130 Scientific Proceedings, Royal Dublin Society. 



compact pinnules would be less injured, and plants possessing tlieni would 

 survive. With tlie transition of a whorl of two to one of three or more fertile 

 segments a sort of strobilus suggestive of Sphenophyllum could arise. The 

 frond of Archseopteris is imparipinnate, and this explains the fact which 

 Nathorst notes in the case of A. Boemeriana, that the main raohis of the 

 frond in all species of Archseopteris is occupied at its distal end by pinnules 

 in pairs like the decursive, rachidial, or aphlebioid pinnules. If the leaves 

 of A.fissiKs were not opposite, but arranged in whorls of tliree or six, they 

 would very much resemble those of Splienophijllum tenervimimi, the earliest 

 of the Sphenophyllums. 



Again, if the apical pinnules, making the frond of Archseopteris impari- 

 pinnate, were fertile — and there is no reason why this should not have 

 occurred occasionally — the Sphenophyllum condition would be realizable, 

 though in Sphenophyllum the strobilus is formed of an axis with whorled 

 sporophylls ; and in Archseopteris it is the frond only with wliieh the com- 

 parison can be made. Tlie lobed sporophyllule just described materially 

 supports Lignier in his attempts (25) to demonstrate the possibility of the 

 Filicinean origin of the Sphenophyllales. The change from the condition of 

 things in Archseopteris to that of a strobilus, as seen in Sphenophyllum, is 

 hardly more striking than that seen in living Cycadacese (e.g. in Cycas and 

 Zamia). In Cycas the pinnate leaves, spirally arranged, show pinnules con- 

 verted into ovules — a condition (thougli with intermediate connections) quite 

 different from that in Zamia, where the ovules are arranged in a cone or 

 strobilus on wliorled peltate sporophylls. 



One of the most fascinating features of the study of the fossil plants is 

 the light cast on the line of descent of groups of plants, and on their inter- 

 relationships. In the case of the vascular Cryptogams or Pteridophyta we 

 have to account for the origin of such main groups as the Botryopteridese, 

 the existing Filicinese (Leptosporangiate and Busporangiate), the extinct 

 Sphenophyllacese, the Equisetacese, tlie Psilotacese and tlie Lycopodiales, and 

 also for the extinct Cordaitacese, as well as for the newly-formed group of 

 seed-bearing plants or Pteridosperms. 



Many attempts have been made to give body to an imagined primitive 

 group of land-plants from which the groups just mentioned might arise in 

 the course of time. The rocks have been eagerly searched — so far in vain — 

 for such an originating type. Several terms for such a group have been 

 suggested. The term should not, by implication, convey the idea of the 

 acceptance by its adoption of the view of a definite source of origin. The 

 topsy-turveydom of the classification of Palseozoic plants, caused by the 

 discovery of the Pteridosperms, should warn one, in the present state of 



