112 A. G. Tansley. 
This ring is nearly flat on the ventral side and shows a tendency to 
separate into a curved dorsal and a straight ventral hand, the 
former containing on the whole the larger tracheids. Enclosed 
within the tracheal ring is thin-walled parenchyma containing 
scattered smaller tracheids, some of which are spiral protoxylem 
elements. Externally to the metaxylem ring is a ring of phloem, 
and outside this again is a many-layered pericycle and an endodermis. 
The type of dorsiventral symmetry found in the Hymeno- 
phyllaceous stele is peculiar to it. Dorsiventrality is not very 
common in fern-steles, in spite of the prevalence of creeping 
rhizomes; it is found however in a good many of the advanced 
dictyostelic forms, and there, as in Hymenophyllaceae, the lateral 
gaps correspond with leaf-gaps, though it is the whole vascular 
ring instead of the tracheal belt alone which is interrupted. 
The general relations of the tissues of this form of stele, apart 
from the dorsiventrality, very strongly recall those of Zygopteris, the 
central type of the Botryopterideae, and except in these two 
families it is not known among the Ferns. 
The structure of the base of the petiole is practically identical 
with that of the rhizome, and in T. reniforme this structure is 
maintained nearly up to the insertion of the simple reniform lamina 
found in this species (Figs. 24, 25). Here a small gap may occur in 
Fig. 24. Trichomcines reniforme. Stele of rhizome giving off leaf-trace 
to right (tracheids alone drawn). The smallest central elements in stele and 
leaf-trace are protoxylem. Some larger metaxylem tracheids lie between the 
two protoxylem. A root-stele is about to be given off from the extreme left 
of the stele, x 175. 
.. , Flg ' 25 ' Trichomaues reniforme. Strand of petiole immediately below 
dichotomy. The small central elements arc protoxylem. x 175 . 
the middle of the upper side of the xylem ring, though this is not 
constant, and the sieve tubes also become scattered on that side. 
The central protoxylem divides into two groups, a vertical bridge of 
xylem is formed across the central parenchyma and the strand 
dichotomises quite symmetrically. Each branch of the dichotomy 
forks again repeatedly and thus the parallel veins of the fiabellate 
