206 Yapp. — Two Malayan ‘ Myrmecophilous ' Ferns. 
described above. The midrib projects considerably on the 
upper surface of the segment, but very little on the lower. 
It contains a single rather small stele, provided with a brown 
sclerenchymatous sheath similar to those of the stem and 
petiole. Between the stele and the epidermis (both upper 
and lower) are thick-walled mechanical elements, completing 
what is thus an efficient I-shaped girder. The stelar sheath 
is continued for a short distance only round the main lateral 
veins, the smaller bundles possessing neither the thick-walled 
sheath nor the strengthening mechanical tissue. In optical 
section it can be seen that the bundles usually contain few 
tracheides, but their number is often increased by the inter- 
polation of a few short curved ones at points where the veins 
branch or anastomose. The free vein-endings are swollen, 
and contain numbers of short scalariform tracheides. In the 
fertile segments the veins supplying a sorus run down the 
proximal side of the sorus-cup, and form an irregular plexus 
under the placenta, containing numerous short tracheides 
(Fig. 46). At the point where the soriferous lobe is sharply 
bent back upon the rest of the leaf-segment, the vascular 
bundles are also bent, even the individual tracheides taking 
part in this curvature. 
The sorus-cup differs somewhat in structure from the rest 
of the lamina. Its mesophyll has a more uniform appearance, 
and possesses only small intercellular spaces. Normal stomata 
occur on the inside of the cup (morphologically the lower 
surface of the leaf), but they are not very numerous. 
A mature sorus (Fig. 39) contains sporangia in all stages of 
development, numerous sporangium stalks, and lastly, a few 
short paraphyses with small, somewhat club-shaped heads. 
The sporangia have long stalks, composed of three rows of 
cells, most of which are extremely elongated (Fig. 44). 
The total length of a mature sporangium and stalk is 
usually about 17 mm. The annulus is vertical and incom- 
plete. The inner and radial walls of the three cells of the 
annulus immediately above the stomium remain comparatively 
thin (Fig. 44), though all the walls of these cells are deep 
