LEAF- ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 707 



megaphyllous origin, holding that the leaf preceded the axis in the evolution of 

 the shoot. His view is, in fact, a phytonic theory akin to that of Gaudichaud, but 

 stated in terms of embryonic development. He bases his argument largely upon 

 facts relating to the Ophioglossaceae, and particularly on the embryology of species 

 of Ophioglossum itself. But it is necessary to point out, as invalidating his argument, 

 first, that Ophioglossum is probably a highly specialised representative of its family. 

 This is indicated by its reticulate venation, and by many facts in its anatomy and 

 morphology. Secondly, in this family the dormant axillary buds observed by 

 Gwynne-Vaughan and Lang are themselves very minute, but none the less the 

 axial nature is recognisable in them, bearing leaves in case of their further develop- 

 ment. And generally, when the facts adduced by Campbell for special cases such 

 as Ophioglossum are placed against those from Pteriodophyte embryology as a 

 whole, it does not appear that his thesis of the priority of leaf over axis is sustained 

 by the actual facts. 



On the other hand, Tansley, in his Lectures on the Evolution of the Filicinean 

 Vascular System, has contemplated a megaphyllous origin of the leaf. He supported 

 his view by comparisons between the vascular anatomy of the adult leaf and that of 

 the axis in certain early fossils. Especially he traced his most convincing parallel 

 in the leaves of those Zygopterids which, with an upright habit of the leaf, bore 

 four vertical rows of pinnae. He remarks : * " This tendency to radial organisation of 

 the frond may perhaps be regarded as a relic of the time when, according to our 

 basal hypothesis, the structure of the fronds of Ferns was but little differentiated 

 from the structure of their stems." On a previous page t he had stated the theory 

 thus : "I am inclined to believe that the Fern-leaf is in phylogenetic origin a branch, 

 or rather a branch-system, of a primitive undifferentiated sporangium-bearing thallus, 

 and not an appendicular organ, differing ab initio from the axis on which it is borne." 

 Miss Bancroft \ finds anatomical support for the theory in Rachiopteris cylindrica, 

 and gives copious references to cognate literature. 



Tansley refers the modern statement of this theory in the first instance to 

 Potonie. § But in 1884 1 had myself stated substantially the same hypothesis 

 thus : || " May we not with good reason think that, just as the phyllopodium 

 gradually asserts itself as a supporting organ among structures originally of similar 

 origin and structure to itself, so also the stem may have gradually acquired its 

 characters by differentiation of itself as a supporting organ from other members 

 originally similar to itself in origin and development? Thus the stem and leaf 

 would have originated simultaneously by differentiation of a uniform branch-system 

 into members of two categories." Finding later that facts were , deficient for any 

 approach to a proof of this hypothesis, I left it, so to speak, in the air ; but now I 

 return to the consideration of it after more than thirty years. 



* L.c, pp. 23-24. f P. 1. + Ann. of Bot., 1915, p. 562. 



S Deutsche Bot. Monatsschrift, xv, 1897. || Phil. Tram., ii, p. 605. 



