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



With regard to the nervation of leaves and ligules as well as respecting the 

 stigma, the rostrum, and the endocarp of the fruit the species is closely related to 

 P. prcelongus. 



My first acquaintance with this plant made me consider it a hybrid P. per- 

 foliatus X prmlongus and I am still convinced that it has risen by cooperation be- 

 tween those two species. Especially the fruit and the denticulation are persuasive. 

 My considering it now a separate species is dependent on the following facts: 

 1) Pollen in the cases examined relatively well developed. 2) Fruit likewise. 3) The 

 presence of some (even though inconsiderable) independent properties (the form of 

 ligules and the nervation of the apex in some instances), and 4) Its occurrence, as 

 I think, like a species, not only on scattered stations like a mongrel. Its distri- 

 bution-area like that of P. prcelongus seems to be Canada and the northern Sta- 

 tes with its centrum round the great Lakes, and as those regions have been covered 

 by the great land-ice during the glacial period it is clear that P. Richardsonii is a 

 relatively new species. 5) The constancy of its properties. All the specimens 

 I have seen and examined from Michigan, Wisconsin, Minnesota and Wyoming de- 

 viate very slightly from each other. Especially the stem-anatomy is fixed, which, 

 I think, were impossible, if it not were about one single cross, which had got the 

 faculty of propagating itself by seeds. None of the Potamogeton hybrids known to 

 me behaves in this way. I particularly think of P. nitens, Zizii, sjKirganifolius 

 and others. P. Richardsonii is, no doubt, a highly interesting form, and I should 

 like to obtain materials from different localities for further anatomical investi- 

 gations. 



Dr. RoBBiNs' specimen, signed »R. Dr. », is preserved at Stockholm (hb. Nat. 

 Hist. Mus.). 



P. perfoliatiis L. 



Sp. pi. 1753, 126. — P. Loeselii Roem. et Sciiult., Syst. veg. 1818, 508. — 

 P. mrfplexicaulis Kar., in Bull. Soc. Nat. Mosc, 1839, 173 (nomen solum). — Figs. 

 116, G, H, 117, 118, A. 



The stem of this species common in the north hemisphere is usually much 

 branched especially upwards, like the type C (Fig. 2), but occurs also unbranched 

 or nearly simple (f. simplex Tis.). Internodes generally short above, or often very 

 short. Rarely they are stretched as far as 100 to 150 mm: 



f. prolixus n. f. : — Internodia superiora sub spica primaria elongata, 100 — 

 150 mm. 



Anatomically it is of no consequence whether the plant grows in currents or 

 in stagna. The stem shows the same non-sclerenchymatous type. Only in very 

 rare cases someone or other subepidermal strand is to be found. Epidermis-cells 

 streched, 4 — 6 times as long as broad or a little more. — In the peduncle also the 

 bundles are running comparatively centrally and but few and small bundles are to 

 be found in the outer septse, whereas P. prcelongus has a great number of strands 



