562 FILICALES 
species the xylem-core is fluted, the protoxylems being mesarch in its 
slightly projecting ridges. The only other widely different type of structure 
found in the genus is the solenostelic, which has been observed in G. 
pectinata alone: here the stele is larger than in any other species which 
have been examined: in addition to the structure as described the centre 
of the fluted xylem is replaced by a mass of sclerenchyma, surrounded by 
a ring of endodermis, pericycle, phloem, and conjunctive parenchyma 
(Fig. 3138). Another type which takes an intermediate position as 
compared with those already mentioned is seen in G. (Platyzoma) micro- 
phyllum, in which the leaves are densely crowded and polystichous on the 
rhizome. Here there is also an inner endodermis surrounding a central 
sclerenchyma, but there is no internal phloem between the xylem and 
endodermis. 
In this last-named species the leaves are small, and the _ leaf-trace 
separates as a small collateral strand from the periphery of the stele 
without disturbance of the underlying tissues. This appears also to be 
the mode of origin in the seedling of the more complex G. circinata; 
but in these larger-leaved species the leaf-trace of the mature leaf takes 
in the petiole an almost cylindrical form bounded by an endodermis, 
with (§ A@ertensia) or without (§ Zugleichenia) an involution on the adaxial 
side (Fig. 314). G. dichotoma is exceptional in § Mertensia in having 
no involution. The whole petiolar bundle may be regarded as a single 
flat ribbon widened laterally, but closely compressed and crumpled so as 
to take a cylindrical form: in that case the condition of § Mertensia with 
the endodermal involution would be more primitive than § Lugdeichenia 
where there is none. But G. dichotoma is an exception in the latter 
section, showing the more advanced state. 
The node of insertion of these larger leaf-traces may be marked by 
complications, islands “of tissue (composed of phloem, endodermis, and 
sclerenchyma) appearing in the xylem of the stele as cut transversely: 
these correspond actually to pocket-like encroachments of those tissues, 
extending down from the centre of the petiolar trace into the stele of the 
axis. Such pockets are only slightly developed in ZLugletchenia, but more 
so in Mertensta, and especially so in G. dichotoma, which leads suggestively 
on towards the continuous solenostely seen in G. fectinata. They have 
their relation to the theory of stelar structure, and on the facts two views 
are possible: either that the protostelic condition of most Glechenias is 
primitive, and that the solenostelic type has been derived from it, or that 
the protostelic Gleichenias might be regarded as showing the reduced 
remnants of a previous solenostelic structure. The former view appears 
the more probable: in the first place the seedling is protostelic, and offers 
no suggestion of reduction to produce that primitive state: analogy with 
Lygodium corroborates this. Further, the nodal pockets may naturally 
be held to be local complications of the stele, directly connected with 
the insertion of the peculiarly complicated leaf-trace of an unusually 
