434 NAIADACEJE, [Potamogeton. 



Rivers near Hereford ; fL July. — Habit between P. pralon'yus and rufes'cevs, 

 sparingly branched. Leaves 4-6 in., 3-nerved with many secondary nerves. 

 Peduncles 2-4 in. ; spike f-l| in. Drupelets not seen in British examples. 

 — Distrib. Sweden, Lithuania. 



12. P. perfolia'tus, L. ; stem stout terete, leaves amplexicaul ovate- 

 cordate obtuse or subacute entire 5-9-nerved tip flat, stipules small 

 subacute caducous, peduncles short stout, spike dense-fid., dry drupelets 

 hardly keeled, beak short. 



Ponds, lakes, and streams, N. to Shetland ; ascends to 1,500 ft. in Wales ; fl. 

 June-Sept. — Branches dichotomous. Leaves \-± in., upper opposite, trans- 

 lucent. Peduncles terminal and in the forks, not thickened upwards. 

 Perianth-segments long-clawed. Drupelets x \ in., compressed. — Distrib. 

 Europe, N. Asia, N.AV*. India, W. Africa, America, Australia. — Forms 

 approaching var. lanceola'tus, Robbing (A. Gray, Man. Bot. N. U. States), 

 are found in Scotland. — Var. Jackso'ni, Lees, from Yorkshire. 



13. P. cris'pus, L. ; stem slender compressed, leaves distichous 

 ^-amplexicaul oblong acute or obtuse crisped serrulate 3-nerved, stipules 

 small obtuse caducous, peduncles curved tapering upwards, spikes few-fid., 

 dry drupelets acuminate, beak long. 



Ponds, ditches, &c, from Orkney southd.; Ireland; Channel Islands; fl. 

 July-Aug. — Dichotomously branched. Leaves H-3 in., close-set, spreading 

 and recurved, margins rarely flat (P. serratus, Huds.). Peduncles stout or 

 slender ; flowers 6-8, lax, very small. Drupelets \ in., obliquely ovoid, 

 compressed.— Distrib. Europe, N. Asia, India, Japan, N. America, 

 Australia. 



Section 2. Leaves all subopposite, margins involute in bud ; sti- 

 pules 0. 



14. P, den'sus, L. ; stem slender brittle, leaves subopposite distichous 

 ovate-cordate serrulate translucent 3-5-nerved, stipules except on the 

 upper pair of leaves, peduncles very short, spikes few-fid., dry drupelets 

 suborbicular sharply keeled, beak short recurved. 



Ponds and streams, from the Forth of Clyde to Somerset and Kent ; ascends 

 to 1,000 ft. in the Lake District; very local in Scotland and Ireland; fl. 

 July-Sept. — Dichotomously branched. Leaves |-1 in., close-set, recurved, 

 acute or acuminate, keeled, the nodes of the subopposite pairs being confluent, 

 but their bases overlap. Peduncles rarely longer than the leaves, at length 

 recurved ; spike laxly 3-6-fld. Drupelets compressed, pericarp thin ; cotyle- 

 donary end of embryo circumvolute. — Distrib. From Denmark southd., 

 Himalaya, N. Africa, America. 



Section 3. Leaves alternate or the upper opposite, all similar, ligulate 

 (uot broader in the middle), margins flat in bud ; stipules free. 



15. P. zosterifo'lius, Schum. ; stem compressed winged, leaves |-am- 

 plexicaul linear abruptly acuminate 3- rarely 5-nerved, stipules large acu- 

 minate, peduncle much longer than the spike, drupelets subreniform 

 3 -ribbed. P. cuspidatus, Sm. ; ? P. ccmpres'sics, L. 



