436 NAIADACEdS. [POTAMOGETON. 



19. P. trichoi'des, Cham, and Sehl. ; stem capillary, leaves |-amplexi- 

 caul setaceous 1- (rarely 3-) nerved, stipules slender acute, peduncles 

 filiform much longer than the few-fld. spike, dry drupelets solitary, beak 

 short. 



Muddy ponds and ditches, Norfolk, E. Suffolk ; W. Ireland; fl. Aug.-Oct. 

 Stem repeatedly dichotornously and divaricatingly branched. Leaves 1-2| 

 in., spreading, acuminate, dark green, rigid. Peduncles curved, longer than 

 the leaves ; flowers 3-6, very minute, monogynous. Drupelets ^ in., obliquely 

 ovoid, compressed, dorsally more or less tubercled, ventrally toothed near 

 the base. DISTRIB. From Sweden southd. and eastd., N. Africa. 



SECTION 4. Leaves all similar, submerged, linear, margins flat in bud ; 

 stipules adnate with the leaf-base into a sheath. 



20. P. pectina'tus, L. ; stem filiform, densely distichously branched, 

 leaves very long linear or filiform acuminate. 1-3-ribbed, peduncles long 

 slender, flowers few remote, dry drupelets large turgid, beak short. 

 Fresh and brackish ditches, &c., from Orkney southd.; Ireland; Channel 



Islands ; fl. June- Aug. Forms dense masses. Hoot a small tuber. Leaves 

 3-8 by ^V~y m -> lower 5-nerved ; stipulary sheath often 1-1J in, tip free. 

 Peduncles not thickened upwards ; flowers interruptedly whorled. Drupe- 

 lets very large for the plant, in., dimidiate-obovoid, obscurely keeled 

 dorsally, ventrally slightly convex. DISTRIB. Europe (Arctic), N. Asia, 

 India, Africa, N. America, Australia. 



P. PECTINA'TUS proper; upper leaves 1-nerved, channelled bifistular, lower 

 flat 3-ribbed, lateral ribs of dry drupelets conspicuous. Common. P. 

 mari'nus, Huds., not L., is a var. with stems naked below. 



Sub-sp. P. FLABELLA'TUS, Bab.; upper leaves 1-3-nerved, lower flat 3-5-nerved, 

 lateral ribs of dry drupelets obscure. P. jimcifo'lius, Kerner. England. 

 Scotland (very rare), Ireland. 



21. P. filifor'mis, Noltc ; stem filiform, branches short, leaves capillary 

 1-nerved, peduncles longer than the leaves very slender, flowers in dis- 

 tant whorls, dry drupelets small, beak very short. 



Lakes and ditches, Shetland to Berwick, local ; Anglesea; Ireland, rare ; fl- 

 July-Sept. Stem branched below, simple above. Leaves all capillary 1- 

 nerved and channelled. Flowers in dense whorls. Drupelets scarcely 

 keeled, beak terminal. DISTRIB. Europe (Arctic), N. Asia, N. India, N. 

 America, Australia. 



4. RUP'PIA, L. 



Slender brackish-water herbs. Leaves alternate or subopposite, sub- 

 merged, filiform, with stipuliform sheaths. Flowers minute, 2-sexual, 

 proterandrous, usually 2 on opposite sides of a filiform rachis ; peduncle 

 ebracteate, terminal, but apparently lateral from being pushed aside by an 

 axillary shoot from the last leaf, elongate after flowering. Perianth 0. 

 Stamens 2, an upper and a lower, filaments short broad ; anthers attached 

 by the back ; cells reniform distant, dehiscence vertical ; pollen a curved 

 tube with 1 median and 2 terminal nuclei. Carpels 4, sessile ; stigmas 



