49 i 
the Monocotyledonous Leaf. 
which the transition from petiole to ‘ lamina * takes place quite gradually— 
but also near the margin. Here there is only a single series of vascular 
strands, among which inversely orientated bundles are very numerous. 
Some of the lateral veins in the ‘ lamina ’ consist of a single, normally orien¬ 
tated bundle, while others consist of a pair of bundles, one normal and one 
inverted (Fig. 25, p. 490). 
Among the Pontederiaceae, we not only find leaves, such as those just 
described, in which there is a differentiation between petiole and ‘ lamina ’, 
but others, which are ribbon-like, with no distinction of blade and stalk. 
For comparison with the more highly differentiated types, sections were 
cut of the ribbon-leaf of Heter anther a zosteraefolia , Mart. Here the midrib 
and main laterals proved to be normal, but the others—i. e. the majority of 
the laterals—were inverted. Fig. 30, p. 490, shows two adjacent bundles 
orientated in opposite ways. The structure of this ribbon-leaf is closely 
similar to that of the ‘ lamina 5 in H. reniformis. 
It may be worth noting that a peculiar submerged member of this 
Family, Hydrothrix Gardneri , Hook, f., described by Goebel, 1 has, on its 
long shoots, leaves with a sheathing base and hair-like upper region, whose 
external morphology distinctly suggests a phyllodic origin. Anatomical 
evidence cannot be looked for here, since the extremely slender leaves are 
said to be traversed by a single bundle only. 
The presence of inverted bundles in four species of Pontederiaceae 
representing three genera—in fact, in all the species of which material has 
been available to the present writer—is a remarkable anomaly which calls 
for some explanation. It is difficult to see how such a structural peculiarity 
can be explained as*an adaptation, since it is common to leaves otherwise 
differing notably in type and mode of life. It is equally conspicuous in the 
very delicate ribbon-leaf of Heteranthera zosteraefolia and in the well- 
defined, thick ‘lamina’ of Eichhornia speciosa\ it occurs both in Heteran¬ 
thera reniformiS) in which palisade parenchyma is confined to the upper 
side, and in Pontederia cordata , in which this tissue is developed towards 
both surfaces. In the present writer’s opinion, this anatomical anomaly is 
best interpreted on the view that the ‘ laminae ’ of the Pontederiaceae, 
instead of being homologous with the laminae of Dicotyledons, are merely the 
expanded apices of pre-existing phyllodes: the inverted bundles are thus 
an indication of the petiolar nature of the organ, and are regarded as an 
ancestral feature rather than as an adaptation. 
But even if the probability of the truth of the phyllode interpretation 
be admitted for the ‘ lamina ’ of the Pontederiaceae, botanists may prefer 
to regard this group as possessing a unique leaf structure from which no 
conclusions can be drawn regarding other Monocotyledons. The Ponte¬ 
deriaceae are not, however, the only Family in which we meet with 
1 Goebel, K. (1913). 
K k 
