198 ANATOMICAL EVIDENCE 



would be the more probable : if the leaf were initially a minor appendage, 

 then the former of the two explanations suggested by Mr. Tansley will 

 naturally follow. The facts are not decisive in indicating either of these 

 alternatives ; but a comparative study of the vascular relations of leaf to 

 axis in the earlier leafy sporophytes will throw a valuable side-light upon 

 the question. Dr. Jeffrey l has distinguished two main types of relation 

 between the vascular supply of the leaf and the vascular system of the 

 axis. The one type he styles "cladosiphonic" : it is characterised by the 

 insertion of the leaf-trace on the periphery of the axial stele, which is itself 

 hardly disturbed at the point of junction (compare Figs. 71 and 99). This 

 is clearly the anatomical expression of the dominance of the axis in the 

 shoot, for the leaf-trace is added as a mere appendage on the periphery 

 of the otherwise cauline stele. In this respect the structure is like that 

 described for some of the larger Mosses. This condition is characteristic 

 of the Lycopodiales, Equisetales, and Sphenophyllales ; all of them small- 

 leaved forms, and of early occurrence in the scale of vegetation : and 

 there is good reason to believe that it is a really primitive condition 

 in these early Pteridophytes. 



The other type recognised by Dr. Jeffrey is the "phyllosiphonic," which 

 is characterised by the profound disturbance of the vascular tissues of the 

 axis at the point where the leaf-supply is inserted : so much so that a 

 distinct leaf-gap is produced, and connection may be established at that 

 point between the central and peripheral tissues (compare Fig. 95). This 

 is the anatomical expression of the dominance of the leaf over the axis in 

 the shoot, and it is characteristic of certain large-leaved Pteridophytes, and 

 is also general in Seed-Plants. As regards the latter, it has already been 

 seen in Chapter XL that certain of the Gymnospermic Seed-Plants have 

 probably been derived, with progressive leaf-production, from a Filicinean 

 ancestry : their phyllosiphonic character supports this view, which is, how- 

 ever, based upon a wide area of comparison on other points besides. It 

 may be held as probable that the seed-bearing plants at large were developed 

 from a large-leaved ancestry, having undergone reduction of leaf-complexity 

 in the course of their evolution. But while we thus recognise a probability 

 of a widespread reduction producing relatively small-leaved forms, it does 

 not follow that all smaller-leaved forms originated thus : and the anatomical 

 and palaeontological facts together make it probable that such small-leaved 

 forms as the Lycopodiales, Equisetales, and Sphenophyllales were primitively 

 small-leaved. 



It has been remarked in Chapter XL that in the individual life, one 

 or the other anatomical character is usually constant : this is true for the 

 mature structure, but the transition from the cladosiphonic to the phyllo 

 M phonic may frequently be traced as the young plant of the latter type 

 passes to the mature state. It has been shown very clearly in the case of 

 Alsophila excelsa by Gwynne-Vaughan (I.e., p. 710) (Fig. 100): here the 



1 Mem. Boston Soc. of Nat. Hist., vol. v., No. 5, 1899. 



