312 



full number of the leaves nor the usual proportions of size, and 

 with rei^ard to tlie air-passages remains in the whole length of 

 the stalk like the middle part of some Danish plants. All these 

 questions can only be settled by cultivation of living material, 

 which unfortunately I have not yet had at my disposal. 



Nor can we learn without experiment, whether this possible 

 reduction has to be ascribed exclusively to the account of the 

 arctic conditions, or further to the littoral condition which is 

 often added here, and whether both contain factors with the same 

 inlluence. At any rate there can be no question of the influence 

 of brackish water where Mr. Porsild has seen Hippuris on Disco 

 Island (cfr. p. 301). 



Halorrhagidaceae. 

 Myriophyllum alterniflorum DC. and M. spicatiim L. 



Herbarium material from West and East Greenland, Ice- 

 land and, for M. alterniflorum, the Faeroes ; M. alterniftorum 

 also in alcohol. 



The lower, branched, in the end leafless stems creep rhizome- 

 like in the bottom of still or running water (cf. Irmisch 1859, 

 Warming 1884, Sylven 1906). 



The plagiotropic shoots are chlorophyllous with usually 

 long internodes. M. alterniflorum from Kvalböejde, Syderö, 

 the Faeroes, differs from the same species from other localities 

 by having short internodes and being tuft-like; it grew here 

 in shallow water on the sandy west side of the lake, where 

 with other water-plants it came almost right in to the margin, 

 M. alterniflorum also in great quantities a little further out 

 (Ostenfeld 1908). Some specimens on the sandy beach of 

 Gurresø, Denmark, have a similar appearance, others not. 



From the bases of the leaves descend branched or un- 

 branched roots, as a rule only from the creepiug internodes or 

 those just above these, but they may also arise in the upper part of 

 the shoot, above a long, rootless part of the stalk, in specimens 



