376 95. POTAMOOETONACE^. 



the back, nut with a prominent keel. — Sy. B. B. 1421. Phytol. iv. 

 1158. P. zosteraems Bab. (not Fr.). — Rhizome spreading, from 

 a tuber that has outlived the winter. Floating st. branched, 

 spreading like a fan. Broad lower 1. usually decayed at the 

 time of flowering. Lateral veins of upper 1. at the margin, of 

 lower 1. distant from it. Spikes slightly interrupted. Back of 

 fr. without ridges, rounded when fresh ; enclosed nut with faint 

 lateral ridges. — Ponds and ditches near the sea. P. VI. VII. 



E. I. 



23. P. pectindtus (L.) ; I. formed of 2 interrupted tubes, lower 

 narrowly linear flattened slightly grooved above, upper setaceous, 

 fr. (2 lii. long) broadly J-obovate inner edge rather convex 

 rounded on the back, nut with 2 lateral ridges but (usually) no 

 Iceel. — E. B. 323. R. vii. 19. — St. branched, forming Hnear 

 masses. L. very gradually acute, all 1-veined, no marginal veins 

 and scarcely thickened there ; upper 1. with an oval section. 

 Spikes slightly interrupted. Back of fr. without ridges and 

 rounded when fresh ; nut with strong lateral ridges. Varying 

 greatly in length and size of st. and leaves. — P. marinus (Huds.) 

 has the stems naked below. Sp. E. B. 1424, — ^Ponds and 

 streams. P. VI. VH. E. S. I. 



24. P. jUifar'mis (Nolte !) ; i Unear^setaceous 1-veined with 

 transverse veins, spikes greeMy interi-upted, fr. (IJ lin.) obovate 

 rugose rounded on the back without keel or ridges when dry, nut 

 round-backed. — Sy. E. B. 1424. — L. like those of the preceding 

 but longer. Fr. smaller. Whorls very distant on the spikes. 

 Peduncles very long. — Rare. Scotland. Lough Cullen and 

 Lough Conn, Oo. Mayo, Ireland. P. VI. VIL S. 1, 



***** X. all opposite, submersed ; stipules none, 



25. P. den'sus (L.) ; I. all opposite pellucid clasping elliptic- 

 lanceolate or lanceolate, spikes shortly stalked ultimately re- 

 flexed. — E. B. 397. P. vii. t. 28. — L. crowded, rather recurved. 

 Spike 4-flowered. Sep. triangular. — Ditches. P. VI. VII. 



E. S. L 



2. Rtjp'pia Linn. Tassel-Pondweed. 



1. iJ. marit'ima (L.) ; common ped. long spirally twisted, anth.- 

 cells oblong Ij times as long as broad, nut ovoid rather ob- 

 liquely erect. — E. B. 136. P. vii. 17. B. spiralis Hartm. — 

 Whole plant stronger than the next. L. very narrowly linear, 

 formed as in Potamogeton pectinattis. Sheaths large, inflated. — 

 Salt marshes in the South. P. VII. VIII. E. I. 



2. P. rostelldta (Koch) ; ped. short not spiral, anth.-cells 

 sqiiarish as long as broad, nut gibbous at base obliquely ascend- 



