374 NAIADES. [PoTAMOOETOir. 



ovoid, obscurely keeled ; beak subterm'nal, stout. DISTRIB. Europe (Arctic), 

 N. Africa, Siberia, N. America. 



VAB. 1, pasil'lus proper ; leaves acute 3-nerved. VAR. 2, tenuis'simus, Fries 

 (sp. ) ; smaller, leaves acuminate 1-3-nerved more spreading. /-*. yracilis, 

 Bab. not Fries. VAR. 3, mucrona'tus, Schrad. ; stem more compressed, 

 leaves broader 3-nerved. P. compressu*, Sin. Seems intermediate between 

 P. pusillus and obtusifolius. 



13. P. trichoides, Cham.; stem capillary repeatedly branched, leaves 

 ^-amplexicaul setaceous 1-nerved, stipules acute, peduncles filiform much 

 longer than the few-flowered spike, drupe solitary shortly beaked. 

 Muddy ponds and ditches in Norfolk ; fl. July-Aug. Stem dichotomously and 



divaricatingly branched. Leaves 1-2 in., dark green, rigid, spreading, acu- 

 minate ; stipules slender. Peduncles curved, longer than the leaves. Fl<>; ,-. 

 3-6, very minute. Drupe (dry) ,, in., compressed, obliquely ovoid, more or 

 less tubercled on the dorsal face. DISTRIB. Europe from Belgium southwards 

 and eastwards to Austria, Italy, and Russia, N. America. 



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

 bud ; stipules adnate to their bases and forming a sheath with them. 



14. P. pectinatus, L. stem filiform, branches dense distichous, 

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

 long filiform, flowers few remote, drupes shortly beaked. 



Ditches, canals, &c., fresh and brackish ; fl. June-July. Hoot a small tuber. 

 Sterns repeatedly branched, forming dense masses. Leaves 3-8 in., &- in. 

 broad, lower sometimes 3-5-nerved, sheathing portion often 1-1 ^ in., tips 

 of the stipules free. Peduncles not thickened upwards. Flowers in inter- 

 rupted whorls. Drupe* very large for the plant, in., dimidiate -obovoid, 

 turgid; ventral margin slightly convex; dorsal obscurely keeled. DISTRIB. 

 Europe (Arctic), N. Africa, Siberia, India, N. America. 



Sub-sp. PECTINA'TUS proper ; upper leaves setaceous 1-nerved channelled 

 (composed of 2 parallel tubes) lower flat 3-ribbed, lateral keels of drupe 

 conspicuous. P. mari'nus, Huds., is a var. with stems naked below. 

 Common in brackish and fresh water. 



Sub-sp. FLABELLA'TUS, Bab. (sp.); leaves flat 3-5-nerved, lateral keels of drupe 

 obscure. P. zostera'ceus, Bab. not Fries. England and Ireland. 



Sub-sp. FILIFOR'MIS, Pert, (sp.) ; leaves all setaceous 1-nerved channelled, 

 drupe smaller not keeled. P. marinus, Linn. Herb. Scotland and Ireland. 



a. RUP PIA. L. 



Slender brackish-water herbs. Leaves submerged, filiform ; stipules 

 sheathing. Flowers minute, 2-sexual, usually 2 on a short filiform 

 peduncle (female greatly lengthening), arising from the spathaceous leaf- 

 sheaths. Perianth 0. Stamens 4, filaments short broad ; anthers attached 

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

 tube with 1 median and 2 terminal nuclei. Ovary of 4 sessile carpels, 

 stigma sessile peltate ; ovules 1 in each carpel, pendulous. Drupes long- 

 pedicelled, ovoid, carried up on the greatly lengthened usually spirally 

 coiled peduncle. Seed pendulous, testa thin ; radicle large ovoid, cotyledon 

 small hooked subternnnal. DISTRIB. Temp, and trop. regions ; species 1 

 or more. ETYM. H. B. Ruppius, a Iwtauical author. 



