POTAMUC.ETOX.] NAIADES. 373 



SECTION 2. Leaves all opposite, margins involute in bud : stipules 0. 



9. P. den'sus, L. ; stem slender, leaves subopposite distichous ovate- 

 cordate 3-5-nerved, peduncles very short, spikes few-flowered. 



Ponds and streams, from the Forth and Clyde to Devon and Kent ; ascending 

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

 -Sept. Branches dichotomous. Leaces -1 in., close-set, recurved, acute or 

 acuminate, keeled, serrulate, translucent, apparently opposite, the nodes of 

 the pairs being confluent, but their bases overlap. .Peduncles rarely longer 

 than the leaves, laxly 3-6-flowered. Ih-upes (dry) oblique, almost orbicular, 

 compressed, sharply keeled, beak recurved. DISTRIB. Europe from Den- 

 mark southwards, N. Africa. 



SECTION 3. Leaves alternate, or the upper opposite, all similar, linear- 

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



10. P. compres'sus, L. ; stem compressed winged, leaves ^-amplexicaul 

 acuminate 3-5-nerved, stipules large acuminate, peduncles longer than the 

 spikes. P. zoster cefolius, Schum. ; P. cuspidatus, Sm. 



Lakes, ditches, &c. rare, from Forfar and Perth southwards ; Co. Down in Ire- 

 land; fl. July- Aug. ^ Stem -in. broad, internodes long, nodes constricted. 

 Leaves 2-8 in., J-^in. broad, dark green, with 3 strong and many slender 

 nerves. Spike slender, interruptedly many-flowered. Flowers small. Drupe 

 (dry) in., subreniform, compressed, obtusely keeled ; beak short, stout, re- 

 curved. DISTRIB. Europe, N. America. I follow Fries and Tuckerman in 

 referring 1 this to P. compressus, L., though not the plant of Linn. Herb. 



Sub-sp. COMPRES'SUS proper ; stem broad stout, leaves 4-8 in. often in. 

 broad, peduncles 2-3 in. many : flowered, drupe in. slightly gibbous at the 

 base. Perth, Forfar, Surrey, and Co. Down in Ireland. 



Sub-sp. ACUTIFO'LIUS, Link (sp.) ; smaller, more slender, less branched, 

 leaves 2-4 in. ^-in. broad more acute, peduncles and spike very short few, 

 flowered, drupe | in. more curved toothed towards the base, beak longer. 

 Norfolk and Lincoln to Surrey and Dorset, rare ; absent from Ireland. 



11. P. obtusifo litis, Mert. and Koch; stem slender compressed 4- 

 ironous much branched, leaves sessile 2-3-uerved subacute or obtuse, 

 stipules very obtuse, peduncles veiy short longer than the spike, drupe 

 (dry) shortly beaked. P. gramincus, Sm. not L. 



Ponds and ditches, rare, from Lanark, and Kincardine to Devon and Kent ; 

 local and rare in Ireland ; fl. July-Aug. Stem flexuous, not winged. Leaven 

 26 in., - in. broad, dark green, rather opaque, with no nerves between 

 the 3 principal ; stipules ^in., tip broad. Peduncles much shorter than the 

 leaves, terminal in the forks, equalling or exceeding the densely few-flowered 

 spike. Drupe (dry) compressed, broadly obliquely ovoid, keeled ; beak 

 straight. In a specimen from Chilsworth (Herb. Borrer) the lateral nerves 

 are absent or very obscure. DISTRIB. Europe, W. Siberia, N. America. 



12. P. pusillus, L. ; stem filiform slightly compressed, leaves ^-am- 

 plexicaul narrow linear i-3-nerred acute, stipules small acute, peduncles 

 terminal in the forks slender, spike few-flowered. 



Rivera, ponds, and ditches, ascending to 1,000 ft. in the Highlands ; fl. July- 

 Aug. .St -M often much branched. Leaves J-2in., ^ a -fa in. broad, rarely 

 acuminate, rather opaque green. Peduncles usually much longer than the 

 spike. Spikf. of 6-10 minute flowers. Drupe (dry) small, turgid, obliquely 



