ABSORPTION AND MOVEMENT OF WATER 115 



the roots, held there by the root-hairs, just as larger lumps 

 of soil are held by the root-branches. 



The function of root-hairs is to absorb nutrient solutions 

 from the soil. They, and their physiological equivalents, 

 the rhizoids of lower plants, are the chief absorbing organs 

 of larger plants. The roots themselves are for the conduc- 

 tion of the solutions absorbed by the root-hairs, and also 

 for the mechanical support cJE the whole plant. The root- 

 hairs should not be regarded merely as structures increasing 

 the surface through which aqueous solutions are absorbed 

 by roots ; the root-hairs are the main surface through which 

 the absorption takes place. But more than this, they are 

 the very perfect means by which the only parts capable of 

 copious absorption — living cells bounded by thin cellulose 

 walls and containing cell-sap of proper composition and 

 density — are brought into the necessary intimate contact 

 with the nutrient solutions adhering to the irregular sur- 

 faces of the small soil particles. In other words, the root- 

 hairs are the means of bringing together, so that they are 

 separated only by a thin permeable membrane, two aqueous 

 solutions of such osmotic pressures that the one enclosed 

 will absorb the one held by surface attraction (p. 105). 



It has been estimated* that the surface of a root is in- 

 creased 5 to 12 times by the production of hairs. From what 

 has just been said, and because the hairs are bounded by 

 walls at least thinner if not otherwise more permeable than 

 the cells between, we see that this does not necessarily 

 fairly indicate the increase in absorbing power even of the 

 part producing the hairs. This may be more than 5 to 12 

 times increased, according to circumstances. 



The life of a root-hair is necessarily brief. Its delicacy, 

 and the fact that it may be torn and broken by the con- 

 tinued forward growth of the part where it is borne, favor 

 this. The root-hairs are formed by the outward branching 

 of epidermal cells on that part of the root just behind the 

 tip which has almost or quite ceased to grow in length. If 

 any considerable growth in length does occur after the for- 



* Schwarz, F. Die Wurzelhaare der Pflanzen. Untersuch. aus dem botan. 

 Institut zu Tubingen, Bd. I., p. 140. 



