86 Gwynne- Vaughan . — Observations on the 
for that of Matonia p'ectinata 1 . On the other hand, in Tri- 
chomanes Prieurii , where these fibres were first discovered by 
Mettenius (1. c.), all the sclerotic elements can hardly be 
regarded as sclerosed sieve- tubes, because, as I have seen, 
although some are elongated and pointed, resembling those 
in Loxsoma , there are also others which are relatively short 
and square-ended ; here, perhaps, both constituents of the 
phloem, sieve-tubes, and parenchyma are implicated in the 
sclerosis. Finally, a very puzzling case is described by 
Thomae 2 3 in the petiole of Adiantum trapeziforme , var. penta- 
dactylon , where the fibrous elements are found not only at 
the margin of the xylem-strand, but also intermingled among 
the tracheides themselves. The most obvious suggestion 
seems to be that the xylem-parenchyma has become sclerotic 
and fibrous. 
The xylem, apart from its first-formed elements, consists 
of scalariform tracheides, which upon treatment with caustic 
potash behave in the same way as those in the stem. The 
largest tracheides are found on the flanks of the xylem-strand, 
and the smallest in the neighbourhood of the protoxylem- 
groups. The protoxylem consists of narrow elements exhibit- 
ing beautiful spiral and angular thickenings, and is situated 
on the internal adaxial surface of the xylem-strand, towards 
the concavity of the horseshoe ; it may therefore be regarded 
as endarch (Figs. 6 and 8, prx). The number of groups 
varies according to the petiole, and in the same petiole 
according to the level at which the section is taken, becoming 
fewer towards the top of the rachis. Generally speaking, 
there are two or three groups in the bay of each hook, and 
from two to six along the dorsal-curved portion of the strand. 
These endarch protoxylems, although perfectly clear and 
distinct in all parts of the free petiole, gradually become less 
definite when the petiolar meristele enters the tissues of the 
stem. Shortly after the leaf-trace has become completely 
1 Seward, Struct, and Affin. of Matonia pectinata, Phil. Trans., B., vol. cxci, 
1899, p. 183. 
3 Blattstiele der Fame, Prings. Jahrb., Bd. xvii, 1886, p. 129. 
