242 J. O. HAGSTROM, CRITICAL RESEEARCHES ON THE POTAMOGETONS . 



now more nodosus-like. In the latter case the cortical bundles again are more nume- 

 rous, (2 — )3 circles, in the former fewer, 1 — 2 circles. 



Tor the rest this hybrid varies with more prominent serrulation (lucens-Mke), 

 wheras the leaves otherwise verge toward P. nodosus, or with weaker, nodosus-like 

 serrulation together with large leaves and short petioles {lucens-\ike), but with, nodosus-like 

 apex. Thus especially the Danish form from Gudena, which is »P, fluitans f. suh- 

 lucens Baagoes in Asch, & Gkaebn., Synopsis 1897, 308, P. olivaceus Baagoe, 

 Prep, des Hydroph. 1900. When I first was studying this plant (1899) I con- 

 sidered it to be the hybrid alfinus x lucens (hence the admission alp. X lucens 

 J. Baagoe, 1. c), though some characters, especially the petioles of the sub- 

 mersed leaves were opposed, I thought me bound to this conclusion, because P. 

 nodosus does not occur in Denmark at present. When establishing the only 

 possible combination, hicens x nodosus, it is clear, that 1 also admit P. nodosus to 

 have formerly belonged to the flora of Jutland, though probably by the disimpro- 

 vement of climate it has now disappeared from it, just as, for instance, Trapa nutans 

 from the southern Swedish waters. By its relationship to P. lucens the hybrid 

 is more adapted to a rough climate and has consequently been able to endure. 



The American examples are somewhat dissimilar among themselves: one spe- 

 cimen without floating leaves (more lucens-like) , the others endowed with such leaves, 

 thus more nodosus-like, but all with longer petioles and more reduced lamina than 

 the Jutland specimens. Anatomically they behave alike. 



Under the name of P. Rugelii Ar. Bennett has distributed some Florida spe- 

 cimens as a hybrid P. americanus X lucens (The Journ. of Bot. 1908, 250, without 

 descr). Although I consider P. americanus to be P. nodosus, I do not, however, 

 use the name of P. Rtigelii, because I suspect the Florida plant to be something 

 other than a lucens-hyhrid. 



Distribution. Denmark, »In amne Gudena ad Kongensbro, Jyllandia», 99, 

 Baagoe (hb, Stockholm., Uppsal., Lund.). — N. America, »In lacu Seneca ad Geneva, 

 N. Y, »84, MoRONG (hb. Stochholm,), labelled: »P, lonchii.es 'i. 



P, lucens L. x perfoliatus L. 



(P. decipiens Nolte in Hansen, Herb, der Schleswig — Holstein — Lauenburgischen 

 Flor no, 715, 1838, et in Koch, Synopsis Fl. germ, et helv., 1844, 779. — P. lithua- 

 nicus GoRSKi, Herb., et in Reichenb., Icones, VII, 1845, t. XXXI. — P. olivaceus 

 Lang in Flora 1846, 472.) 



On this beautiful hybrid the following observations may be made: 1) In the 



description of Koch (1. c): foliis omnibus — basi rotundatis sessilibus ovali- 



bus oblongisve obtusis mucronatis, margine Isevibus, pedunculis sequalibus etc., the 

 last two characteristics are incorrectly given, in as much as the leaf margin is some- 

 what rough by small, but often very soon decaying, or abraded teeth, most distinctly 

 visible in very ^^'oung, not yet quite full-grown leaves, and the peduncles often or 



