24 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOOBTONS. 



by the colour of the leaves, their fragility and apexes (in general), the colour and 

 structure of the sheaths and their quality of being connate when young, and the 

 mode of branching with only one branch in each axil. Finally the pollengrains, 

 style and stigma belong to P. filiformis, to which may be added that the fruit 

 when developed is intermediate or often hardly larger than in a macrocarp P. fili- 

 formis. The central cylinder of the stem is rather slightly differed from that of the 

 just mentioned species. 



Distribution. Denmark, Jutland, Kongeaa, 96, Baagoe (hb. Stockholm.). — 

 Tirolia sept, in rivo Giessen, Hellweger (hb. Lund.), Zimmeter et Zarntheim (hb. 

 Lund.), Innsbruck in Giessen, 83, Tiselius (hb. Stockholm, et Uppsal.). In lacu 

 sPillersees ad St. Ulrich 834 m s. m., 10, Keller (hb. Stockholm.). 



P. filiformis Pers. X vaginatllS Turcz. — P. vaginaius X filiformis ex Fontell, 

 Von ein. Potam. Hybriden 1902, 2. — P. filiformis X vaginaius ex Fontell, Beitr. 

 Anatom. Bau Pot. -Art. 1909, 30. — P. fennicus Hagstr. 



This bastard has a greyish tone of colour that points to P. vaginaius as the 

 one of the parents. This parentage is confirmed by the richer ramification, thus 

 differing from X P. siiecicus, in which the evolution of branches more agrees with 

 P. filiformis and the colour moreover inclines to brown or green. It differs from 

 P. pectinatus X vaginaius, the more slender forms of which it bears strong resemblance 

 to, through the slender, only at the base somewhat thicker stem, the usually shorter 

 weaker basal sheaths, the always obtuse, narrow and often longer, upwards less 

 tapering leaves and a less number of cortical strands in the stem, commonly 6 — 8 

 in one circle at the middle internodes. For the rest the sheaths are intermediate, 

 but even when very young open as in P. vaginaius; ligules as in P. filiformis, but 

 more persistent and of stronger structure, leaves elongated and narrowly linear, pe- 

 duncles thin and more elongated, spikes of four verticils or more. A cross-section 

 of the stem shows a central cylinder with four lysigene channels, an endodermis of 

 typical M-cells and usually a few narrower air-cells among the wider in the uttermost 

 circle. The leaf-structure is more like that of P. vaginaius than of P. filiformis, and 

 the marginal nerves retire more or less from the border just as in that. In the 

 peduncle the four central vascular bundles appear conspicuously, and in the whole 

 the hybrid in its hitherto known forms approaches more the P. vaginaius than the 

 P. filiformis. The slenderness, the thin elongate peduncle and intermediate mode of 

 branching makes it at the first glance rather easily recognizable. 



Distrihuiion. Sweden, West-Boihnia, Lofanger, 1870, L. Andersson (hb. 

 J. O. Hagstrom). — Finland, Ostrob. bor., Karlo, 84, Sandman (hb. Uppsal.). The 

 third locahty is Pedersore, Ostrob. med., coll. C. W. Fontell 1901. 



A plant from Canada. Winipeg Valley, 1859, E. Bourgeau (hb. Stockholm), 

 labelled »Pallisers Brit. N. Am. Expl. Expedition. P. pectinaius var. » I as well 

 refer to this hybrid. According to A. O. Kihlman (in Meddel. Soc. Fauna et Fl. 

 fenn. 1888, 113) P. vaginaius is said to have been collected by Botjrgeait, on the 

 Expedition of Pallisee in 1858 in Saskatschavan and communicated as P. peciinaius 



