692 Gwynne- Vaughan. — Observations on the 
will serve for them all. The gaps formed in the solenostele 
by the departure of the leaf-traces never overlap. The xylem- 
ring is surrounded, both externally and internally, by a 
complete ring of phloem and pericycle, and the whole is 
delimited from the ground-tissue on both sides by a well- 
marked endodermis. The leaf-trace departs from the soleno- 
stele of the stem as a single continuous vascular strand, 
usually curved so that it has a section similar in form to 
a horse-shoe or an arch. This curved strand is so attached 
to the solenostele that its concavity faces the median dorsi- 
ventral plane of the rhizome, either directly, as in Dicksonia 
punctiloba (PL XXXIII, Fig. 1), Pteris incisa, &c., or else more 
or less obliquely, as in Dicksonia apiifolia (Fig. 2), Davdllia 
Spelnncae (Fig. 3), &c. In Pteris ludens and Jamesonia 
imbricata the leaf-trace faces directly towards the apex ; 
a position it also occupies in L ox soma (Part 1 1 , Fig. 4) ; in this 
Fern, however, the leaves are inserted along the upper surface 
in a single median row. For some time before it actually 
departs, that portion of the solenostele destined to form the 
leaf-trace is usually well defined as a protrusion which is some- 
what thinner than the rest of the vascular ring (Figs. 2 and 3). 
The leaf-gap generally closes up at the same time as the 
acroscopic flange of the leaf-trace departs, or immediately after- 
wards. It may, however, remain open for a short distance 
above the leaf, as in Davallia Spelnncae (Fig. 3), Pteris ludens, 
Jamesonia imbricata, and others. The free margin of the 
leaf-gap is usually of the same thickness as the rest of the 
solenostele, but in some cases it becomes more or less en- 
larged : Dicksonia apiifolia, cicutaria, Davallia hirta. The 
enlargement is entirely due to the increased thickness of the 
xylem-ring in this region ; in the two Dicksonias it is often 
twice as thick as elsewhere in the stele, so that the free 
margin of the leaf-gap projects markedly towards within. 
This marginal thickening is a feature of considerable interest, 
and those Ferns in which it reaches a more conspicuous 
development will be trea'ted separately later on. When 
1 Ann. of Bot. vol. xv. PI. III. 
