KUNGL. SV. VET. AKADEMIENS HANDUNGAR. BAND 55- NIO 5. 149 



P. alpinus Balb. x prselongus Wulf. (P. nerviger Wolfg. ap. Schultes, 



Mantissa in Vol. III, 1827, 359. — P. Griffiihii Ar. Bennett, in the Journ. of 

 Botany, 1883, 65, t. 235). 



I have had an opportunity to see and examine some specimens, determined by 

 Wolfgang himself and labelled: Potamogeton nervigerus mihi J. W.» They are 

 conspicuously the hybrid written above. As to the stem-anatomy P. alpinus is traced 

 in the O-endodermis and in the obvious reduction of the number of bark-bundles, 

 otherwise originating from P. prcelongus. The latter appears in the pseudo-hypoderma, 

 in the stout stele, and in just mentioned cortical bundles. They occur as only one 

 interlacunar circle but never subepidermally. The English specimens are also evidently 

 this hybrid, but the stem-anatomy comes much nearer to P. prcelongus than it does 

 in the preceding. P. alpinus, however, has influenced both the endodermis, which 

 exhibits only faint M-cells, and the cortical strands, the number of which always is 

 less than in P. prcelongus. Especially the subepidermal strands are few. 



As to the leaf-form the two parent-species appear in the hybrid according to 

 the descriptions. 



P. nerviger has been considered by several authors (Kunth, Enumer., Richter, 

 Pl. Eur., Aschers. & Graebn., Syn., A. Bennett in the Journ. Bot., 1889 and 1891) 

 now identic with P. alpinus now as a subspecies or variety of it. The occurrence, 

 however, of cortical bast in the stem as is shown above does not allow such a pre- 

 sumption. — Wolfgang himself wrote the name nervigerus, yet I use the first published 

 form of it. — As to P. Griffiihii, A. Fryer (Pot. Brit. Isl.), after Bennett, has con- 

 ceived it as an independent species, låter on as a supposed polygonifolius-h&st&Yå (The 

 Journ. Bot., 1907, 300). Its hybrid origin, however, is beyond all doubt and may 

 nowadays be disputed in earnest by nobody. Graebner still writes (1907): Planta 

 originis dubiosa?. 



Distribution. Lithuania, Vilna, Horaninow (hb. Uppsal.), leaves lanceolate, 

 sessile or with the base sloping stalk-like, top gradually tapering, obtuse, 8—9 X 1,5 

 —2 cm. — England Llyn-an-afon, near Aber, Carnarvonshire. N. Wales, 82, 85, 86, 

 87, 88, Griffith (hb. Stockholm.). — No more stations hitherto known. 



P. alpinus Balb. x pusillus L. (P. lanceolatus Smith, Engl. Bot. Vol. 28, 

 1809, t, 1985, Engl. Fl. I, 1824, 232, as species. — P. rivularis Gillot in Magnin, 

 Scrin. VI, 1887, 118). Fig. 68. 



In the course of time P. lanceolatus Sm. has been much written of and much 

 misunderstood. It was only rather late that its hybrid origin has been called into 

 question. A. Fryer is the first who, on account of its narrow distribution, has 

 considered it to be a hybrid: the case in favour of the hybrid origin of P. lanceolatus 

 is strongly supported by the local distribution of the species (The Journ. of Bot., 

 1894, 339). On occount of the glands at the leaf-base and the habit of the plant 

 in young state this author regarded it as a Friesii-hyhviå. The Anglesea-form is 

 admitted to be a pusillus-hybrid. As the other co-active species P. heterophyllus has 



