189 
The Evolution of Dictyostely. 
verse commissures ( c , Fig 53, A, B), a structure would result such 
as is actually found in very many dorsiventral Ferns. In most 
cases, however, the connexions with the simpler types are to some 
extent obscured by the fact that in place of a single leaf-trace two 
or more separate strands are given off to each leaf, and these 
usually run forward for a greater or less distance in the leaf-gap 
before turning out into the petiole (Fig. 53, 54, A). The dorsal and 
ventral cauline meristeles are generally clearly distinguishable from 
the leaf-traces by their greater breadth. Often, however (e.g., Platy- 
cerium alcicorne), the ventral meristele is broken up into an irregular 
meshwork of strands (Fig. 53, B), and these are very frequently no 
A 
c 
B 
Fig. 53. Diagrams of relatively simple dorsiventral dictyosteles. d.m., 
dorsal meristele ; v.m., ventral meristele ; c., commissure connecting the two ; 
l.t., leaf-trace ; l.g., leaf-gap. After Mettenius. 
A. Asplenium obtusifolium The vascular system is supposed to be divided 
longitudinally along the mid-ventral line, and spread out so that the two halves 
of the ventral meristele appear at the top and bottom of the figure. Three 
diagrammatic transverse sections are placed opposite the points at which the 
sections are taken. 
B. Platycerium alcicorne. One half of vascular system, seen from the 
side. The ventral meristele is seen to be perforated so as to form a meshwork. 
thicker than the constituent strands of the leaf-trace (Fig. 54, A, B), 
so that the whole wall of the dictyostele is resolved into a number 
of branching and anastomosing strands, in which the distinctive 
