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 OpMoglossum 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 Psilotitm : the structure of the young spike in Ophio- 

 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 Ophioglossum 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 Ophioglossum 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 



