8 THE PHYSIOLOGY OF PLANTS 



cells of the root-tip, and the increase in length is therefore 

 also due to apical growth. But this root-tip is not free ; it is 

 covered over by a cap of parenchymatous cells called the root- 



FlO. 



-Lateral Boot sbowino out prom the Stem op a Potato. 



G, young central cylinder, bounded by rows ol wood vessels ; R, cortex ; E, epidermis ; B, 

 hairs. The separation of these tissues can be traced into the youngest portions of the 

 root-tip. in this meristematic region. PI (Plerom) is the tissue which will continue the 

 formation of the central cylinder ; P {P&nbtem) the seat of formation of. the cortex,and 

 D (Dermatogen) the initial cells of the epideimis. The delicate meristematic apex of the 

 root itaelf is covered by the root-cap ( W), the outermost cells of which (a) are gradually ■ 

 becoming disorganised while internally (within b) new root-cap cells are being formed, 



cap (Fig. I WW). The layers of cells forming the latter 

 are so arranged that the oldest layers (a) lie nearest the out- 

 side, the youngest layers (6) next to the root-tip; here the 

 root-cap is connected with the root-tip by a layer of cells 

 common to both. 



