104 



BOTANY 



PART I 



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 imlgaris (Fig. 115), in ^\'hich 

 the embryonic tissues are arranged in layei's which, as was first 

 noticed by Sachs i}^^^), form confocal parabolas. The outermost 

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

 leaves, is distinguished as the dermatogen (^^''') (d) ; the cells of the 

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

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

 the dermatogen and plerome are called the periblem {p>r). In the 



Fig. 114. — -1, Apical view of the vegetative 

 cone of Equisehiin ari'ense; B, optical 

 section of tlie same, just below tlie ai)ical 

 cell ; I, lateral walls of the segments. 

 Further explanation in text, (x 240.) 



Fio. 115. — Median longitndinal section of the 

 vegetative cone of Hippuris vulgaris, d, Der- 

 matogen ; j)r, periblem ; pi, plerome ; /, leaf 

 rudiment, (x 240.) 



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. 



Wliile Sachs regarded the insertion of new cell walls at right angles to pre- 

 existing ones as the principle of their succession, Bertholu and Eureka have 

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

 obej' the same laws as do weightless films of liquid. The cell wall tends at the 

 moment of its origin to assume the same position as a film in a mass of soap-suds 

 would under similar conditions. On the other hand, K. Giesexhagen has 

 attempted to prove experimentally that the position of the segment walls is 

 determined by the two daughter cells between which it forms. The relative 

 positions of the daughter cells in the cavity of the parent cells will, however, be 

 detcrmimd bj' tiie surface tension resulting from their cohesion (""')■ 



