126 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGBTONS. 



P. pusilliis L. X triclioides Cham. & Schlecht. (P. fraiicoiiicus Fischer, 

 Die bayer. Potamogetonen, in Ber. bayer. Bot. Ges. XI, 1907, p. 29, 123). 



This hybrid is by far not so difficult to distinguish as some of the preceeding, 

 because the parent-species have rather different qualities both in the vegetative system 

 and in the fructification. The stem of the hybrid is thread-like, terete and branched. 

 In habit it resembles now P. triclioides with numerous spike-bearing branches shoot- 

 ing out from the stem beneath the primary spike, now P. pusillus forkedly branched 

 above the first spike. Of the two buds in the axUs of the involucral leaves now 

 the undermost (P. trich.), now the upper one is the strongest (P. pus.). The turios 

 consist of transformed tops of branches of an intermediate shape. The leaves are, 

 like the stem, now of darker green (P. trich.), now of a rather light-green colour, 

 the leaf -width intermediate, or more or less approaching to the one of the parents, always 

 stiffer than in P. pusillus, on account of the strong nerves and always with stronger 

 marginal sclerenchyma than in P. pusillus, and with a sharp point, in their lower 

 part more or less lacunous, the broader ones reminding of P. pus. v. Berchtoldi, but 

 distinguished through their rigidity. The peduncle is often more elongate than in 

 P. pusillus, up to 75 mm in the danish form from Villa Gallina (see below !). Pollen 

 mostly or wholly barren. 



Beside the Danish specimens I have seen a few specimens from Germany, 

 France, England, and Sweden. They are conveniently to be divided into two series: 



a spicosus n. var. : — Pars superior caulis ut in P. trichoide, vide supra ! 



Distribzition. England, Hedge Court Mill Pond, Surrey, 86, Beeby (hb. 

 Stockholm.), Glastonby, Somersetshire, 82, Rev. Murrey (hb. Stockholm.). — France 

 Ecausseville (Manclie), 86, Corbiere (hb. Stockholm.), with the leaf -base of the same 

 structure as in P. pusillus. — Siveden, Gothenburg, 77, Winslow (hb. C. Alm). 



P dichotomus n. var.: — Ramificatio ut in P. pusillo, vide supra! 



Distribution. Germany, Brandenburg, Rudomer, Wiesen, 78, Potonib (hb. 

 Stockholm.), internodes 3 — 5 cm long. I have seen the same form from Freijen- 

 stein (Brandenburg), leg. Rietz, 1895, from Cadolzburg at Niirnberg, leg. Fischer, 

 1903, and from Kirchfarnbach and Wittinghof (Fiirth), the latter, however, a little 

 different. — Denmark, Sjselland, Villa Gallina, 02, Baagob (hb. Stockholm.), an 

 extremely beautiful form that could be called 



f. loiigipes n. f. — Planta gracilis, altissima. Internodia elongata, 5 — 15 cm; 

 pedunculus longus (3 — 7 cm). 



Mr Baagob took this plant for P. pusillus capitatus Ar. Benn. 



P. pusillus L. X V.iseyi Robe (P. lateralis Morong in Coult., Bot. Gazette, V, 



51, 1880, as species). 



That the plants here alluded to really are of hybrid origin does not admit of any 

 doubt, nor that they are Fase^/i-hybrids. The rather frequently existing floating 



