182 KJELLMAN, THE ALG^ OF THE ARCTIC SEA. 



sporangia. Among its forms f. lapponica approaches most nearly to A. boreale by the 

 branches of the last and next to last order being more generally one-sided, by the 

 systems of short branches being spreading or recurvate, and by the branches of the 

 last order being coarser and stififer. The typical form of the species, according to my 

 opinion, is that which has the branches of the last and next to last order longer and 

 more slender, rarely one-sided, but sometimes opposite, sometimes alternate, or some 

 being opposite or alternate, others one-sided. When now and then they are one-sided, 

 they occur on the inside as well as the outside, of their main axis. This form stands 

 very near A. americanum, as has been correctly stated by Gobi. The principal diffe- 

 rences between them are as follows:. A. americanum is larger and more tufted, more 

 violet in colour, with longer cells — even ten times as long as thick — and fewer, 

 longer, and more flaccid, branch-systems of the last order, with longer and finer side- 

 branches. The form corallina differs from the typical form by having branches and 

 branch-systems densely crowded into dense, button-shaped fascicles at the tops of the 

 main axis and the long branches, and by four branch-systems issuing from most 

 of the articular cells of the main axis and the long branches. In these respects 

 it approaches A. cruciatuin, from which it differs by the longer cells and the finer, 

 more flaccid and elongated branches of the last order, and by the tetrasporangia being 

 situated not at the base of the secondary branch-systems supplying their branches of 

 the first order, but on the branches of the first order representing the axes of the 

 second order of these branch-systems. 



From the supposition that ^4. plumula belongs properly to the Atlantic and the 

 Mediterranean and that it occurs, as he thinks, less frequent and luxuriant in the Arctic 

 Sea, Gobi concludes that this species has immigrated into the Arctic Sea and become 

 strongly changed there. It is impossible, of course, to determine with certainty how 

 this has been, but for my own part I should be more inclined to adopt a quite contrary 

 opinion, that is to say, that A. boreale has originated within the Arctic Sea and that A. 

 plumula and other species have issued from it and been developed in a southAvard direction. 

 A. boreale is so widely distributed in the Arctic Sea, that it can hardly be assumed to be 

 an immigrant. I have found it commonly diffused in all the parts of the Arctic Sea 

 that I have investigated. It is, indeed, often but little luxuriant and seldom occurs in 

 greater numbers; but I have pointed out above and already mentioned in Spetsb. Thall. 1, 

 p. 27 that this is not always the case. Touching A. plumula, it may be remarked that is often 

 difficult to arrive at any certain knowledge about the frequency of a species bymeans of 

 the terras generally used, particularly with regard to the occurrence of the species in 

 question I must admit that I know but very little. Akeschoug states that on the coast of 

 Scandinavia it is Bminime infrequens". On the ground of my own experience, I should trans- 

 late this expression by saying that, like A. boreale, it is commonly diffused, but seldom 

 or never appears in greater masses nor surpasses the last-mentioned species in luxuriancy 

 of growth when this is most luxuriantly developed. On the coasts of Britain the present 

 species according to Harvey is wnot uncommon)), which need not mean anything else 

 than that it is rather commonly spread. It is rare on the north-west coast of France 

 at Cherbourg according to Le Jolis, and on the north-east coast of America according 



