148 OSMUNDACEAE [ch. 



statement of the grounds for an opinion upon it seems desirable. The data 

 will be arranged under the headings of the twelve criteria laid down in Vol. I, 

 and summarised in the Introduction to this volume: 



(i) The upright radial shoot, and the equal distal dichotomy of the 

 Osmundaceae indicate a primitive state, as well as the winged leaf-base 

 and commissure {Toded) comparable with those seen among the Coeno- 

 pterids, Pteridosperms, and Cycads, 



(2) The unstable structure of the apical meristems, the three-sided initial 

 of the leaf-apex, and the massive structure of its wings: the fluctuation of 

 the root-tip between the type with a single initial and that with four: and 

 the hovering between the Eusporangiate and Leptosporangiate segmentation 

 in the young sporangium all indicate a place between the Eusporangiate 

 and the Leptosporangiate Ferns as regards cellular construction. 



(3) The progression in the sporeling-leaves from equal dichotomy to 

 dichopodial branching of the veins, and uniformly open venation are also 

 primitive characters. 



(4) The simple stelar structure, related to that of the protostele, and most 

 markedly so in the Permian fossils, together with the undivided trace, and 

 the mode of its origin in the early fossils, all indicate a primitive character. 



(5) The simple uniseriate hairs form a feature common in primitive Ferns. 



(6) The non-soral character, and the marginal insertion of the sporangia 

 in Osimmda on narrow segments of the sporophyll, agree with what is 

 seen in many Coenopterids ; while the superficial insertion in Todea, corre- 

 sponding to that of the Marattiaceae, may be interpreted by abnormal 

 sporophylls of Osmunda, and is probably a derivative state consequent on 

 expansion of the leaf-surface. 



(7) The absence of indusial protections in the Osmundaceae is also shared 

 by the Coenopterids. 



(8) The relatively bulky sporangium, opening by a median slit, with 

 a non-specialised mechanism of the annulus, corresponds to what is seen in 

 the Coenopterids, and also in the Schizaeaceae and Gleicheniaceae. 



(9) The spore-output per sporangium (512-128) is intermediate between 

 the large figures for Eusporangiate and the smaller for Leptosporangiate 

 Ferns. 



(10) The relatively bulky prothallus is comparable with that of the 

 Marattiaceae. 



( 1 1) The sexual organs project from the prothallus as in Leptosporangiatae, 

 but the antheridia have a larger number of spermatocytes than in these 

 Ferns, excepting the Gleicheniaceae; this feature runs parallel with their 

 large spore-output. 



(12) The embryogeny is prone as in Leptosporangiate Ferns, and the 

 segmentation is on the same plan, but with differences in detail which 



