G. R. Wieland — Origin of Dicotyls. 453 



it is now the aim to bring together in the simplest possible 

 manner. 



The early dicotyl leaves of the Potomac, like Celastrophyllum 

 and Cissites, quite the oldest that are well known, yield no 

 clues of direct relationship to either cycads or gnetaleans. 

 They only show that the primitive dicotyledonous forms, what- 

 ever their flowers, soon became modern in type. Whence we 

 are forced to note more general facts. 



Ferns are frequently net-veined, and the magnificent meter 

 broad palmate and lyre-shaped net-veined fronds of the Dictyo- 

 phyllum and Camptopteris type flourished notably in the cli- 

 matic conditions of the Mesozoic ; although there is, with the 

 sole exception of the net-veined cycad Dictyozamites, a singu- 

 lar absence of gymnospermous types with net-veined leaves. 

 But that many such could have existed is sufficiently attested 

 by Dictyozamites, which occurs in India, England and Antarc- 

 tica ; while a most important chain of possibilities is suggested 

 by the pinnately net-veined leaf of Gnetum gnemon differing 

 in no essential from leathery dicotyl leaves (Coulter and Cham- 

 berlain), and by the huge Welwitschia blades with a venation 

 type similar in most respects to monocotyls, but also inter- 

 mediate between that of the mono- and dicotyls (De Bary). 

 Moreover, it must be a fundamentally significant fact that the 

 plants of the great Williamsonian alliance had early in the 

 Mesozoic developed a free branching habitus with elongated 

 leaf-bearing nodes, and numerous and rapidly dehiscent Cycad- 

 olepis scales or just plain bud scales. 



Though, so far as I know, no one has called direct attention 

 to the further simple quantitative fact of a virtual microphylly 

 in the Jurassic and in some Triassic cycadophytans. Even in 

 the early Mesozoic great numbers of the cycadophytans were 

 no longer megaphyllous and had as distinctly reduced frond 

 contours as any dicotyledonous types. Many Otozamites and 

 Pterophyllum leaves had, too, all through the Jura, elliptical, 

 ovate and obovate forms which, could the pinnules have grown 

 together and net venation arisen, would at once have produced 

 leaves in size and form like those of oaks, chestnuts, magnolias, 

 and much smaller-leafed plants. Indeed a leaf like the Indian 

 Pterophyllum fissum of Feistmantel (fig. 1), a small peduncu- 

 late obovate form which may have fallen within the same 

 family as Dictyozamites but also resembles Anomozamites, 

 must always have its venation well conserved to prevent being 

 mistaken for some lesser angiospermous form, like a small 

 Dryophyllum. Furthermore in the small Wielandiella leaf 

 the petiole expands basally, so that a second conceivable 

 method of derivation from Pterophyllum, blades would be by 



