46 BOTANY: PRINCIPLES AND PROBLEMS 



factuie, growth, or reproduction. The plant body is composed of 

 a multitude of cells, bound firmly together by cementing sub- 

 stances to form an entire, coherent organism. As we consider 

 the various tissues and organs in detail, we shall have occasion 

 to describe the particular characteristics which their cells display. 

 Internal Structure of Roots. — Internally, roots show a very 

 marked structural differentiation. In a young root, three main 

 cell-systems or tissues are distinguishable — the epidermis, the 

 cortex, and the fibro-vascular cylinder (Fig. 28). 



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 6C 



Dc. 



Fig. 28. — Transverse section of the fibro-vascular cylinder of a young root. 

 The bundles of thick-walled wood cells alternate with bundles of thin-walled 

 bast cells. The whole is surrounded by an endodermis, outside of which is the 

 cortex of the root. (From De Bary). 



The epidermis of the root, like that of all other plant organs, 

 is a single layer of cells in thickness. These cells are normally 

 protective in function, but in the root-hair zone many of them 

 produce on their outer surface a characteristic projection, the 

 root-hair itself. 



The cortex lies just under the epidermis and is of varying 

 thickness. In the young root, its cells serve to transmit water 

 and dissolved substances from root-hair to fibro-vascular cylinder, 

 and, in the older roots, to store food. Most of the fleshy portion 

 of storage-roots consists of enlarged cortex. The innermost 

 layer of cortical cells is often especially modified and is then 

 known as the endodermis. 



