MANTON: PTERIDOLOGY 309 



first World War, by the almost simultaneous discovery of both Psilophyton and 

 Arthrostigma, along with fragments of other plants, notably the putative bryo- 

 phyte sporogonites, in Lower Devonian rocks of Roragen in Norway (Halle, 

 1916) and by the report of the wonderfully preserved Rhynie fossils from Scot- 

 land (Kidston and Lang, 1917 et seq.). 



The successful study of the plants of the Lower Devonian and other early 

 floras is probably the most important contribution which the Pteridophyta have 

 made to botanical thought since Hofmeister. Some higlily instructive comments 

 on it may be quoted verbatim from Halle's 1916 paper. Under the heading 

 "General Botanical Conclusions" he says (p. 35) : 



The botanical interest presented by the oldest known land-floras, of which the Roragen 

 flora is one of the most typical representatives, is naturally connected with the question 

 of the relative antiquity of the different phyla of land-plants. The Pteridophyta stand 

 naturally in the foreground; and in regard to these the interest centres round the prob- 

 lem whether the microphyllous or the megaphyllous forms, the Lycopsida or the Pterop- 

 sida, are the more primitive. This is a question on which information may well be 

 expected to be gained from the fossils, provided the record goes sufficiently far back. It 

 is the general opinion that the Devonian floras are already too far advanced to throw 

 any light on this question. In the well developed floras of Kiltorkan, Bear Island, etc., 

 to which attention has usually been confined, both megaphyllous forms such as Archaeop- 

 teris and microphyllous forms such as Cyclostigma, occur as dominant elements. These 

 floras, however, belong to the Upper Devonian. The Lower Devonian floras, from reasons 

 mentioned in the Introduction, have mostly been neglected, although it would appear that 

 a critical review of the available material would lead to the recognition of some note- 

 worthy facts. In the following pages the evidence for the occurrence of respectively the 

 Lycopsida, Pteropsida and the Bryophyta are shortly discussed. 



We cannot unfortunately follow the whole discussion here. The essential 

 facts indicating the presence of microphyllous forms, and the apparent absence 

 of any fernlike or megaphyllous forms both from Roragen and from Gaspe, are 

 reviewed and then the author goes on to say : 



It might perhaps be suggested, although this is pure speculation, that megaphyllous 

 forms may be evolved from a type like Psilophyton Goldschmidtii. The lateral branches 

 of this form already appear to have a bilateral or dorsiventral symmetry. The rapid 

 tapering of the segments of isolated branch-systems similar to the lateral branches of 

 Psilophyton Goldschmidtii, suggests a limited growth in some cases. Such lateral branches 

 of limited growth may be imagined to develop laminae by a process of cladodification. A 

 similar development has been suggested by the late Professor Lignier (1903, 1908-11) in 

 his interesting speculations on the flrst evolution of the" different branches of the pteri- 

 dophytic stock. Lignier even used Psilophyton princeps as a starting point. He adopts 

 the view of a diphyletic origin of the leaves of the Pteridophyta, starting from a "pro- 

 hepatic type" derived from algae. The leaves of the Lycopodiales are distinguished as 

 "phylloids" and regarded as developed phylogenetically by "enation" in the manner of 

 emergences. The frond of the megaphyllous forms, on the other hand, are true leaves 

 formed by differentiation of thallus branches in accordance with current opinion. It 

 would seem that what little is known at present about the Lower Devonian flora is well 

 in accord with Lignier's views. We have in Psilophyton princeps, imperfectly though it is 

 known, a plant which has actually existed and which answers well to the type theoreti- 

 cally required as a starting point. Similar plants, with well developed stems and small 

 lateral appendages which may be compared either to emergences or leaves, were dominant 

 in the Lower Devonian flora; and there is reason to regard them as primitive. The 

 geological record available at present indicates that they existed before the fern-type 

 with large fronds, as exemplified by the Upper Devonian Archaeopteris. On the other 

 side there is Psilophyton Goldschmidtii, which is probably closely related to Psilophyton 



