734 Boodle . — Anatomy of the Gleicheniaceae. 
and some have arranged themselves in chains, the external 
cells being in contact with the conjunctive parenchyma 
bounding the xylem. They become more scattered again, 
the second leaf-trace is given off about opposite the 
first one, and a little higher up the third root is attached. 
The xylem of the stem is now roughly circular in outline, 
and consists of tracheides of fairly uniform size (probably 
all scalariform) and a few xylem-parenchyma-cells amongst 
them. Thus, except for the absence of protoxylem-groups, 
the type of structure of the mature stem is already attained. 
Where the third leaf-trace was given off, the stele was incom- 
pletely differentiated, and, at a short distance above, became 
procambial. It is a pity that no older seedling was obtain- 
able, but as a structure practically resembling that of the 
mature stem of Gleichenia was arrived at without any more 
complicated stage being passed through, it seems probable 
that such complication (e. g. solenostely) would not be found 
higher up. Thus, as far as the data go, the seedling-structure 
gives no evidence of the protostely of Gleichenia being due to 
reduction. The petiolar bundle of the third leaf has a small 
band of about seven tracheides, and apparently has phloem 
on the outer side only, i. e. it is collateral. 
PLATYZOMA, habit. 
The stem of Platyzoma microphyllum , which is the only 
species of its genus, is a creeping rhizome, with roots on 
its lower side. It differs from the rhizome of Gleichenia in 
bearing densely crowded leaves, which are not restricted to 
the upper side of the stem, but have polystichous arrange- 
ment. The leaves are 12 in. or less in length, and are simply 
pinnate. The pinnae are very small (about 2 mm. in length), 
ovate-orbicular in outline, and have their margins revolute 
as in Erica hiemalis , so that the stomata, which are restricted 
to the lower side, are placed in a nearly closed cavity. The 
plant is evidently strongly xerophytic, and the leaf has no 
doubt become simply pinnate by reduction. As mentioned 
