150 



BOTANY 



in the wedge-shaped marginal cells which form its surface layer (Fig. 

 162, /). This is succeeded at a lower level by other and older 

 leaf-whorls (/', /"). An initial cell (g) may be distinguished in the 

 axil of the second leaf-whorl, and this is destined to become the 

 three-sided apical cell of a side branch. In the Lycopodinae, the most 

 highly developed of the Pteridophytes, a distinct apical cell can no 

 longer be recognised, while in the Phanerogams the cells of the 

 vegetative cone are arranged as shown in the accompanying figure of 

 Hippuris vulgaris (Fig. 164), in which the embryonic tissues are arranged 

 in layers which, as was first noticed by Sachs, form confocal parabolas. 

 The outermost layer, which covers both vegetative cone and also the 

 developing leaves, is distinguished as the dermatogbn (d) ; the cells of 



pi P'- 



Fig. 163. — A, Apical view of the Fig. 164. — Median longitudinal section of the 



vegetative cone of Equisetum vegetative cone of Hippuris vulgaris, d, Der- 



arvense ; B, optical section matogen ; pr, periblem ; pi, plerome ; /, leaf 



of the same, just below the rudiment, (x 240.) 

 apical cell ; I, lateral walls 

 of the segments, (x 240.) 



the innermost cone of tissue, in which the central cylinder terminates, 

 constitute the plerome (pi) ; while the layers of cells lying between 

 the dermatogen and plerome are called the PERIBLEM (pr). In the 

 same figure may be noticed the uniformity with which the dividing 

 walls of the different layers intersect at right angles. This arrange- 

 ment was regarded by Sachs as characteristic of the whole plant 

 structure. The anticlinal walls at right angles to the surface form a 

 system of orthogonal trajectories for the periclinal walls. 



True roots are first found in the Pteridophytes, and possess an 

 apical cell in the shape of a three-sided pyramid (Fig. 165, t). In 

 addition to the segments given off by the apical cell parallel to its sides, 

 it also gives rise to other segments (k) parallel to its base. It is from 

 the further division of these latter cap-like segments that the ROOT-CAP 



