65 



The varieties of this species are very varying and not 

 sharply defined. While the frond of M. fuscum is cleft in an 

 early stage of development, without regard to environmental 

 influences, this species always remains tubular in sheltered 

 situations (/? and y) , and I think that the tubular 'frond must 

 be regarded as normal in this species, as the split varieties 

 (a and ;-), which grow in the littoral region exposed to the 

 dashing of the waves, in sheltered places occur with tubular 

 frond; such specimens from Iceland have been shortly men- 

 tioned by me in Icel. alg. p. 350. The splitting of the frond is, 

 I think, exclusively due to the surroundings, and the variability 

 of the species seems to be caused by environmental influences. 

 According to our present knowledge of these varieties I think 

 it most natural to regard them as belonging to one and the 

 same species, and 1 cannot admit Collins (1. c. cfr. my foot- 

 note p. 63) to be right in dividing M. Grevillei K. Rosenv. 

 into two species: M. Grevillei Collins including var. Vahlii 

 K. Rosenv., and M. arcticum Collins including var. intestini- 

 formis K. Rosenv. The limit between the two species as under- 

 stood by Collins, is as indistinct as the limit between the main 

 form of the species and the included varieties. If closely 

 related forms, which run into each other, are not to be regarded 

 as belonging to one and the same species, we had better take 

 as a species every form that can be described plainly enough to 

 be recognizable, than form species of artificially grouped forms, 



Gr. or.: Ikerasak (d), Tiningnekelak (d), Angmagsivik (a, j-), 

 Smalsund (;-), Nualik (ß, â). 



104. M. groenlandicuin J. Ag., K. Rosenv. Grl. Havalg, p. 954, 

 Deux. Mém. p. 117. 



it has been collected in June — July togetlier with Ulothrix 

 flacca, Monostroma Grevillei d. intest iniformis and Bhodomela 

 lycopodioides. Fructiferous plants were gathered in July. 



Gr. or.: Ikerasak, Nualik. 



6 



