EXTERNAL CHARACTERS 629 



In 1884 I formulated a theory of origin of the " phyllopodium," or rachis 

 of the leaf, chiefly based upon comparative study of the leaves of Ferns. 1 

 It was pointed out how in an enlarging dichotomous system a main rachis 

 asserts itself as a supporting organ among parts similar in origin and structure 

 to itself. This theory of genesis of the Fern-leaf may now be restated as 

 follows : the Fern-leaf was originally a limited structure of flattened form, 

 endowed with growth at its distal end : this growth was conducted with fan-like 

 segmentation, but it was apt to be localised at points which diverge dichoto- 

 mously. Sometimes the margin remained entire, and the fan-like expansion 

 is then traversed by dichotomising veins : it is not improbable that this is 

 really a secondary condition of condensation of a branch-system. More 

 commonly the margin grows out dichotomously, the veins following, and 

 a fan-like forking is produced such as is actually seen existent in many 

 Ferns. But frequently with the enlargement of the branch-system the 

 equality of the forking was not maintained : certain branches took the 

 lead, and a sympodial development resulted in a rachis being produced, 

 as a strong support for the rest, though primarily it was of similar origin 

 with them. It is but a slight modification which would establish the rachis 

 thus initiated as the definite "phyllopodium," upon which the earlier, and 

 sometimes also the later branches would arise monopodially, being lateral 

 from the first : and thus a Pecopterid-type of leaf would result. The facts 

 certainly indicate that such a transition has been effective in descent, though 

 it may be a question whether all pinnate types, for instance the Marattia- 

 ceae, originated in this way. Lastly, it may be noted that the winged 

 structure, so prevalent in Fern-leaves as lateral lines leading even to the 

 base, still indicates the ultimate origin from a flattened expansion : the 

 margins may often still be traced in this way even where the petiole is 

 almost cylindrical in form. 



A theory of the whole shoot based upon similar progressions was also 

 suggested in the same Memoir in 1884, viz. that just as the phyllopodium 

 gradually asserts itself as a supporting organ among structures 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 similar to itself in origin and development. A similar idea has been 

 subsequently expanded into Potonie's theory of origin of the Fern-shoot. 2 

 There seems to be no sufficient foundation in fact for its acceptance. In 

 the first place, there is in Ferns no known case where the axis and leaf 

 appear as the two branches of a dichotomy, so that the suggestion is purely 

 hypothetical : it is based only on analogy with what is learned from the 

 comparative study of the leaf. The strength of the argument referring 

 the branching of Fern-leaves to an original dichotomy rests on the fact 

 that that mode of branching commonly appears at the apex, and is specially 

 apparent in the lateral branchings. There are no such examples showing 



1 Phil. Trans., 1884, part ii., pp. 604-5. 



2 Palaeophytologie, pp. 156-159. 



