114 PLANT PHYSIOLOGY 



longest and most strongly upon the soil particles, and not 

 between them, no smooth cylindrical organ of the size of 

 even the smallest roots will be able to bring enough of 

 those cells capable of absorption into sufficiently intimate 

 contact with a large enough number of soil particles to 

 ensure the osmotic absorption of water from the soil parti- 

 cles into the root. For osmotic transfer, as we have seen 

 before (p. 109), both of the two liquids concerned must be 

 in contact with the permeable membrane. Furthermore, as 

 is the case on soil particles, water strongly held as a thin 

 film over an irregular surface will not rapidly move from 

 part to part of that surface. To ensure the absorption of 

 much water from such a surface, there must be the most 

 extended proximity possible of the osmotically active 

 liquids. The permeable membrane must therefore cover the 

 irregular surface as widely and as closely as possible. The 

 most intimate contact of absorbing cells and liquid to be 

 absorbed will be effected when hairs of such size and length 

 that they will fit the soil particles develop on the root. 



The length, thickness, and number of root-hairs will vary 

 according to the medium in which they develop, and also 

 according to the amount of water given off by the plant. 

 The root-hairs will be numerous directly in proportion to 

 the difficulty of getting enough water. This can be easily 

 demonstrated by cultivating young seedlings of corn with 

 their roots in moist air, in soil, and in water. The root- 

 hairs will be most numerous in the air, less in the soil, and 

 there will be exceedingly few if any in the water. The length 

 of the root-hairs will also differ strikingly ; they will be 

 longest in the air, shortest in the water. The diameter of 

 the hairs is necessarily limited by the size of the epidermal 

 cells of which they are branches, but within this limit the 

 hairs certainly vary according to the size of the soil parti- 

 cles among and around which they must grow. That the 

 root-hairs not only grow between the soil particles, but 

 actually apply themselves very closely to them, is abun- 

 dantly proved by the common experience of up-rooting 

 plants grown in loose soils. When such plants are pulled 

 up gently, numberless soil particles of minute size cling to 



