MORPHOLOGY 



159 



the stems. It will, accordingly, be sufficient to describe a root of 

 one of the Gramineae (Fig. 168) as a representative of one of these 

 types ( 154 ). The vegetative cone of this root differs from that of the 

 stem previously described (Fig. 166) in the possession of a root-cap. 



Kio. Ki8. Median longitudinal section of the apex of a root of the Barley, Hordeum vulgare. If, 

 calyptrogen ; rf, dennatogen ; c, its thickened wall ; pr, periblem ; pi, plerome ; en, endoUerniis ; 

 ', intercellular air-space in process of formation ; , cell row destined to form a vessel ; r, 

 exfoliated cells of the root-cap, (x 180.) 



The dermatogen (d) and periblem (pr) unite at the apex in a single 

 cell layer, outside of which lies the CALYPTROGEX (k) or layer of cells 

 from which the root-cap takes its origin. In many other roots, 

 however, the formation of the root-cap results from the periclinal 

 division of the dermatogen itself, which, in that case, remains distinct 

 from the periblem. In the apices of Gymnosperms the dermatogen, 



