473 
the Monocotyledonous Leaf\ 
character of their leaves \ The stereotyped external form and the lack of 
lateral veins receive a ready explanation if the leaves of the Coniferae are 
interpreted as phyllodes, equivalent in some cases to petiole and leaf-base, 
and in others, possibly, to the leaf-base alone. If this view be accepted, the 
leaves of Cordaites and the Coniferae will be regarded as bearing the same 
relation to those of the Pteridosperms and Cycadophyta as the leaves of 
Monocotyledons bear to those of Dicotyledons. It is impossible here to 
enter upon a detailed discussion of the subject, but it may be suggested 
that the Gnetales offer a special case which is of interest from this stand¬ 
point. The leaf of Gnetum may be compared with that of a Dicotyledon, 
while the scale-leaf of Ephedra and the parallel veined leaf of Welwitschia , 
with its continued basal growth, would be regarded as petiolar phyllodes— 
or possibly as leaf-bases only—and thus morphologically comparable with 
the leaves of Monocotyledons. 
The chief general result of the application of the phyllode theory to the 
Gymnosperms, is that the Coniferae come to be regarded as microphylions 
by reduction , unlike the Lycopodiales, whose microphylly is probably a 
primary character of the group. 
II. The Bearing of Anatomical Evidence upon 
the Phyllode Theory. 
i. Introduction. 
(i) The nature of the evidence. 
The evidence hitherto brought forward for regarding the Mono¬ 
cotyledonous leaf as of a phyllode nature has, apparently, been based 
entirely upon external morphology. For some years the present writer 
unsuccessfully pondered the question whether it might not be possible to 
bring anatomical data to bear on the problem, but she was completely 
baffled until Solereder, 1 in 1913, reported the discovery of vascidar bundles 
of inverted orientation in the leaves of various Hydrocharitaceae. He com¬ 
pared the structure thus revealed to that of petioles, Acacia phyllodes, and 
various isobilateral equitant leaves, but he did not, apparently, attach any 
theoretical importance to it, or regard it as an indication of the true morpho¬ 
logical nature of the leaves of this Family. His results, however, gave the 
clue the present writer was seeking, and in their light she returned afresh 
to the problem of how anatomical evidence could be used to test the 
phyllode theory of the Monocotyledonous leaf. 
(ii) The anatomy of Dicotyledonous petioles , phyllodes , and scale-leaves. 
As a preliminary to a general survey of the leaves of Monocotyledons 
it is necessary to refer very briefly to the anatomical characters of Dicotyle- 
1 Solereder, H. (1913). 
