470 Boodle.—Anatomy of the Hymenophyllaceae. 
section. The drawing includes the phloem (//£), and excludes 
the pericycle. The protoxylem in this section is centrally 
placed, and the two groups of tracheides (r, r) are the ends of 
the lower xylem-band, which develop early in connexion with 
the roots, as in Hymenophyllum. The group on the right is 
cut near the insertion of a root-stele, as shown by the phloem 
being curved out into a prominence opposite to it. 
In the petiole, at its base, the xylem forms a continuous 
ring, enclosing central parenchyma, with protoxylem and a 
little metaxylem included in it, the whole being surrounded 
by a continuous ring of phloem. Thus the structure in this 
region is almost identical with that of the rhizome. High up 
in the petiole a small gap may occur on the upper side in the 
xylem-ring, and the sieve-tubes become scanty and scattered 
on the same side. The protoxylem comes to abut on the 
inner surface of the upper band of xylem (or is embedded in 
it), and becomes separated into two groups. This is just 
below the lamina. Some metaxylem then forms a bridge 
across the central parenchyma, in the median plane of the 
petiole. The xylem thus resembles two circles pressed 
together so as to be flattened where they touch. In the very 
base of the lamina the two circles of xylem split apart, and 
phloem is formed on the sides where they face one another, so 
that two perfectly similar concentric bundles are formed. 
They are the two primary veins of the leaf, and are thus formed 
by dichotomy of the petiolar bundle. By their repeated 
forking they produce the venation of the leaf. 
The retention of basal petiolar (stem-like) structufe up to 
the top of the petiole seems to be connected with the sub¬ 
sequent division of the bundle by dichotomy. In species of 
Hymenophyllum with a similar structure at the base of the 
petiole, the xylem-ring opens into an arc, a little higher 
up, the bundle assuming a form which is probably more suit¬ 
able for the detachment of bundles for the pinnae right and 
left. As the dichotomous venation has been regarded as 
a primitive character x , a similarly shaped leaf, namely that of 
1 Prantl, 1. c., p. 57. 
