53 2 Brebner. — On the Anatomy of 
been already stated, no ‘pith’ appears in the centre of the 
haplostele, but the ground-tissue is embraced by the conversion 
of the rod-like haplostele into a crescentic one. Then by the 
formation of leaf-gaps and their subsequent closure the ‘ extra- 
stelar * ground-tissue comes to lie in the centre of a more or 
less complete ring of meristeles, as seen in transverse section 1 . 
In the series of sections from which Figs. 28 and 29 were 
drawn, the very transitory solenostelic condition did not 
appear till after the leaf-trace had become dimeristelic, but 
in two other cases (one of which is illustrated by Fig. 19, PI. 
XXIII) it took place while the leaf-trace was still mono- 
meristelic. A little above the level at which diagram XI 
was drawn, the solenostelic ring opens out at the side 
opposite to the anastomosis of the central strand, and 
then from the tip of each horn of the crescent a leaf- 
trace meristele is cut off, one apparently much ahead of 
the other 2 . While these changes have been going on, the 
central strand again frees itself. Its nature is well expressed 
by Farmer and Hill’s term ‘commissural column.’ At the 
most peripheral part of its course it comes to occupy the 
centre of a leaf-gap, and it, with the other meristeles, forms 
a sort of ring, Fig. 29, diagram XV. It seems that this is 
the nearest approach to internodal structure attained, and, 
from the time it first appears, it repeats itself at intervals 
with but slight modifications. Shortly after the stage just 
described has been reached, certain of the meristeles pass off 
as leaf-traces and others anastomose and form a long slightly 
curved meristele (gamomeristele), as seen in transverse section. 
This is usually more or less directly associated with the 
incoming of a root. The central strand fuses with, or be- 
1 This central, so-called extrastelar, parenchyma is developed from the central 
parenchyma of the differentiating meristem, so that it might not be so far-fetched 
to call it a pith in the ordinary sense. Whatever was its phylogeny, the pith of 
the normal Dicotyledon is also formed from the central part of the apical meristem. 
It is difficult to understand, except on account of the fetish endodermis, why the 
one should be called intrastelar and the other extrastelar. 
2 The meristeles of a leaf-trace do not always depart simultaneously from the 
dictyostele ; but a little obliquity in the plane of section, which is often unavoid- 
able, considerably emphasizes the discrepancy. 
