78 A. H. Graves, 
(3) Endodermis. 
The endodermis is a fairly regular layer of cells surrounding the cen- 
tral vascular region (PI.I, fig.3). On the application of concentrated 
sulphuric acid all the walls of the endodermal cells become beauti- 
fully undulated, the radial walls showing a stronger suberization. In 
many cases the ring of cells outside of the endodermis is also quite 
strongly suberized and therefore withstands to a considerable extent 
the action of the sulphuric acid 
(Text-fig. 4). No U-shaped stripes 
of thickening appear on the walls, 
such as were seen by Schenck 
(1886) in Potamogeton pectinatus 

Figure 4.—Cross section of portion 
of endodermis and next outer ring 
of cells treated with concentrated 
sulphuric acid, showing slightly 
and other species. Although a 
banded appearance is produced in 
a slightly oblique view, a longi- 
tudinal section shows that it is 
thickened radial walls. >< 400. due merely to the slightly undu- 
lating walls, essentially as has 
been shown in the cortical cells. The radial walls, however, are 
slightly thickened uniformly. 
(4) Vascular System. 
One of the most interesting features of the stem is its remarkably 
simple vascular system. This comprises two small cortical bundles 
and a larger central bundle (PI. I, fig. 4; Pl. Ill, fig. 7). I shall 
describe first the course of these bundles, and then turn to a de- 
scription of their anatomical structure. 
(a) Course of Vascular Bundles. 
The course taken by the vascular bundles in the stem is very 
simple. The main central bundle travels in the center of the stem, 
and at each node sends off two branches in the manner figured 
(Pl. V, fig. 17)—a large branch to the lateral member which almost 
invariably occurs at each node, and a smaller contribution to its 
subtending leaf. This is repeated at each succeeding node until the 
floral region is reached. Here the main bundle continues to its 
termination in the rhachis, but gives off on each side a couple of 
branches to the lower and upper subfloral leaves with their axillary 
members in turn. This is simply a repetition of the process occurring 
at then odes, except that here, between the lower and upper subfloral 
leaves, the node is practically obliterated (Pl. V, fig. 18). This 
arrangement, at least in the vegetative region, is much like that of 
Potamogeton pectinatus, described and figured by De Bary (1884, 
p. 273). 
