472 Boodle.—Anatomy of the Hymenophyllaceae . 
times seen. They are connected with a root, and occur on 
the lower side. 
The rhizome bears distichous leaves and roots, but is very 
stout compared with T. reniforme or Hymenophyllum , and, as 
seen in the figure just described, it contains a very much larger 
stele, but though the amount of xylem is greatly increased, it 
is really of almost the same type as T. reniforme. As in the 
latter plant, there is internal protoxylem consisting of either 
one or two groups, according to the distance from the node, 
and lying in the horizontal plane. T. reniforme has several 
metaxylem elements in the central parenchyma as well as the 
protoxylem, and in T. radicans the central parenchyma is 
mostly replaced by metaxylem, so as to leave only a small 
group of parenchyma accompanying each protoxylem. Ac¬ 
cording as one regards the one type or the other as the more 
primitive, parenchyma must have been phylogenetically re¬ 
placed by tracheides, or tracheides by parenchyma. 
In the petiole the metaxylem has the form of an arch, with 
its ends turned in, and inclining to a triangular outline. This 
is seen in Fig. 26, Plate XXVII, which is a diagram made in 
the same manner as the previous diagrammatic drawings. 
The protoxylem is not, as might have been expected, 
restricted to the ends of the two incurved arms of the xylem 
(which is the position it occupies in the petiole of Hymeno¬ 
phyllum dilatatum , Fig. 7), but it is distributed at a few 
points (represented by dots in the diagram) in the central 
parenchyma. In this tissue in the mature petiole one recog¬ 
nizes the protoxylem as small and partly collapsed tracheides, 
some being in contact with the ends of the xylem-arc on 
their inner surfaces. The phloem is almost continuous, but 
much scantier on the upper side, where it is interrupted by 
about one or two parenchymatous cells at the median point. 
P'ig. 27 is a diagram of the structure a short distance above 
the insertion of the axillary branch. The xylem is continuous, 
the arms having united 1 i and the central parenchyma contains 
1 This conversion of the xylem-arch into a closed ring at the base of the petiole 
is mentioned by Prantl ( 1 . c., p. 19). 
