KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. NIO 5. 217 



of the branch-leaves, the soon decaying ligules, and the short zigzag-placed internodes. 

 P. lucens has likewise evidently influenced the leaf-apexes, which are sharper and 

 longer than in X P. nitens, beside which the leaf-size, the length of the ligules and 

 the occurrence of two circles of interlacunar cortical bundles also point toward the 

 same species. The subcoriaceous structure of the floating leaves, again, and the 

 presence of stomata undoubtedly show influence and cooperation of P. gramineus. 

 The above said especially refers to the plant from the Sillen Lake which, besides, in 

 habit reminds of X P. nitens as well as of X decipiens. But the rest of the below 

 noted forms also, in habit most reminding of x P. Zizii, and with short-petioled stem- 

 leaves, point towards P. perfoliatus by their sessile branch-leaves with rather broad 

 base, and by the central stele of the stem with two median xylem canals, and (the 

 plant from Vologne) with three free lateral bundles. 



Distribution. Sweclen, Sudermania, Vårdinge in the Sillen Lake, 93, Torssander 

 and others (hb. Stockholm., Uppsal., Lund., Gothenb.). Upland in the Fyris River at 

 Ekeby mill, 59, Tiselius (hb. Norrköping); Norrsunda in the outlet of the Oxunda 

 Lake, 94, Almqvist (hb. Stockholm.) together with x P. decipiens and x P. Zizii. 

 Westgothia, Trollhättan, Skärbo, 88, Landtmanson (hb. Uppsal.). — Denmark, Susa 

 ad Hjelmsölille S. S., 95, Baagöe (hb. Stockholm.) leaf-point dec i j)i ens-like, upper 

 leaves petioled like those of P. Zizii. 



France, Vologne, Lebel (hb. Uppsal.). This form has weaker nitens-Yike serru- 

 lation, perceptible only in the very youngest leaves. The ligules again are lucens- 

 like, brown-coloured, conspicuously bicarinate. The narrow (5 — 7 mm) branch-leaves 

 and long-petioled upper stem-leaves witness of an influence from P. gramineus. The 

 specimen from Trollhättan resembles the Vologne specimens, but has shorter and 

 broader branch-leaves (60 — 70 X 7—10 mm) with a little more prominent serrulation. 

 The French form, in habit and morphologic facts least reminding of P. perjoliatus, 

 has, on the contrary, this species best of all represented in the stem-anatomy (see 

 above!). 



A plant from Benmark, Ghenö, Ssell., 97, leg. J. Baagöe, is very likely the 

 above hybrid. In habit and as to leaf-size (stem-leaves ca. 50 x 10 mm) and ligules 

 it resembles much P. nitens and is distributed by Ar. Bennett as P. nitens var. tj 

 cuspidatus in Graebner, Potamog. 1907, 91. On closer examination, however, it 

 presents great deviations, which all point to P. lucens: leaf-apex with a long, sharp 

 cusp; serrulation lucens-Yike; nervation and bases of the lower stem-leaves likewise; 

 anatomical stem-diagram like that of P. gramineus except that the endodermis-cells 

 are more lucens-like. 



P. gramineus L. x natans L. 



(P. sparganijolius LiESTADius ap. Fries, No v. Mantissa I, 1832, 9. — P. Kirkii 

 Syme, Engl. Bot. ed. 3, vol. 9, 1869, 31, t. 1403. - P. Tiselii Richter, Plantse 

 Eur. I, 1890, 13. P. dubius Tiselius, Pot. suec. exs., I, 1894, no. 19.) - Fig. 



104. — 



K. Sv. Vet. Aktul. Handl. Band 55. N:o 5. 28 



