412 A. H. Graves, 
d. Vascular System. 
This is represented by a single vascular bundle in the center of 
the root, with a structure similar to that of the stem (Pl. VI, fig. 27; 
Pl. VII, figs. 46, 47). No longer a typical radial root bundle, its 
present concentric structure admits of the same interpretation as 
regards adaptation as does the simplified bundle of the stem (see 
pp. 81-82). It differs from the stem bundle in only a few minor 
details. Barely as large as that of the stem, in all cases observed 
the root bundle retains intact the axial vascular area composed of 
a few tracheae. These tracheae are peculiar in that they possess 
no thickenings of any sort. Their walls, although extremely thin, 
can be distinctly seen in cross and longitudinal sections (Pl. VIII, 
figs. 46, 47, tra). Schenck has observed a similar peculiarity in 
the root of Potamogeton densus. He says (1886, p. 61), “Die Ge- 
faf}e aber erfahren keine deutliche Differenzierung der Wandung ; 
auf Langsschnitten sieht man keine Verdickungen oder nur schwache 
Spuren, so daf die Gefafse als Gange erscheinen. Uberhaupt er- 
fahren in den Wurzeln der monocotylen submersen Gewidchse die 
Gefafse, obwohl sie iiberall sofort zu erkennen sind, keine weit- 
gehende Ausbildung. Die Resorption der Querwdnde tritt meist 
ein, bevor Verdickungen angelegt werden.” 
Five or six sieve tubes, with their companion cells, are situated 
at more or less regular intervals just inside the endodermis, forming 
an irregular ring. These can be distinguished in cross-section by 
their generally larger, nearly empty cell-cavities, their position 
just inside the endodermis, and their accompanying companion 
cells. In longitudinal section the sieve plates may be seen, as also 
the accompanying companion cells with their protoplasmic contents 
(Pl. VIII, figs. 46, 47 s). 
The remaining tissue of the bundle is parenchymatous, and on 
account of its disposition in all parts of the bundle, it is impossible 
to distinguish phloem and xylem parenchyma. It may therefore be 
termed “connecting tissue” in accordance with Van Tieghem’s (1870—71) 
“tissu conjonctif” or Schenck’s “ Verbindungsgewebe” (1886, p. 59). 
e. Brief Comparative Study of Roots of Other Potamogetonaceae. 
In general, Potamogeton pectinatus and Zannichelha palustris agree 
with Ruppia in the vascular structure of their roots. The main 
difference is the presence of only one central trachea, which is 
considerably enlarged (Schenck, 1886, p. 61, figs. 80, 81). I find 
no reference in the literature to the cortex or epidermis of these 
plants, but see no reason why it should differ to any extent 
from that of Ruppia. Very probably also other slender sub- 
