Fi lie ales. 
27 
elaborations (4). The substantial though inexact parallelism shown 
by this botanist to exist between the number of sterile lobes and of 
fertile spikes is equally compatible with a process of gradual reduction 
as with one of gradual elaboration ; and though Professor Bower 
regards the simpler forms as the more primitive, yet in discussing 
the origin of the leaf-trace he admits, in a foot-note, that the simple 
O. pendulum and O. simplex belong to a section of the genus believed 
to be highly specialized (9). But it may be argued that the spikes 
are usually adaxial, marginal ones being rare (4). Against this it 
may be urged that the fronds of many Botryopterideae branched in 
more than one plane (40), and that branching into dorsal and ventral 
lobes occurs in the sterile leaf of Azolla and the fertile frond of the 
Marsileaceae, though here the adaxial sporocarp is said to originate 
marginally (25), (26). 
The gametophyte of the Ophioglossaceae is subterranean, 
completely saprophytic and colourless (except in Ophioglossum 
pedunculosum), and more or less radially constructed. On typical 
dorsiventral fern-prothalli, the gametangia are on the lower surface, 
in HelminthostacJiys and Ophioglossum they occur all round the radial 
prothallus, while in species of Botrychium the somewhat flattened 
prothallus bears gametangia on its upper surface only (12), (30). 
This distribution of the gametangia suggests that the prothallus of 
Botrychium, instead of being intermediate between that of most 
Ophioglossaceae and of other ferns, is a modification of the radial 
type, and is advantageous in that its gametangia probably arrest 
the water percolating the soil more easily and increase the 
probability of fertilization (30). 
The Ophioglossaceae show certain resemblances to the Botryo¬ 
pterideae. In both, the wall of the sporangium is more than one cell 
thick and the frond frequently branches in more than one plane; in 
the Adder’s Tongues, however, this branching in a dorsiventral 
plane is confined to the fertile fronds. Multiseriate pitted tracheae 
occur in both orders and may well he primitive, for though rare in 
recent ferns they are characteristic of the Pteridosperms and these 
probably had a common origin with the Filicales from fern-like 
plants. Indications of the development of parenchyma at the centre 
of the stele occur in Zygopteris corrugata ; an increase of this 
tendency might easily give rise to the stele of Helminthostachys (the 
most primitive in the Ophioglossaceae) with its central pith and ring 
of wood. The Ophioglossaceae seem to be vegetatively more reduced 
than the Botryopterideae and the frequent dichotomy or dichotomous 
yenation of their fronds is a primitive character showing that, 
