246 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



lotigus a eu-decipiens Asch. & Graebn., Synopsis, 1897, 331, Grabbn. Potamog. 1907, 

 137, Syn. 1913, 505, but no character of P. prcelongus is liere present; they are all 

 very clear lucens X perfoliatus. »Tilsit in Memela, 1866, Heidbnreich» (hb. Stock- 

 holm., Berolin.)j /. memelanus, is P. Heidenreichii Asch. et Gr. The Berlin speci- 

 mens are labelled: » Flora Borussiae orien talis. Potamogeton longifolius Gay. Tilsit 

 in Memela fluvio Junio 1886 legit Dr. F. Heidenreich Tilsit» »Herbarium A. Eng- 

 LER». »Potamogeton lucens X gramineus (P. Heidenreichii) A. u. G. Syn. I. 327. 

 (1897). 1897. P. Graebner det. »: Leaf margin strongly serrulated, apex of deci- 

 2nens-type, see above! base, see fig. 113! Ligules 40—60 mm, thin, fugitive; ana- 

 tomy: subepidermal strands lacking or almost lacking (influence from perfol.), cen- 

 tral stele lucens-Vike. It is impossible to discover anything of P. gramineus in this 

 plant. — Sivitzerland, Geneva in Rhone, Chavin (hb. Stockholm., Lund.). — Lithua- 

 nia in lacu Kroman ad Naliboki, 93, Dybowski (hb. Stockholm.), f. lithuanicus; 

 »Im Flusse Vilia (Nevis Lithuanorum) bei Vilna. Prof. S. B. Gorski» (hb. Stock- 

 holm., Uppsal.), »P. lithuanicus GoRSKi»! Only the upper portion of a stem with a 

 branch at the primary spike. No tendency of forming floating leaves; Leaves lan- 

 ceolate, sessile, serrulate; involucral leaves rounded at base, 100 X 12 — 15 mm; Li- 

 gules pretty durable, without ridges, 30 mm long; Anatomy: 2 cortical circles of 

 bundles and scattered subepidermal strands. On account of these poperties the 

 plant can neither be lucens X prcelongus (GoRSKi), nor alpinus X lucens (Asch. & 

 Graebner, Ar. Bennet), nor gramineus x perfoliatus, but is an evident and beauti- 

 ful decipiens-iovra. approaching to the Swedish form from the Fyris River at Jasevad, 

 see above! 



This hybrid is not yet discovered in N. America. 



P. lucens L. X praeloiigus Wulf. 



(P. Bahingtonii Ar. Bennett, Notes on Potamogetons in The Journ. of Bo- 

 tany, 1894, 204-205.) 



In its Danish forms this hybrid comes partly very near X P. decipiens, partly 

 P. lucens, with which two it has also been mixed up. From the latter, however, 

 it is easily distinguished by the sessile leaves with only rudimental, only in not 

 yet full-grown leaves microscopically perceivable teeth. Leaf-base most frequently 

 narrower than in X P. decipiens, often stalk-like; Ligules longer and more strongly 

 bicarinate. The very tip of the leaves is always a little broader and shorter than 

 in P. lucens, flat or somewhat bent upward by influence from P. prcelongus; in 

 the two just mentioned (P. X decip. and P. luc.) the tip is somewhat canaliculate 

 and recurved. Peduncles also usualty more elongated than in X P. decipiens, and 

 the spikes larger. The cortical bundles of the stem are present in several circles be- 

 side the subepidermal one. The bundles of this circle have an inclination to bending 

 inward, from the epidermis into the radial septums (influence from P. prce- 

 longus) which never or at least extremely seldom may be met with in x P. decipiens. 



