354 ROYAL SOCIETY OF CANADA 
the rhizome in transverse section and in each of these bundles, in the 
smaller xylem elements, there is a patch of thin-walled, evidently pa- 
renchymatous tissue, the cells of which overlap one another in places. 
and show irregular outlines. These patches undoubtedly consist of 
thyloses of the same nature as the ones to be described below, although 
they are represented somewhat diagramatically in the figure. No refer- 
ence is made by Thomé to this part of the bundle, either in the lettering 
of the figure or in the text, so that although observed by him, still it did 
not strike him as peculiar and he therefore gave it no interpretation. 
The first attempt at an interpretation of these structures is found 
in Russow’s Vergleichende Untersuchungen. In treating of the Poly- 
podiaceæ (15, 101) he remarks that in transverse sections of vascular 
bundles of certain stipes, the cells adjoining the protoxylem stand out 
from the surrounding cells by reason of their wide lumen, thus differing 
from the other phloem cells. In longitudinal section these cells are 
noticeable owing to the fact that their walls are not regular, but bend 
in and out, the cells being connected with each other in such a manner 
that large lacunæ or intercellular spaces arise. The protoxylem elements 
bordering these cells possess very irregularly formed spirals. In mature 
bundles the walls of these lacunar parenchyma cells (Luckenparenchym- 
zellen) appear to be brown in color. 
From the above description it is plainly evident that Russow re- 
garded these cells as a portion of the phloem, and that he considered 
the lacunæ or intercellular spaces to owe their origin to the curious 
shape of certain primary phloem cells. As will be seen below this in- 
terpretation is wholly incorrect. 
In describing the Cyatheaceæ he remarks as follows (15, 104): 
“In the bundles of the stipe the strands of lacunar parenchyma, which, 
in the longitudinal direction of the xylem border on the protoxylem, are 
always composed of very broad cells; in thin transverse sections these 
give the impression of large lacunæ.” This appearance of the cells in 
question is very common in the mature stipes which will be dealt with 
below, and in these the individual cells form large vessels. 
Russow also mentions cells of wide lumen in the bundle of Osmunda | 
regalis which he describes as follows :— 
“In the xylem of the C-shaped petiolar bundle in transverse sec- 
tion, the tracheids form a compact mass; on the concave side of the 
bundle there are certain cells of the phloem which appear in groups at 
ten or twelve points and which are remarkable for their wide lumen, 
exceeding that of their neighboring cells four or five times; they con- 
tain starch, like the other conducting cells (geleitzellen).” These 
