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 : Todea 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 

 Lycopodhun 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. 1 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. 



1 De Bary, Comparative Anatomy, p. 15. Treub, Sda^inclla .Martcnsii, Lcide, 1877, 

 PI. I., II., in. 



