86 A. H. Graves, 
and in places the subepidermal layer is thick-walled and constructed 
much like the epidermis (Pl. IV, fig. 16). Asa whole, the structure 
shows greater strength than the peduncle. 
4. Brief Comparative Study of the Stem Anatomy of the Potamo- 
getonaceae. 
Zannichellia (Campbell, 1897) and Althenia (Prillieux, 1864) perhaps 
most closely resemble Ruppia in their cauline structure, although 
they possess no cortical bundless. Polamogeton pectinatus (Schenck, 
1886) also, which externally resembles Ruppia so closely, internally 
reveals a very similar stem anatomy. Moreover, the Potamogetons 
possess cortical bundles lacking in Zannichellia and Althenia. 
Cymodocea (Bornet, 1864), appears to differ mainly in having a 
thicker stem and a ring of small cortical bundles with lacunae 
between these and the axial vascular area. 
In its vascular system Zostera (Sauvageau, 1891. I.) shows close 
relationship to Ruppia. Z. capricorni and Z. nana have one cortical 
bundle on opposite sides of the stem, as in Ruppia (Sauvageau, 
1891). Zostera marina (Chrysler, 1907. p. 172, fig. 29) is also 
similar. In other species these bundles are more numerous. The 
bundles run free the whole length of the internode, unite with the 
central area at the node and send out branches which become the 
lateral nerves of the leaves. The central vascular area appears 
composed of four fibrovascular strands, the phloem parts isolated 
and separated, and the xylem bundles united into an axial strand. 
LEAF 
A. Enumeration of Kinds of Leaves 
Of Strasburger’s (1908, p. 26) four categories of leaves, three 
occur in Ruppia:—the scale leaves, small specialized structures at 
the base of every branch (PI. Il, fig. 6, vs/; Text-figs. 19-20, vs/, 
fsl); secondly, the ordinary long, narrow foliage leaves; and finally, 
the subfloral leaves, belonging properly to the category of bracts, 
a pair of which is borne at the base of the peduncle (Text-figs. 19-20, 
up. sfl, low. sfl). Since the flower is naked, there are no floral 
leaves. 
B. Arrangement 
An investigation of the position of the leaves throughout the 
plant reveals the following: 
