74 



SOIL AND ITS RELATION TO ROOTS 



Cross section of a young taproot; 

 a, a, root hairs ; b, outer layer of 

 bark; c, inner layer of bark; 

 d, wood or central cylinder. 



the length of the main root. In their development the cells men- 

 tioned have grown together in such a manner as to lose their small 



ends, and now form continuous 

 hollow tubes with rather strong 

 walls. Other cells have come to 

 develop greatly thickened walls ; 

 these cells give mechanical sup- 

 port to the tubelike cells. Col- 

 lections of such tubes and sup- 

 porting woody cells together make 

 up what are known as fibrovascular 

 bundles. 



Root Hairs. - - Careful examina- 

 tion of the root of one of the seed- 

 lings of mustard, radish, or barley 

 grown in the pocket germinator 

 shows a covering of tiny fuzzy 

 structures. These structures are very minute, at most 3 to 4 milli- 

 meters in length. They vary in length 

 according to their position on the root, 

 the most and the longest root hairs 

 being found near the point marked 

 R. H. in the figure. These structures 

 are outgrowths of the outer layer of the 

 root (the epidermis), and are of very 

 great importance to the living plant. 



Structure of a Root Hair. - - A single 

 root hair examined under a compound 

 microscope will be found to be a long, 

 round structure, almost colorless in ap- 

 pearance. The wall, which is very flexi- 

 ble and thin, is made up of cellulose, a 

 substance somewhat like wood in chemi- 

 cal composition, through which fluids 

 may easily pass. Clinging close to the 

 cell wall is the protoplasm of the cell. 

 The interior of the root hair is more or less filled with a fluid 



Young embryo of corn, show- 

 ing root hairs (R. PI.} and 

 growing stem (P.). 



