254 J. O. HAGSTRÖM, CRITICAL RESEARCH KS ON THE POTAMOGETONS. 



With regard to t lic ncrvation of leavea and lignles as wcll as respecting thc 

 stigma, the rostrum, and the endocarp of the fruit the species is closely relä t ed to 

 P. prcelongus. 



My first acquaintancc with this plant made me considcr it a hybrid P. pcr- 

 folidtus X prcelongus and I ani still convinced that it bas risen by coopcration be- 

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

 .My considering it now a separate species is depcndent on thc following facts: 

 1) Pollen in the cases examined relativclv wcll developed. 2) Fruit likewise. 3) The 

 prescnce of some (even though inconsiderablc) 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 covcrcd 

 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, wcrc 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 vvay. I particularly think of P. nitens, Zizii, sjxiiyinifolins 

 and others. P. Richardsonii is, no doubt, a highly interesting form. and 1 should 

 like to obtain materials from different loealities for furthcr anatomical investi- 

 gations. 



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

 Hist. Mus.). 



1\ perfoliatus L. 



Sj). ))1. 1753, 126. P. Loeselii Roem. et Sciiult., Syst. \ cg. ISIS, 508. — 

 /'. nm />!< xicn ni is Kar., in Bull. Soc. Nat. Mosa, 1831), 173 (nomen solum). — Figs. 

 110, Q, 11, 117, 118, A. 



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

 branched especially upwards, like thc type C (Fig. 2), but occura also unbranched 

 or nearly sini])lc (f. simple. v Tis.). Internodes generally short abovc, or oftcn vciv 

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



f. prolixus n. f.: — Internodia Buperiora sub spira primaria elongata, 100- 

 150 mm. 



Anatomically it is of no consequence whether the j)lant grows in curreats ar 

 in slagna. The stcin shows fche sann; non-sclcrcnchyinatous ty|)c. Only in \ v\ \ 

 rare cases someone or othcr subepidermal strand is to be found. Epidermis-oella 

 streched, 1 <> times as long as broad or a little morc. In thc pedunole also the 

 bundles are running coraparativoly oentrally and bul lr\\ and small bundlee are to 

 be found in thc outcr BeptfiB, whereas /'. prcelongus has a great number of strand.- 



