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- 



FiG. I. —Lateral root growing out from the Stem of a Potato. 

 G young central cylinder, bounded by rows of wood vessels ; R, cortex ; E, epidermis ; H, 

 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 (Periblem) the seat of formation of the cortex, and 

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

 root itself is covered by the root-cap (IF), the outermost cells of which (a) are gradually 

 becoming disorganised while internally (within;?;) 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 (h) next to the root-tip ; here the 

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

 common to both. 



