78 A. H. Graves, 



(3) Eiidodermis. • 



The endodermis is a fairly regular layer of cells surrounding the cen- 

 tral vascular region (PL 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 

 and other species. Although a 

 Figiire 4. — Cross section of portion banded appearance is produced in 

 of endodermis and next outer ring ^ slightly obHque view, a longi- 

 01 cells treated witli concentrated j • i • , • - 



sulphnric acid, showing slightly tudmal section shows that it is 

 thickened radial walls, x 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 remarkabty 

 simple vascular system. This comprises two small cortical bundles 

 and a larger central bundle (PI. I, fig. 4; PL III, 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). 



