332 
Arber . — On the Nature of the ( Blade ’ m 
described as passing through the leaf-sheath ; the section next below showed 
fusion between leaf and axis, while that next above revealed an early stage 
in the detachment of the stipula adnata — a detachment whose plane is 
already indicated in Fig. i A. The section drawn in Fig. i B passes through 
the petiole (pet.) and shows the stipule (s.a.) as a free, sheathing structure, 
open on the side remote from the petiole. Fig. i c shows the initiation of 
the 1 lamina ’ by the development of wing-like marginal outgrowths, which 
become coiled as they elongate. In Fig. i D the definitive form of the 
limb is reached. For comparison, I have cut serial sections through the 
apical buds of a second species of Potamogeton (unidentified) ; it was of 
a somewhat different vegetative type from P. natans , as it had broad 
submerged leaves, but I found that its leaf development proceeded on 
essentially the same lines as in that species. 
HELOBIEAE — Aponogetonaceae. 
Aponogeton. 
The developing leaves of the apical bud of Aponogeton distachyum , 
Thunb., have at the base a conspicuous sheathing region (Fig. 3 A), which 
is succeeded by a petiole, more or less triangular in section (Fig. 3 B). 
The ‘ lamina ’ is formed by outgrowths from the lateral margins of the 
ventral surface of the petiole (Fig. 3 c), which become coiled as they 
elongate (Fig. 3 D). Towards the tip, the blade, as is so often the case in 
Monocotyledonous leaves, loses its dorsiventral character, and at the extreme 
distal end it enters on a solid cylindrical phase (Figs. 3 E, and F). 1 
Helobieae — Alismaceae. 
Sagittaria. 
In the case of Sagittaria sagittifolia , L., I have studied serial sections 
through buds consisting of leaves of the mature ‘ arrow-head 5 type. In 
Figs. 2 A-G, drawn from a single leaf belonging to such a bud, only the 
three main vascular strands are indicated — m.b ., the median bundle, and 
l. x and l', lt the main laterals. Fig. 2 A shows the sheathing structure at the 
leaf-base, which passes up into a triangular petiole (Fig. 2 b). Fig. 2 C is 
from a section traversing the distal region of the stalk ; the tips of the 
auricles (a. and a!) with their bundles (/. 2 and l'. 2 ) are cut on either side. 
Fig. 2 D shows the connexion of the auricles with the petiole. In Fig. 2 E 
the origin of the vascular supply of the auricles can be traced ; the lateral 
bundles (/ n and !.\) bifurcate — the outer half in each case being destined 
to pass down into an auricle. This section also indicates how the main 
part of the ‘ blade ’ comes into existence by the development of lateral 
wings from the petiole. At a slightly higher level there is a fusion between 
the median bundle arid the laterals (Fig. 2 f) ; the general arrangement of 
the developing veins is perhaps not inconsistent with the morphological 
] Arber, A. (1922 2 ). 
