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! 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 XI. that certain of the Gymnospermic Seed-Plants have 
probably been derived, with progressive leaf-production, from a Filicinean 
ancestry: their phyllosiphoni@ 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 XI. 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- 
siphonic 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 (4c, p. 710) (Fig. 100): here the 
+ Mem. Boston Soc. of Nat. Hist., vol. v., No. 5, 1899. 
