86 A. H. Graves, 



and in places the subepidermal layer is thick-walled and constructed 

 much like the epidermis (PI. IV, fig. 16). As a 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. Potauiogeton pedinatus (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 (PL II, fig. 6, vsl\ Text-figs. 19—20, vsl, 

 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: 



