Roots. 



35 



of 37,700 square feet. The greater surface presented by 

 the smaller particles is of further advantage in giving the 

 water increased opportunity to dissolve out from them cer- 

 tain substances necessary to the food of plants. 



22. Action of Root Hairs. — The root hairs place them- 

 selves in close contact with the soil particles, and conform 

 to their irregularities of surface 

 so completely as to embed and 

 hold them fast. This accounts 

 for the difficulty of washing away 

 the soil from the roots without 

 breaking off the hairs. The close 

 relation of the root hairs to the 

 soil is of great importance, for 

 long after the capillary spaces 

 have been emptied of their water 

 by evaporation, the soil particles 

 still retain a film of water about 

 them from which the root hairs 

 are able to draw suppUes for the 

 plant, even when the soil ap- 

 pears dry. 



A better comprehension of the 

 absorptive action of the root hairs 

 will be obtained after an exami- 

 nation of their structure and the 

 circumstances governing their ac- 

 tion. They are really greatly elongated outer or epidermal 

 cells of the roots (see Figs. 1 1 and 12). Their outer wall, a 

 (Fig. 12, A and B), is quite thin, and composed of cellulose, 

 a substance readily permeable to water. Within the cell 

 wall is the live part of the cell known as the protoplast 

 (all of the granular part in A), consisting of the outer lin- 



FlG. 12. 



A, diagrammatic representation 

 of a Root Hair; B, a more 

 highly magnified detail, a being 

 the outer cellulose wall, d the 

 plasma membrane, and c the 

 cytoplasm, d is the nucleus 

 suspended in the cytoplasm. 



