492 OPHIOGLOSSALES 
and of the enlargement of the leaf culminating in monophylly: both these 
factors will have tended towards the dominance of the leaf, and so it is 
not surprising that the structure of the shoot should be phyllosiphonic 
from the first. 
Reasons have been advanced above for not accepting the view of the 
fertile spike as a modified pinna, holding a median position. The 
alternative is that it is a substantive part not referable in origin to any 
vegetative structure previously present. Such substantive parts are seen 
in the Psilotaceae and Sphenophylleae, occupying a position corresponding 
to that of the Ophioglossaceous spike, viz. the sporangiophores. The 
smallest spikes of Ophzoglossum or Botrychium are little in advance of 
these. From them, by seriation of specimens of the same species of 
different ages, and by further seriation of different species, the steps 
leading to the most complex forms of spike may be represented: while 
its branching, where present, is matched by the increasing complexity of 
the sterile leaf. The advance thus contemplated in the spike involves. 
.continued apical growth, and branching, together with growth and septation 
of the sporangia. , Apical growth of limited duration already exists in the 
sporangiophore of Fszlotum: the structure of the young spike in Op/zo- 
glossum, and less clearly that of Botrychium, is such as to be perfectly 
compatible with septation, a process for which there are demonstrated 
precedents elsewhere. Further, it has been shown that with the growth 
and septation of the sporangium the simplest branching of the spike of 
Botrychium is very closely allied. In Helminthostachys a further elaboration 
is present, which may be referred to the replacement of the sunken 
sporangia of Ophioglossum by dense ranks of sporangiophores: and it has 
been shown that this mode of origin is reflected in the individual develop- 
ment of the sporangiophore. Lastly, the spike, like so many other parts, 
is liable to fission or®chorisis. The numerous sporangiophores of the 
Sphenophyllales seen in some species probably owe their origin to such 
chorisis. In Ophzoglossum it appears occasionally in common species, such 
as O. vulgatum: branching or fission of the spike occurs not uncommonly 
in O. pendulum, but in O. palmatum it has become habitual, though there 
is reason to think that it is only attained in that species when the plant 
becomes fully mature. The various types of spike in the family thus 
readily lend themselves to interpretation as an upgrade series 
As regards the development of the sporangium the Ophioglossaceae 
form a series, from Ophzoglossum with its large, ill-defined sporangia to- 
the larger-leaved Botrychia and Helminthostachys with smaller and more 
definite sporangia. It has been shown that in the indefiniteness of 
limitation of the sporogenous tissue,.and in the absence of a marked 
tapetum, Ophioglossum, Tmesipteris, and Psilotum agree more nearly than 
other Pteridophytes. It seems highly improbable that such indefinite 
characters would be the result of specialisation along parallel lines in two 
distinct series. This similarity may more probably be held as indicating 
