A. G. Tansley. 
for regarding them as anything but (temporarily) arrested con¬ 
tinuations of the rachis. Each bud contains a direct continuation 
of the primary C-shaped vascular strand of the rachis, and the 
vascular strands of the primary branches clearly arise from the 
sides of this. Boodle (’01 B) has pointed out the possibility of 
deriving an ordinary pinnate fern-frond from the Gleichcnia- type, 
the first pair of primary pinnae of the former being considered as 
equivalent to the first branches of the latter type of frond ; with 
this view of the homology I should entirely agree. The rhizome 
(except in Stromatopteris ) is always creeping and usually subter¬ 
ranean, the leaves being typically inserted at comparatively long 
intervals. 
The rhizome has a single “solid ” vascular cylinder; the tracheae 
of the xylem, which fills up the centre of the stele, being inter¬ 
spersed with parenchyma. The position of the spiral protoxylems 
is characteristic. They are situated just below the surface of the 
metaxylemat about equal intervals round the periphery of the stele, 
which is sometimes lobed in relation to the insertion of roots (Fig. 
34). This mesarch “ protostely ” is probably secondary in ferns, 
r 
Fig. 34. Gleichcnia linearis. T. S. of stele of internode, e., endodermis ; 
/>//., phloem, indicated by an interrupted line ; .r., metaxylem, cross-hatched ; 
f>x., protoxylems indicated by circles ; r., root stele attachment. From Hoodie. 
The conventional indications of phloem, xylem and protoxylem arc used 
in all the following diagrams. 
and may, as we have already seen (p. 117), be derived from the 
endarch type and be dependent on the relation of the size and 
shape of the leaf-trace to that of the stele. In the rhizomes of 
some species spiral protoxylems are quite absent. 
There are two well-marked sub-genera of the more typical 
Gleichenias— Mertensia , which has its pinnules more or less elon¬ 
gated, and Eugleichenia, in which the pinnules are small and 
rounded. The latter sub-genus is to all appearance distinctly more 
xerophilous than the former. The cross-section of the leaf-trace 
is distinctive in the two sub-genera. That of Mertensia is typically 
C-shaped as a whole, i.e., the curved band of xylem is covered 
