MORPHOLOGY 



151 



is derived. In the roots, as in the stems of the Lycopodinae, no apical 

 cells are found. In like manner the roots of Phanerogams, although 

 exhibiting several different types of root-growth, follow the same law 

 in the arrangement of their elements as the vegetative cone of the 

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

 Gramineae (Fig. 166) as a representative of one of these types. The 

 vegetative cone of this root differs from that of the stem previously 

 described (Fig. 164) in the possession of a root-cap. The dermatogen 



Fig. 165. — Median longitudinal section of the apex of a root of Pteris cretica. t, Apical cell ; 

 k, initial cell of root-cap ; *,-». root-cap. ( X 240.) 



(d) and periblem (pr) unite at the apex in a single cell layer, outside 

 of which lies the calyptrogen (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, periblem, and calyptrogen 

 are not marked out as distinct regions. In roots, as in stems, the 

 plerome cylinder (pi) almost always terminates in special initial cells. 

 At a short distance below the growing point the embryonic tissue 



