GROWTH AND WORK OF PLANTS 



the tip of the root some of the epidermal cells grow out into the 

 long root hairs. The root hairs occupy a rather definite area of 

 the root a little distance back from the tip. As the root elongates 

 new root hairs are constantly being developed from newly formed 

 epidermal cells, while all the older root hairs behind are con- 

 stantly dying and disappearing. They are long, slender, tubular 

 cells, and since they serve the important purpose of absorbing 

 water and mineral food solutions from the soil, they must be 

 fresh and new. 



60. Internal structure of the root. Fig. 39 represents a 

 cross section of a root. The outer layer of cells is the epidermis 



from which the root hairs arise. 

 Just inside of the epidermis is 

 the cortex, which consists of 

 several layers of cells more or 

 less rounded and with air spaces 

 between where they touch ad- 

 jacent cells. Inside of this is a 

 ring of a single layer of rather 

 thin cells (the pericycle). This 

 can be located by observing 

 that it lies just outside of groups 

 of cells so highly differentiated 

 in the center. These groups of 

 cells, together with the thin-walled fundamental cells which 

 are between them, make the central cylinder (fig. 40). There 

 are often eight groups of special cells seen in cross section 

 of a young root. Four of these, the more prominent ones, are 

 the fibro -vascular bundles (paragraph 104). They are broad 

 inward and narrowed outward as seen in cross section. The 

 larger cells are the vessels or tubes through which the water 

 or "sap" flows in the root. Besides the vessels there are thick- 

 walled wood fibers, and on the outer side also some young thin- 

 walled living cells which divide and grow to form additional cells 

 for the bundle as the root increases in diameter. The tissue of 

 these bundles is woody and is the xylem (paragraph 104). Alter- 



Fig. 39- 



Section of corn root, showing root hairs 

 formed from elongated epidermal cells. 



