160 



J. O. HAGSTEOM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



been set up but without argumentation. Fryer has also first of all pointed out 

 the near relationship between P. lanceolatus and P. riimlaris. 



The stem of this plant is terete and capillaiy without the compression charac- 

 teristic to the body of the pusilloids. This being the case, there can be thought of 

 only three linear-leaved species which possibly may have produced P. lanceolatus and 

 P. rivularis, viz. P. pusillus, panormitanus, and trichoides. The last mentioned is 

 excluded partly on account of the short peduncles of the hybrid, partly for its lively 

 green colour, partly for the weak nervation and obtuse broad apex of the submersed 

 leaves, and finally for the very faint subepidermal strands in its stem-anatomy. 

 The nervation and leaf-apexes taken together with the behaviour of the stem-prolonga- 

 tion are against P. panormitanus also, which, besides, usually has longer peduncles. 

 The ligules, however, are decisive to my opinion. They are always open 

 and convolute and as to the nervation intermediate between the above two species 

 by being 4 — 6-nerved in the front-field. 



If we proceed from the leaves, we must as the one of the 

 parents think of a species with floating leaves and many-nerved 

 submersed leaves, thus P. gramineus, alpinus, polygonifolius or 

 coloratus. The first mentioned is excluded partly on account of 

 the smooth margin of the submersed leaves, partly and absolutely 

 for the obtuse leaf-apexes with their peculiar nervation, as the 

 side-nerves do not bend out into the point but end with a curve 

 in the midrib. P. gramineus X pusillus and still more P. gram. X 

 mucronatus must necessarily have cuspidate submersed leaves since 

 this character is essential to both these species. Other circum- 

 stances also are agahist P. gramiyieus, for instance the form of the 

 peduncle, the 0-endodermis of the stem, the absence of cortical 

 interlacunar bundles in the stem, the feeble ligules, etc. The sessile submersed 

 leaves exclude also undoubtedly the suggestion of P. polygonifolius and coloratus, 

 whereas everything points in the direction of P. alpinus, the stem-anatomy also by 

 the epidermis consisting of large-roomed cells without hypodermatic la3'er and the 

 endodermis of 0-cells. Cf. under P. variifolius! Above all it is the nervation of 

 the submersed leaves together with their blunt points, the form of the floating leaves 

 and their stalks (always shorter than the blades) and the feeble ligules without rid- 

 ges that favour the supposition that P. alpinus has been active in this case. 

 The stem's mode of prolonging itself corresponds entirely to that of P. pusillus 

 and P. alpinus. — Turios are not seen by me and are not hitherto described by 

 anj^ author. 



The central cylinder of the stem is the same as in P. pusilhis, which holds 

 good for both the English and French forms. Concerning the description and figures 

 of the former I refer to Ar. Bennett, P. lanceolatus Sm., in the Journ. of Bot., 

 1881, t. 217. 



Distribution. England, Angelsea, N. Wales, 1808, Davibs (hb. UppsaL), 

 !> Potamogeton sp. nov. Rev. Hugh Davies to Smith LS08 from North Wales » ; 1836, 



Fig. 68. P. lanceolatus 

 Sm. a. Top of a submersed 

 leaf, showing tlie nervation, 

 -f. B, Floating leaf, |. C, 

 Pistil, side-view, ^-^. 



