270 SUMMARY OF RESULTS [CH. 



sometimes be so similar to that of the petiole, as to suggest their representing 

 unequally developed branches of a dichotomy {B. cylindricd), thus throwing 

 a side-light on a possible mode of origin of the leaf. The vascular system 

 is very simple in accordance with their small size, and the sporangia bear a 

 general resemblance to those of Etapteris (Scott, I.e. p. 344). 



The Coenopteridaceae thus show among themselves a considerable range 

 of size, form and structure, and collectively afford suggestions of value in 

 relation to various features seen in more typical, but yet primitive Ferns. 

 Their early occurrence, coupled with the small size and simple structure of 

 some of them: their prevalent dichotomous branching: the indeterminate 

 distinction of axis and leaf: the simple though large hairs oi Botryopteris, 

 enlarged basally to stiff bristles in Zygopteris: their primitive sporangia, and 

 numerous homosporous spores collectively indicate generalised types. Their 

 complex leaf-structure based on bifurcation of narrow segments, and with 

 circinate venation: the vascular structure, simple in small types but in- 

 creasing in complexity with size: the grouping of the sporangia from the 

 solitary distal capsule to the radial sorus; these are all pointers towards 

 more definitely Filical types. The Coenopteridaceae may fairly be held as 

 Ferns, and as representing more than any other known organisms an ap- 

 proximate source from which the whole Class of the Filicales has originated. 

 On the other hand, that problematical plant Staiiropteris gives the most 

 valid basis for comparison with the Devonian Psilophytales: together they 

 suggest that an upright, profusely branched indeterminate frond may have 

 provided the raw material for the elaboration of the Fern-leaf, and that a 

 solitary distal sporangium of radial structure, almost Bryophytic in its 

 character, heralded the aggregated sori with dorsiventral sporangia seen in 

 most of the true Ferns. 



The Ophioglossaceae (Vol. II, Chap. Xix) appear as a blind evolutionary 

 series, in the sense that they do not link phyletically with any living Family 

 of Ferns. But they present certain features of advance parallel to those seen 

 elsewhere. In contrast to the Coenopteridaceae, their gametophytes are well 

 known. Fungal infection affects both generations, particularly their massive 

 underground mycorhizic prothalli, and this has probably promoted their 

 survival. Both generations are constructed on a massive scale, essentially 

 Eusporangiate, as are all of the Coenopterids. But the single initial cell of 

 the stem, and the presence of a suspensor in H e Imint host achy s and in a 

 section of the genus Botrychium, may perhaps be held as traces of far-off 

 filamentous origin. These two genera, from their lobed lamina, open vena- 

 tion, coherent stele and leaf-trace, and the presence of a suspensor, appear 

 to be relatively primitive : while Ophioglossum, with its blade usually entire, 

 reticulate venation, dictyostelic axis, its sometimes divided leaf-trace, and 

 absence of a suspensor, may be held as advanced. A peculiar line of reduc- 



