1921] ARBER—LEAVES OF HELOBIEAE 35 
type is the leaf with a sheathing base and more or less radial limb 
with phyllodic anatomy. This leaf form is found in Cymodocea 
isoetifolia Aschers. (fig. 11), described by SAUVAGEAU (16) and 
OSTENFELD (14). A ring of bundles surrounds the median strand. 
The same genus also includes leaves in which the limb is flat and 
furnished with only one series of bundles (as C. nodosa Aschers., 
fig. 12), while C. manatorum Aschers. (fig. 10), with its more or less 
terete leaf, traversed by three strands only, forms an intermediate 
type. In each species there is a ligule, clearly delimiting the 
sheath from the petiolar region. 
In Potamogeton the leaves of some species, like the air leaves of 
many Alismaceae, possess pseudo-laminae. In P. natans L. there 
are, in addition, bladeless, terete, phyllodic leaves, corresponding 
exactly in structure and anatomy to the petioles of the fully 
developed ‘leaves (16), and which may be regarded as equivalent to 
the leaves of Cymodocea isoetifolia. P. natans also has leaves 
showing a further degree of reduction, but instead of being ribbon- 
like, as in the Alismaceae, they consist almost entirely of the highly 
developed axillary stipules or ligules. 
Naiadaceae 
The leaves of the Naiadaceae are much reduced, but they have 
a sheath and a thin flat limb, and thus correspond to the ribbon 
leaves of Sagittaria. 
Aponogetonaceae 
Only the one genus, A ponogeton, is included in the Aponogetona- 
ceae. Most of the species have leaves with a differentiated sheath, 
petiole, and pseudo-lamina, but A. vallisnerioides Bak. has ligulate 
ribbon leaves, described (7) as resembling those of Vallisneria, while 
in A. spathaceum E. Mey. var. junceum Hook. f. (12) semiterete 
leaves with a sheathing base are found. A piece of a leaf of this 
variety which was examined did not include the distal part of the 
leaf, but showed the structure of the transition region between 
sheath and limb. In addition to the three main bundles, which 
HOoKeEr indicates in a slightly magnified transverse section which 
he figures, there are a number of small peripheral bundles. The 
Structure is thus closely equivalent to that of the petiole of such a 
species as A ponogeton distachyum Thunb. 
