103 
Hillii and P. obtusifolius, distinguishing by the 3 vascular 
areas in the central cylinder, 0-endodermis and subepider- 
mal bastbundles in the cortex. A transverse section of the 
stem shows that it is compressed and slightly furrowed at 
least in the one side. The shape of the leaf-apex remindes 
of P. rutilus almost more than of P. Hillii; the latter of 
which with the leaves more abruptly acute, often almost 
aristate and the lateral nerves nearer the margins than the 
midrib (Morong, A revision of the am. Naj. 1893. 38). P. 
orientalis has the main lateral nerves quite in the middle 
between the midrib and the margin. The broader leaves 
have moreover on each side a faint nerve outside of the 
main lateral one. 
7 — 8. Pot. orientalis Hagstr. 7. Top of a stem-leaf. 8. Top 
of a branch-leaf. (Both magnified). 
The species forms a natural group with P. pusillus, 
obtusifolius, Hillii etc., which all have flexible leaves and 
differ from P. rutilus, mucronatus etc with stiffer leaves 
and more prominent nerves. Specimens are seen from 
»Assam: Brahmaputra. 7. 1896, D:r King’s Collector» and 
from Japan »R. Tama, Musashi, 1888, T. Makino». The latter 
is by its collector determined as P. pusillus L. Possibly many 
Japanese »pusillus»-forms, if closely examined, might be P. 
orientalis. 
Potamogeton alpinus Balbis + perfoliatus L. ( P . prus- 
siens n. hybr.) 
Caulis 0,5—1 m altus simplex vel ramosus teres, inter- 
nodiis 6—15 cm longis; folia submersa elongata basi ± 
rotundato vel ± cordato-rotundato, apice semper rotundato, 
margine sparse denticulato; stipulæ intermediæ vel ad P. 
