THE VEGETATIVE SYSTEM 679 
comparative study of the meristems. of root, stem, and leaf in Ferns indicates 
that the most complex meristic condition is found in the Marattiaceae, a 
series of Ferns known to have been well represented in Palaeozoic times 
(see p. 650, etc.) The Osmundaceae are now being more and more firmly 
established in relation to the ancient Botryopterideae: their characteristic 
structure is recorded back certainly to early Mesozoic times, and possibly 
earlier: they show in their meristems an intermediate condition, while 
that of the roots is variable: Zodea often has the structure characteristic 
of the Marattiaceae: Osmunda has sometimes a single initial in the 
root, but often more, with curious irregularities of the segmentation. 
Its stem-structure shows a similar state, while the leaf in the Osmun- 
daceae is alone among Ferns in possessing a three-sided _ initial 
with regular segmentation: the leaf (except in the filmy Todeas) 
also shows a structural complexity of the wings similar to that of 
the Marattiaceae. All other Ferns, including even such early forms as 
Schizaeaceae and Hymenophyllaceae, have the single initial in all their 
parts, while the wings of the leaf also have a single marginal series of cells 
with definite segmentation. From this it is concluded that in the Filicales 
there has been a progression from types which were more primitive—where 
the meristic structure was more complex, with the centre of construction 
more deeply seated, and as a consequence with a plurality of initials of 
prismatic form—to those characteristic of more modern times, where the 
meristic construction is less complex, the centre of construction less deeply 
seated, and as a consequence with a single initial having the form of a 
three-sided, or even a two-sided, pyramid. The progression has been from 
a more massive to a less massive construction, and from less definite 
to more definite segmentation. It has been shown above (p. 637) that 
a similar progression may be traced in the sporangial character. 
A parallel progression, though less definitely indicated, is to be traced in 
the Lycopodiales. At the apices of stem and root in the ancient genus 
Lycopodium the tissues are not referable to a single initial cell. In 
Selaginella spinulosa also, that species which on grounds of its radial shoot 
and its anatomical structure we have recognised as a relatively primitive 
type of the genus (pp. 300, 332), there is from the first stages of the embryo 
a small-celled meristem, without any single initial in stem or root. But in 
the dorsiventral species, which on grounds of form and structure are held 
to be derivative types, there may be a single initial both in axis and root, 
though variable in the details of form.!. This mode of apical growth makes 
its appearance in the very first stages of the embryo (p. 356). It seems 
therefore probable that here again, as in the Filicales, there has been a 
phyletic progression from a less definite segmentation with several initials 
at the apex of stem and root to a more definite segmentation with a single 
initial cell. 
1De Bary, Comparative Anatomy, p. 15. Treub, Selaginelia Martensii, Leide, 1877, 
PL 1, I, Il. é 
