1 4. root-hairs free from 



solved salts (food substances) mto the root- g^ii ^f^er Sachs. 



hairs and so into the roots, it has to be 

 dispersed, and the question of the ascent of this fluid, 

 now caUed sap, is a difficult one. It travels in minute cells 

 called tracheids or in vessels which are formed from rows of 

 superimposed cefls whose adjacent, transverse walls have 

 broken down, thus producing minute capillary ducts. So 



Fig. 12. Seedlings of 



SL 



