ECOLOGY OP ISLE ROYALE. 221 



It is to be regretted that more borings and measurements cojild not 

 have been taken in the limited time at our disposal. A summer spent 

 with suitable boring tools in making an extended series of borings over 

 various parts of several of the Isle Royale bogs and bog-lake margins 

 would doubtless bring to light some very interesting data. 



Now as to what gives strength to the bog cover, Wince the sphagnum 

 is so predominant on bog areas, covering large parts of the surface, 

 and often extending out almost to the water's edge, one is apt to think 

 only of the sphagnum surface and fail to consider the important net- 

 work below that gives su-ch strength to the ^trembling bog carpet out 

 almost to the very water's edge. It is scarcely necessary to add that 

 the ' delicate sphagnum alone is not sufficient to make a strong bog 

 cover. The weakness of the individual sphagnum plants to resist strain, 

 the lack of interlacing parts, or of even "felting" properties are clearly 

 shown in that one can reach down a foot or more into the loose, soft, 

 sphagnum and pull out a ha'ndful of it without seriously disturbing 

 the adjacent plants. Moreover, the sphagnum does not grow along the 

 water's edge in advance of its supporting mat — at least we failed to 

 find a single instance of this on Isle Royale — while in many cases it 

 did not extend out to within several feet of the water edge of the 

 supporting mat. 



On pulling up large masses of the floating mat at the water's edge 

 it was found to consist of a dense tangle, or network, of tough fibrous 

 roots and rhizomes of sedges, Menyanthes trifoUata and Comarum pal- 

 ustrCj all so tightly interlaced that it was very difficult to separate any 

 part of the tangle from the rest. Such tough parts are in striking 

 contrast to the delicate sphagnum, as is also the manner of growth; 

 and furnish the platform, on which the sphagnum works out toward the 

 lake margin. 



In the last, or wholly covered, division of bogs a wet and a drier stage 

 may be recognized; the former may be characterized by the Sarracenia 

 purpurea, Menyanthes trifoliata, Comarum palustre, and one or more 

 species of Drosera. Sphagnum hummocks may occur in both of these 

 covered stages, or the surface may be comparatively smooth. These hum- 

 mocks, of varying size up to 4 feet in height, seem to be due in most 

 cases to the sphagnum growing up around tree trunks, shrubs, or other 

 objects. Instances were noted of where the rapidly growing sphagnum 

 had so nearly covered the Ledwin grocnlandicum that only the ends of 

 the upper branches were to be seen. It is possible, however, that some 

 of the hummocks may be formed in other ways, e. g. one large hummock 

 was noted that was, inhabited by ants. This suggested that possibly the 

 sphagnum had overgrown a large ant mound, although it is also possible 

 that the ants may have inhabited the mound only after its foi'mation in 

 some other way. 



The pioneer trees to appear in Isle Royale bogs are the Larix laricina 

 (Tamarack), and Picea martano/ (Black Spruce) , which appear simultan- 

 eously, and seem equally Avell adapted to bog conditions. Owing to the 

 advance of the bog cover from the margin toward the centre, one nat- 

 urally expects to find the youngest trees farthest in, and this is strik- 

 ingly well illustrated in many of the bogs. 



By counting the rings of trees cut in the bogs, and comparing with 

 the same species just outside, it Avas found that the growth of those in 



