43 
A natomy of the Ophioglossaceae. III. 
suggested in the case of the Marattiaceae when the young plants are con¬ 
sidered. The most interesting comparisons are, however, with the extinct 
Zygopterideae and Botryopterideae, and with the Osmundaceae, which have 
been shown by recent investigation to show signs of a common origin with 
these. It would lead too far to follow the comparisons in detail at present, 
but it may be pointed out that the closest comparisons can be drawn between 
Helmintkostackys and those forms in which the inner xylem consisted of 
elongated tracheides mixed with parenchyma, rather than with those in 
which a gradual conversion of actual elements of the inner xylem into short 
parenchymatous cells is suggested. The two processes are, however, probably 
not fundamentally distinct. Osmunda appears to still possess a zone of 
inner xylem around its pith, and is thus comparable with Helminthostachys. 
The parallel is even closer between Helminthostachys and some Zygopterideae, 
notably Metaclepsydropsis duplex, Ankyropteris Grayi, and A. corrugata, 
although a central pith is not present in them. The steles of these 
plants with mesarch decurrent protoxylems are, in fact, paralleled by 
the condition found in the smaller stems of Helminthostachys with a mixed 
pith, the inner tracheides being histologically different from those of the 
outer xylem. The condition in Helminthostachys with a mesarch but solid 
xylem may be compared with what is sometimes found in Bo try op ter is . 1 
Whether the comparison can be extended to some other Botryopterideae 
and to the Hymenophyllaceae, where the narrow inner elements of xylem 
appear, on our present knowledge, to be really protoxylem, is not so clear. 
The protoxylem may possibly be central in some very slender young plants 
of Helminthostachys where the inner metaxylem is practically wanting, but 
in all strong plants the metaxylem was mesarch as described. 
The parallel may be extended to the primary structure of some 
Cycadofilices, where the inner xylem is of the Heterangium type, and outer 
primary xylem is seen in the leaf-traces. The significance of smaller remains 
of centripetal xylem, as in Lyginodendron , will be referred to below in 
connexion with the leaf-trace structure. 
(b) Secondary thickening. In Helminthostachys there is normally no 
secondary growth, though (as is also seen in the Zygopterideae and some 
other Ferns) there may be an approach to an irregular radial serration 
of the tracheides. As is shown by the pieces of rhizome which bore 
branches, however, a localized development of tracheides in the normal 
position of secondary xylem can take place long after the primary structure 
was mature. Though there is no regular persistent cambium, this must be 
regarded as a special case of secondary thickening, all the more interesting 
because something is known of the stimulus that starts the process. It may, 
on the one hand, be compared and contrasted with the more regular 
secondary thickening in Botrychium , and, on the other, with the secondary 
1 Cf. Benson, Ann. of Bot., xxv, PI. LXXXII, Fig. 13 A. 
