470 Agnes Arber .— The Phyllode Theory of 
reduction of a normal leaf to a phyllode seems, at first sight, to have taken 
place in three well-known aquatic Dicotyledons— Littorella , Subularia , and 
Lobelia Dortmanna , L. But it must be remembered that the phyllode 
nature of these leaves is not at present proven. To consider their morpho¬ 
logical value would occupy space that cannot be spared in the present 
paper, but the writer hopes, later on, to deal with these leaves, as well as 
those of certain other Dicotyledons which are of interest from the stand¬ 
point of the phyllode theory. 
4. The * Lamina ’ in certain Monocotyledonous leaves. 
So far we have only considered Monocotyledonous leaves in which no 
lamina is differentiated, but a large number of Monocotyledons existing at 
the present day possess a distinct lamina, e. g. Sagittaria , Smilax , various 
Dioscoreaceae, Araceae, Pontederiaceae, Scitamineae, Palms, &c. How did 
this lamina arise, and what are its homologies ? On the present writer’s 
view of the phyllode theory, the leaf of the ancestral Monocotyledon con¬ 
sisted only of the leaf-base and petiole, and was entirely lacking in lamina. 
If the Monocotyledons are—as seems most probable—monophyletic, two 
explanations of the ‘lamina’ are open to us ; it must either be a revival of 
that organ as it occurs among the Dicotyledons, or an organ which has 
arisen de novo , as a modification of the apical part of the pre-existing 
phyllode, and thus not strictly homologous with the blade of a Dicotyledon. 
Professor Henslow, 1 who—without formulating the problem quite in this 
way—appears to accept the second of these alternatives, has propounded 
the theory that the so-called lamina of those Monocotyledonous leaves 
which possess a distinct stalk and blade is merely an expansion of the 
apical region of the petiole, and thus that the ‘ aerial reticulated leaf-blades 
of Monocotyledons are not identical , but only imitative of the fibro-vascular 
system of an ordinary dicotyledonous leaf ’. This interpretation certainly 
accords well with the venation of many Monocotyledonous leaves. The 
arrangement of the veins in Eichhornia speciosa , Kunth (Fig. 24, p. 489), for 
instance, looks decidedly as if the lamina had arisen through a spreading 
of the apex of the petiole. The transitional leaf forms produced in Sagit¬ 
taria between the band and arrow-shaped types have also all the appear¬ 
ance of merely representing different degrees of expansion of the upper 
region of the petiole with correspondingly varying degrees of outward 
curvature and apical detachment of the veins. This series affords an illustra¬ 
tion of the way in which the development of the e pseudo-lamina ’—as the 
present writer proposes to term the leaf-blade of the Monocotyledon—may 
conceivably have occurred in the course of phyletic history. 
One merit of Henslow’s theory is that it seems to contain the germ of 
an explanation of the curious fact that there is a certain general similarity 
1 Henslow, G. (1911). 
