258 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



Although this variety p, in the locality stated, probably is only one cross, 

 it nevertheless appears in two rather different forms: one with large, broad leaves, 

 in shape more perfoliakcs-like, 90 — 130x40 — 55 mm: 



f. Varmingii Baagoe in herb. J. 0. Hagstrom 1902: — Folia magna lata P. 

 perfoliati similiora. 



The other has narrower but comparatively more elongated, i. e. more frcelongus- 

 like leaves, 110-130 X 30— 35 mm: 



f. subpriEloiigus Baagoe, 1. c. 1897: — Folia quam in praecedente angustiora 

 et longiora, iis P. prcelongi similiora. 



This var. occurs small-leaved in Norway, Hindoen, 10, NoTo (lib. Christian.). 



The Canadian specimens, again, determined by Ar. Bennett under the name 

 of P. intermixtus, should be ranked under P. cognatus as a var. or f. intermixtus, 

 provided that they really are P. perf. X prcel. 



Neither the Canadian specimens nor those from »Waddainsee bei Serbigal» 

 (KuPFFER, in M. v. zuR MOhlen, Pot. Ostbalt., 1906, 172) are described as 

 regards the leaf-margin. 



It is striking that P. prcelongus and perfoUatus, the stigmas and pollen of 

 which are much the same, so very rarely produce mongrels between themselves, 

 whereas P. perfoUatus and gramineus, for inst., dissimilar in the afore-named respects 

 scarcely can grow together without producing crosses. This rare occurrence is also 

 stated respecting the other prcBlotigus-hyhrids {alpinus X prcel., crispus X prcel., gra- 

 mineus X prcel., and lucens X prcel.). The explanation of this peculiar fact lies in the 

 early florification of P. prcelongus, bj' which the pistils of P. prcel. are prevented 

 to receive pollen from its coexistents. Only the last emitted pollen of P. jJrcel. can 

 be thought to hit the stigmas of its companions. 



P. bupleuroides Fernald in Rob. and Fern., Gray's New Manual of Bot., 1908, 75. 



According to specimens (hb. Stockholm.) from the lakes Fresh Pond and Mystic 

 Pond, Mass., U. S. A., near the Atlantic coast, and a specimen from Connecticut, 

 leg. A. Gray (hb. Christiania) this species is characterized by a stem with very short 

 and very short-jointed branches, and by short and rounded leaves like those of the 

 f. rotundifolius of P. perfoUatus. Their apex and basis, nervation and serrulation 

 as in perfoUatus. Ligules as to form and nervation as in perfol. Peduncle, spike and 

 pistil scarcely deviating. Fruit smaller, 2.5 X 2 mm (perfol. 3.25 X 2,2 — 2,5 mm), 

 rounded at the apex and with a thin rostrum. Anatomy like that of perfoUatus. 



Without fruit this species will be very difficult to distinguish from P. perfol. v. 

 rotundifol. Wallr. as you have only the short- jointed, small-leaved branches to con- 

 clude from. Fernald states, it is true, that the leaves are »flat, not crisped*, but 

 also the said form of P. perfol. has such leaves, at least sometimes, which is even 

 acknowledged by Fernald when saying of P. perfol. » leaves . . . usually obtuse and 

 crisped* (1. c. 74). 



