the Monocotyledonous Leaf . 
theory 5 (though the present writer is not aware that 
it has ever been so used) is that of Phormium 
tenax , Forst. The leaf of this plant is vertically 
flattened for a short distance, but opens out above 
into an expanded portion. It is possible, however, 
that we have here an instance of an isobilateral leaf 
which has developed a ‘ pseudo-lamina 5 at its apex. 
It is perhaps significant that, though most of the 
other Hemerocallineae have no lamina, one genus, 
Hosta ( Funkia ), is characterized by possessing a blade 
whose venation distinctly suggests that it may have 
been developed by an expansion of the upper part 
of the petiole (Fig. 22). 
5. Cases of ‘ phyllodic ’ anatomy which occur among Dicotyledons 
and the question of adaptive interpretations. 
The anatomical argument in the present paper is based upon the 
assumption that the occurrence of inverted bundles, which characterizes 
the leaves of a large number of Monocotyledons, is a feature which is 
normally absent from the laminae of Dicotyledons (excluding the principal 
ribs). If it were a common characteristic of both Dicotyledons and Mono¬ 
cotyledons, the contentions here advanced would fall to the ground. It is, 
of course, well known that the vast majority of Dicotyledonous laminae 
exhibit, in transverse section, only a single series of bundles. But it might 
possibly be argued that extra, inverted bundles are not an ancestral feature, 
as here maintained, but represent a structural adaptation, and might thus be 
expected to occur in Dicotyledons as well as Monocotyledons if the thick 
or succulent nature of the leaf rendered such a development advantageous. 
It is not possible here to treat this question at length, but it may be pointed 
out that many succulent leaves among the Dicotyledons, and some among 
Monocotyledons, are furnished with normally orientated bundles only, so 
that there is obviously no inevitable connexion between succulence and 
inverted bundles. Certain Dicotyledonous leaves, on the other hand, in 
which the anatomy is more or less radial and a peripheral series of bundles 
occurs, may possibly be interpreted either as laminae reduced to the midrib 
alone, or as true petiolar phyllodes. 1 There is no difficulty in supposing 
that phyllodic leaf-structure may have arisen more than once in the 
phyletic history of the Angiosperms—though never with such far-reaching 
consequences as when it appeared in the ancestral Monocotyledon. 
1 The writer hopes to consider certain of these cases in a later paper, as well as the various types 
of reduced leaf found in the perianth of the Angiosperms. 
487 
grandijlora, Sieb. and 
Zucc. Upper part of 
petiole and ‘ lamina ’ to 
show venation (reduced). 
