20 GROWING POINTS 



At the growing apex of the root four meristematic layers are 

 often distinguishable. Three, the dermatogen, periblem, and 

 plerome, serve the same purpose as in the stem ; but external to 

 the dermatogen is a fourth layer, the calyptrogen, which cuts off 

 segments towards the outside only, and thus gives rise to the 



FIG. 9. Root-tip of the Hyacinth (Scilla) in longitudinal section showing 

 the growing point and the root-cap. Some of the nuclei (black) 

 exhibit phases in nuclear division (cf. p. 22). 



root-cap (Fig. 9). The latter is constantly replenished by the 

 growing point, as the older cells in front become disorganised. 

 In many roots, however, the epidermis and root-cap arise from 

 a common meristematic layer, and there are other modifications 

 of the growing point of the root which it is unnecessary to describe 

 in detail. 



