6 MINNESOTA BOTANICAL STUDIES. 



portance, as giving character to the atoll, was a dense growth 

 of Ledum latifolium Ait. — Labrador tea — which covered almost 

 the entire island. A complete list of plants observed on An- 

 derson's atoll is as follows: 



Sphagnum, three undetermined species. — Peat-moss. 



Panicum crus galli var. hispidum (Muhl.) Torr. — Cock's- 

 comb grass, (four plants, only. ) 



Gyrostachys romanzowiana (Cham.)MacM. — Tress orchis. 



Limodorum tuberosum Linn— Purple orchis. 



Cornus canadensis Linn." — Dwarf cornel. 



Ledum latifolium Ait. — Labrador tea. 



Andromeda polifolla Linn. — Andromeda. 



Gentiana sp. und. — Gentian. 



Campanula aparinoides Pursh. — Bellflower. 



Picea mariana (Mill.) B.S.P. — Black spruce. 



Curiously, Sarracenia and Oxycoccus, very commonly found 

 in peat-bogs, did not appear on this atoll — or, at least were not 

 observed. In general the texture of the atoll mass was much 

 firmer than that of Ballard's atoll. One could stand anywhere 

 upon it without sinking in above the insteps. This was doubt- 

 less due to the firmer interknittlng of the roots of Picea and 

 Ledum. The lagoon of this atoll, unlike that of the other, was 

 somewhat invaded with floating vegetation — mostly Utricularia 

 Intermedia Hayne, with a few plants of Panlcularia. 



Origin of tlie atolls. To frame a reasonable explanation of 

 the way in which these sphagnum atolls have developed is not 

 altogether easy. Various hypotheses present themselves, some 

 of which win not be discussed here. That the ponds In which 

 the atolls are formed are certainly not fed by springs Indicates 

 that no connection between these peculiar ring formed masses 

 of sphagnum and the domed or raised peat bogs ^ can be as- 

 sumed. Were the atoll lagoons fed by springs from the bottom 

 It might be supposed that the original form of the sphagnum 

 mass was that of a dome and that through pressure or variation 

 in water level the dome had been broken and thus a depression, 

 occupied by the lagoon, had appeared. This hypothesis would, 

 however, be rendered untenable by the greater depth of the 

 lagoon than of the surrounding pond. Nor would It be clearly 

 apparent why the submergence of the region outside of the 

 atoll — postulated by such an hypothesis — should have taken 

 place. 



2. Ganong, W. F. Qu raised peat-bogs in New Brunswick. Bot. Gaz. 16 : 123 

 1891. 



