Boodle . — A natomy of the Gleicheniaceae. 7 1 1 
xylem-prominences connected with them, the outline of the 
xylem being rounded or nearly flat on this side, while the 
remainder of the protoxylems are found below well-marked 
lobes. This structure was seen, for instance, at a distance 
of about | inch from a node. 
A similar rounding or flattening of the upper side of the 
xylem occurs as one approaches the node in G. dichotomy 
but in G. dicarpa and some other species it is found through 
most or the whole of the internode, so that the xylem only 
bears prominences below and at the sides. 
G. dicarpa , var. alpina , has a much smaller rhizome than the 
type, and a rounded-quadrangular xylem, with a protoxylem 
embedded in each angle. 
Once more comparing the structure of the rhizome of 
Gleichenia with that of Lygodium , and leaving the striking 
differences in the protoxylem out of the question, a few points 
of difference in detail may be mentioned. In Gleichenia there 
is usually a greater proportion of parenchyma in the xylem, 
and it more often occurs as 2-3-seriate chains of cells 
(instead of uniseriate) ; the tracheides are often broken up 
into shorter chains or groups, are often larger in proportion 
to the size of the stele, and frequently have a shorter common 
wall, where they are in contact with one another. Owing 
to the latter point and to the tracheides being in contact 
with a larger number of parenchyma-cells, the tracheides of 
Gleichenia in longitudinal section less frequently show the 
scalariform pits running right across the width of the elements 
than is the case in Lygodium. All the above small differences 
are probably liable to exception. 
Having given a general description of the structure of the 
rhizome of Gleichenia , some peculiarities of certain protostelic 
species should be mentioned. In a section of the rhizome 
of G. dicarpa a few of the xylem-parenchyma-cells were seen 
to be thick-walled and lignified, and others showed traces of 
lignification at their corners in the middle-lamella, so it is 
quite likely that a general lignification of these cells may take 
place in quite old parts of the rhizome. Poirault (’ 93 , p. 173) 
