MORPHOLOGY 



157 



the formation of cell walls inclined alternately inwards and outwards 

 in the wedge-shaped marginal cells (Fig. 164/). This is succeeded 

 at a lower level by other and older leaf-whoris (/', /"). 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 thePteridophytes, 

 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. 166), in which 



<l 



Pio. 1C5. A, Apical view of the 

 vegetative cone of Equisetum 

 orven.se ; B, optical section 

 of the same, just below the 

 apical cell ; I, lateral walls 

 of the segments. ( x 240. ) 



Fio. 160. Median longitudinal section of th 

 vegetative cone of Hippuris vulgaris. d, der- 

 matogen ; pr, periblem ; pi, plerome ; /, leaf 

 rudiment, (x 240.) 



the embryonic tissues are arranged in layers which, as was first 

 noticed by SACHS ( 149 ), form confocal parabolas. The outermost 

 layer, which covers both the vegetative cone and also the developing 

 leaves, is distinguished as the DERMATOGEN ( 15 ) (d) ; the cells of 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 (j>r). 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 with the periclinal walls. 



While SACHS regarded the insertion of new cell walls at right angles to pre- 

 existing ones as the principle of their succession, BERTHOLD and EIIREEA ( 151 ) have 

 endeavoured to show that the curvature and mode of insertion of a partition wall 



