POTAMOGETON.] NAIADAOE^. 431 



Mr. A. Bennett, F.L.S., of Croydon,has given me the benefit of his unrivalled 

 knowledge of this difficult genus by revising my MS. for this edition. 



SECTION 1. Leaves alternate, or the upper opposite, oblong obovate or 

 lanceolate, not truly linear, margins involute in bud ; stipules free. 



* Flowering-stem without barren branches below ; upper leaves oblong floating, 

 lower (rarely all} submerged ; peduncles axillary, many- and dense-jld. 



1. P. na'tans, L. ; leaves long-petioled, submerged or reduced to 

 phyllodes, floating elliptic to lanceolate coriaceous, stipules very long 

 acuminate, peduncle stout, spike dense-fld., dry drupelets large keeled 

 dorsally, beak short. 



Lakes and ponds, N. to Shetland ; Ireland ; Channel Islands ; fl. June-Sept. 

 Stem terete. Floating leaves 2-6 in., mostly alternate, blade very shortly 

 decurrent on the petiole, with the margins minutely incurved or auricled ; 

 submerged 6-12 in., with very rarely a limb. Drupelets ^ in., ventral margin 

 convex, dorsal ^-circular. DISTRIB. Europe, N. Asia, India, Africa, N. 

 America, Australia. P.polygonifo'lius, var. linea'ris, Syme, with submerged 

 leaves 12-16 by |-| in., from Killarney, is probably referable here, but the 

 fruit is unknown. 



2. P. polygonifo'lius, Pourr. ; leaves long-petioled, submerged 

 narrowly lanceolate, phyllodes 0, floating obovate to narrowly lanceolate 

 rather membranous, peduncles and spike slender, dry drupelets very small 

 not keeled, beak very short. P. oblon'gus, Viv. 



Lakes and pools on heaths, &c., N. to Shetland ; ascends to 1,600 ft. in the 

 Lake District ; Ireland, rare ; fl. June-Sept. More membranous than P. 

 na'tans, with well-developed submerged leaves, blade often very decurrent 

 on the petiole, base acute rounded or cordate ; upper opposite. Drupelets 

 r T x o m - red, margins rounded. DISTRIB. Europe, Asia, Canada ?, N. Zealand. 

 VAR. flu'itans, Syme, is a deep-water form with very long submerged 

 leaves and subcoriaceous floating ones. 



3. P. plantagineus, Du Croz ; leaves long-petioled chiefly submerged 

 all membranous translucent from orbicular and cordate to elliptic-lanceolate 

 upper opposite, stipules short broad obtuse, peduncle very slender, spike 

 dense-fld., dry drupelets very small rounded not keeled green, beak 

 obsolete. 



Ditches, fens, &c., from the Clyde and Haddington to Somerset and Kent 

 Ireland ; Channel Islands ; fl. June-Sept. Leaves broader, more mem- 

 branous and translucent with more slender petioles than P. polygonifo'lius 

 and na'tansj and with very different stipules. Drupelets ^ in. long. 



4. P. mfes'cens, Schrad. ; leaves short-petioled translucent, lower 

 submerged linear-lanceolate many-nerved, upper floating or erect broader 

 subcoriaceous, petiole short, stipules large, peduncle stout, spike stout, 

 dry drupelets ovoid acuminate red. P. flu'itans, Sm., not Schrad. 



Ponds, canals, and ditches, from Caithness southd. ; ascends to 3,000 ft. in 

 Perthshire; Ireland; fl. July-Sept. Stem 1-4 ft., terete. Leaves reddish, 



