THE FOSSIL OSMUNDACEiE. 473 



account of the leaf-trace of Zygopteris {Diplolabis) Rbmeri on its passage through the 

 cortex of the stem.* As it passes in, the arms of the peculiar-shaped xylem strand 

 gradually become less conspicuous, and the islands of parenchyma become smaller and 

 smaller until it assumes the shape of an hour-glass, then the form of Dineuron itself, 

 and finally the islands disappear entirely and it is almost identical in outline with our 

 hypothetical ancestral trace. 



It is clear that Dineuron and Clepsydropsis would serve excellently as primitive 

 types from which to derive the leaf-traces of their respective groups. It would be a 

 very easy matter to obtain all the various types of xylem strand that they exhibit by 

 the simple application in varying degree of the principles previously laid down. Again, 

 the great similarity in form between both Dineuron and Clepsydropsis and our 

 assumed ancestral trace gives weight to the supposition that some such form is really 

 the common starting-point for both groups. 



There is, however, one very pertinent objection still to be overcome. Our theory 

 assumes, in the first place, that the Zygopteridese are an ascending series of forms 

 derived from a common ancestor with the Osmundacese. It further assumes that the 

 type of frond possessed by the Osmundacese and by all other Filicales was primitive, 

 and the Zygopterid frond has been derived from it. Now, the ordinary fern leaf has a 

 single row of appendages on each side of the rachis, and appendages of all orders 

 face the same way — towards the stem. To put it otherwise, the principal planes 

 of symmetry of the appendages are practically parallel to that of the main rachis 

 (text fig. 6). (In the text figure of our hypothetical ancestral trace the arrows 

 indicate the planes of principal symmetry.) This brings us face to face with one of 

 the most striking characteristics of the Zygopteridese ; for even in those cases where 

 there is only one row of appendages on each side of the rachis they face totvards 

 the rachis and not towards the stem. In fact, they bear the same relation to the 

 petiole of the leaf that leaves normally do to an erect stem. To state it otherwise, 

 again, their planes of principal symmetry are at right angles to that of the main rachis 

 (cf. text fig. 7). 



In attempting to visualise this extraordinary state of affairs, it is of some assistance 

 to consider what would happen if an ordinary fern leaf were to grow vertically erect 

 throughout its whole rachis. In the first place, the lateral appendages would need to 

 twist round through an angle of 90 degrees in order to expose the laminar expansions 

 directly to the light. It is clear also that a two-ranked arrangement of the appendages 

 would no longer be a good plan, for they would overshadow one another. It would be 

 far better to arrange them in several rows, for the same number of appendages could 

 then be borne with much less mutual interference. As a matter of fact, the rachis and 

 its appendages are now affected by the general conditions in precisely the same way as 

 an erect stem with its leaves, and a tendency towards a radial symmetry is only to be 

 expected. This state of affairs is actually realised in the Zygopterid petioles, and the 



* Gordon. See note, p. 468. 



