144 Dr. F. 0. Bower [April 30, 



analvsis and their rootlessness arc in themselves evidence of the 

 primitive state of these fossils. We are, in fact, in the presence of 

 what evolutionists call Synthetic Types — that is, such as link together 

 groups which have diverged. The Lower Devonian plants and the 

 Psilotaceae show us just those forms which might have been antici- 

 pated as a consequence of comparative study, and some of their 

 characters were actually forecast by Dr. Treub. 



Though it may be difficult to place the parts of these synthetic 

 types in the categories of stem, leaf, and root, as those terms are 

 applied to more advanced forms, still they will serve to illuminate 

 the probable origin of these parts. The rhizomes of Asteroxylon 

 suggest an origin of roots from branched, leafless rhizomes. Its 

 " leaves " suggest a relation with the leaves of Lycopods ; but its 

 most significant feature is the branch-system ascribed to Asteroxylon, 

 bearing the distal sporangia, which is so like that already described 

 for the enigmatical Carboniferous fossil Stauropteris. This com- 

 parison has already been pointed out by Kidston and Lang. On the 

 other hand, approaching the question from the side of the living 

 ferns, I indicated in 1917 that "the distal and marginal position of 

 a sorus, often monangial, is prevalent among primitive ferns, and 

 that more complex sori are referable in origin to it." Comparison 

 of the distal sporangia of the Psilophy tales with those of Stauropteris, 

 Botryopteris, the Ophioglossaceae, Osmunda, and the Schizaaaceaa, 

 gives a sequence which sketches in broad lines, though not rnono- 

 phyletically, a probable origin of marginal sporangia for the ferns. 

 It is accompanied by reduction of size and spore-number in the 

 later and derivative types, which is continued on to the most advanced 

 of living ferns. A reduction of the distal branchlets to a single 

 plane, and webbing of them laterally together, would give a type of 

 sporophyll and fructification known in certain primitive ferns. But 

 if this were the real course of their evolution, the sporophyll so 

 constructed would be a different thing from the "leaves" seen in 

 Asteroxylon, This was the vision of the prophetic Lignier, who has 

 not lived to see his ideas tested by these new discoveries. But such 

 comparisons still leave in doubt the origin of the axis in fern-like 

 types. It is not clear yet how near the truth for them my suggestion 

 of 1884 may be: that "the stem and leaf would have originated 

 simultaneously by differentiation of a uniform branch-system into 

 members of two categories." Nevertheless, the important new fact, 

 which now gives reality to this theory, is that a uniform branch- 

 system has been shown to have existed in these early vascular plants. 

 A sympodial development of it, after the manner shown in the 

 leaves of living ferns, would provide at least one type of foliar 

 appendage, which would bear a relation to the axis similar to that of 

 the pinnae to the phyllopodium or rachis of the leaf.'"' 



* Phil. Trans. 18S4, p. 565. 



