Affinities of Helminthostachys zeylanica . 437 
arrangement of the cells themselves is not so conclusive as is 
commonly the case. This latter circumstance is doubtless 
to be attributed to the slowness with which the new meristem- 
tissues are formed, coupled perhaps with the relative rapidity 
with which the tissues are differentiated ; consequently one 
might perhaps expect to find that the slowly developing 
meristem would be affected by these conditions, and show 
some variation in its disposition which would depend in 
a measure on the time which had elapsed since the last 
segmentation. And indeed from whatever cause, we find this 
to be a fact The apical cell was recognized with probable 
certainty in one case, when its form was seen to be triangular ; 
in another, as Rostowzew appears to have found for Ophio - 
glossum 1 , it seemed to have the form of a truncated prism. 
But these less usual forms are not very difficult to reconcile 
with the more common type of a tetrahedral cell, when the 
long protracted growth of the segment-cells and the resulting 
disturbance of the space relations are borne in mind. 
Following the development of the tissues, the vascular 
region is early distinguished both from the pith and the 
cortex, owing to the numerous tangential divisions and the 
somewhat more regular arrangement of its cells, which are 
smaller than those of the adjacent tissues. The phloem is 
differentiated very early, and shows the characteristically 
swollen walls between the numerous sieve areas almost up to 
the apex of the strand. The lignified elements of the xylem 
are slow in differentiating. A number of isolated protoxylem- 
groups, or even single elements, are dotted about in a manner 
which at once betrays the mesarch character of the strand 
as a whole ; but the formation of the full complement of 
tracheids is only effected at a considerable distance behind 
the apex. But these are all present ab initio as rudimentary 
structures ; there is no evidence of any new ones arising as 
the result of a secondary cambial activity. 
1 Rostowzew, Ophioglosseae, Moscow, 1892 (Russian). Figs.. 8 and 10 in 
the text. 
Gg • 
