142 
r»R JOHN M'LUAN THOMPSON ON 
are well developed in Gl. circinata, and in this species bear tufts of stiff hairs 
instead of scales, as in Stromatopteris. In themselves these emergences may be 
regarded as indicative of a relationship between Stromatopteris and Gleichenia. 
The axis of Stromatopteris is protostelic, the stele consisting of a broad pericycle, 
a narrow zone of phloem, and a relatively bulky parenchymatous xylem (fig. 21). 
At some points the xylem-parenchyma is fairly evenly distributed between the 
tracheides, while at other points the bulk of the latter is peripherally disposed and 
the parenchyma is centrally aggregated (compare figs. 21. and 28). The former 
condition is typical of those portions of the stele which lie between the points of 
branching of the stem ; the latter is usually seen at or near these points. The 
relative proportion of parenchyma to tracheides is liable to vary considerably, and, 
Text-fig. 15. 
while at some levels — and particularly beneath the points of branching — the 
parenchyma may be considerable, at other points, particularly the bases of the 
branches, the stele has an almost solid core of tracheides. These variations will be 
readily understood when it is recognised that it is chiefly by parenchymatous 
increase that the stele is widened preparatory to a branching, and that at such 
points of preparation the bulk of the tracheides is not really diminished, but is, as 
a rule, augmented. On the other hand, the condensation of tracheides and reduction 
of parenchyma, shown in the stele at the base of a branch, may be interpreted on 
mechanical grounds as necessary to ensure rigidity in the branch. 
The tracheides themselves may vary considerably in size, but no general state- 
ment can be made on this point beyond that where the bulk of the parenchyma is 
centrally aggregated, groups of small narrow tracheides are usually lodged within 
the parenchymatous mass, and that at other points conspicuously narrow elements 
may be absent, or, if developed at all, they may be peripherally disposed. These 
