436 NAIADACEsE. [Potamogeton. 



19. P. trichoi'des, Cham, and Sehl. ; stem capillary, leaves J-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 dichotomously 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 J s 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. Root a small tuber. Leaves 

 3-8 by ^g-£ in., lower 5-nerved ; stipulary sheath often 1-1| in 1 , tip free. 

 Peduncles not thickened upwards ; flowers interruptedly whorled. Drupe- 

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

 dorsally, veutrally slightly convex.— Distrib. Europe (Arctic), N. Asia, 

 India, Africa, N. America, Australia. 



P. pectina'tcs 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'tcs, Bab. ; upper leaves 1-3-nerved, lower flat 3-5-nerved, 

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

 Scotland (very rare), Ireland. 



21. P. filifor'mis, Nolle ; 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. RUPPIA, 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 



