330 Arber . — On the Nether e of the 4 Blade ’ m 
I. Introduction. 
I N the course of the last four years I have discussed, in a sequence of 
papers in this and other journals, 1 the interpretation of the Monocotyle- 
donous leaf in terms of the phyllode theory. In certain cases this inter- 
pretation presents little difficulty. There is nothing strained, for instance, 
in the view that such a leaf as that of Triglochin maritimum is a petiolar 
phyllode ; its sheathing base is separated by a ligule from a limb, which is 
more or less cylindrical in form with a slightly flattened ventral surface — 
both in its appearance and in its ‘ radial * anatomy, this limb is distinctly 
petiolar. But the extension of the phyllode theory to those Monocotyledons 
which have a leaf-blade showing a general resemblance to the lamina of 
a Dicotyledon is attended with more difficulty. To many minds there 
seems, a priori , to be an element of improbability in the view, first 
suggested by Henslow, 2 that these blades are not true laminae, but are 
elaborations of the distal region of the petiole. In previous communications 
I have considered the nature of the leaf-blade in certain special cases, but 
I propose now to take a wider survey, and to submit the subject to a more 
rigorous analysis, paying special attention to the comparison between the 
ontogeny of Monocotyledonous and Dicotyledonous leaf- blades. The 
observations in the present paper relate to the ‘ lamina ’ as it appears in 
the leaves of examples from among the Helobieae, Principes, Synanthae, 
Spathiflorae, and Liliiflorae, and also of certain Dicotyledons selected for 
study because of their apparent resemblance to some of the Monocotyledons 
in question. In the case of the Glumiflorae I am contenting myself with 
a passing reference, since I hope to discuss the leaves of the Gramineae 
and Cyperaceac in a later paper. 
I am indebted for material to the Director of the Royal Botanic 
Gardens, Kew ; to the Director and to the Superintendent of the Cambridge 
Botanic Garden ; to Dr. H. E. Durham and to Mr. J. H. Maiden, F.R.S., 
of Sidney. Two of the illustrations in the present paper are drawn from 
preparations made by the late Miss Ethel Sargant. 
II. Observations on the Origin of the ‘Lamina’ in the 
Leaf of certain Monocotyledons. 
H elobieae — Potamogetonaceae. 
Potamogeton. 
Sections of the apical buds of shoots of Potamogeton natans , L., bearing 
leaves of the floating type, show that these leaves in their young stages 
have at their base an extremely short sheathing region. In the case of the 
leaf illustrated in Figs, i A-D, the section represented in Fig. i A was the 
only one, in a series cut to a thickness of 14 pi, which could accurately be 
1 Arber, A : (1918), (1919*), (1920 1 ), (1920 2 ), (1920 3 ), (1921 1 ), (1921 2 ), (1921 3 ), (1922 1 ), (1922 2 ). 
2 See reference in Arber, A. (1918), p. 470. 
