Comparative Anatomy of the Casuarineae . 259 
following peculiarities of its stem-structure : ( a ) the great 
development of xylem-parenchyma in the form of concentric 
bands ; ( b ) the very broad medullary rays (sometimes occu- 
pying one-third of the circumference of the stem) as well as 
narrow ones one cell thick. These very broad ones are con- 
nected with the passing in of the bundles of a branch ; the 
latter, by their very oblique course inwards, give a character- 
istic appearance to the stem. Such characters as these, 
however, do not appear capable of leading to any important 
systematic conclusions. 
Of the more detailed characters, the tracheal part of the 
xylem, consisting of vessels with both numerous and single 
perforations, together with fibrous tracheides, which are largely 
fibre-like in appearance, resembles that of the Cupuliferae 
and other lower forms of Dicotyledons, as also a few higher 
ones. 
A point which has not been fully noticed by former writers 
is the following : — that the vessels in the primary and early 
secondary wood are different from those in the later-formed 
wood, and show characters of a preceding type of structure. 
The fibrous tracheides appear to be originally derived from 
tracheides such as constitute the bulk of the wood of the 
Gymnosperms, and seem to precede the type of fibre-tissue of 
the higher Dicotyledons, which is probably parenchymatous 
in origin \ 
The phloem is dicotyledonous in structure. 
Transfusion-elements occur in the ridges of the young stem 
and in the leaves. They have evidently a distinctly different 
origin and nature from the transfusion-tissue so well known 
in the leaves of Gymnosperms, as is shown by their possessing 
simple pits and no reticulate or spiral thickening ; they are 
adapted to the position and orientation of the cortical bundle 
in the stem with regard to the assimilating tissue of the ridge, 
and to the peculiar structure of the leaf-sheath. 
Another feature, which we think has not been noticed 
Strasburger, loc. cit. 
