434 NAIADACEJE. [POTAMOGETON. 



Rivers near Hereford ; fl. July. Habit between P. pralon'yus and rufes'cens, 

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

 Peduncles 2-4 in. ; spike -l|? in. Dru'peltts not seen in British examples. 

 DISTEJB. 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 1-4 in., upper opposite, trans- 

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

 Perianth-segments long-clawed. Drupelets T T g in., compressed. DISTRIB. 

 Europe, N. Asia, N.W. India, W. Africa, America, Australia. Forms 

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

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



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

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

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

 dry drupelets acuminate, beak long. 



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

 July- Aug. Dichotomously branched. Leaves l|-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-fld., 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 

 (not broader in the middle), margins flat in bud ; stipules free. 



15. P. zosterifo'lms, Schum. ; stem compressed winged, leaves J-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. ; 1 P. ccmpres'sics, L. 



