466 Boodle.—Anatomy of the Hymenophyllaceae. 
Hymenophyllum, petiole. 
The structure of the petiole of Hymenophyllum dilatatum , 
var. Forsterianum , is shown diagrammatically in Plate XXV, 
Fig. 6 . The pericycle is thick as in the rhizome, and the 
phloem is missing on the upper side of the petiole, which 
is the lower side in the diagram 1 . The xylem has the form 
of an arch with its ends turned inwards. Fig. 7 is a drawing 
of the petiolar bundle of the same plant. A protoxylem- 
group ( px 1 ) is seen at the end of each of the two arms of the 
xylem, and one of the arms is separated, in this section, from 
the rest of the metaxylem by parenchyma. Figs. 6 and 7 
represent the structure of the petiole as one finds it for a con¬ 
siderable part of its length. A marked difference is seen, 
however, for some little distance above the axillary branch 2 . 
The section shown in Fig. 8 was cut through this region. 
Comparing this with Fig. 6, the difference observed is that 
the phloem has become continuous, the xylem has also formed 
a closed ring, and the two protoxylem-groups have become, 
at any rate partly, internal to the xylem-ring. The type of 
structure of this region of the petiole is therefore practically 
stem-like. In the disposition of its elements it is scarcely 
different from the stem of the same species (cf. Fig. 9, right 
side). 
The petiole of H. cruentum (Plate XXVI, Fig. 20) is of the 
same type as H. dilatatum , var. Forsterianum , but the bundle 
is much smaller. The protoxylem (px) is at the ends of the 
two curved arms of the xylem, and the phloem is interrupted 
on the upper side (the lower in the drawing). At s.t. a sieve- 
tube lies in contact with a tracheide. 
Most species of Hymenophyllum , which have a sub-collateral 
structure in their rhizome, have a collateral structure in their 
petioles, the arrangement of elements in the two being very 
similar, the difference being that the phloem is continuous in 
the rhizome, but interrupted in the petiole. Fig. 17 repre- 
1 All the petioles are drawn orientated in this way. 
2 The occurrence of axillary branches will be described below. 
