12 MINNESOTA BOTANICAL STUDIES. 



tation around the shore line of the pond at its season of low 

 water must be postulated as a condition of atoll formation. As 

 stated above the shore-plants may have been largely sphagnous, 

 but they might, too, have consisted of small sedges, Campanula 

 and slender grasses — such as are important components of so 

 many shore-floras and floating-bog floras in the lakes of 

 Minnesota. 



Slight lateral tension of winter ice. A not unimportant 

 condition of atoll formation and persistence is to be looked for 

 in the absence of strong lateral, shoreward ice-pressure, caused 

 by the expansion of ice as formed in winter upon the pond 

 surface. In ponds of considerable size and depth this is sufii- 

 cient to modify the shore vegetation and its distribution to a 

 very marked degree. It would evidently, too, exert a distort- 

 ing and destructive influence upon an annular floating bog if it 

 were of sufficient size, enclosing a lagoon of suflicient diameter 

 for such a force to become considerable. The depth, surface- 

 area, contour and cubic contents of the basin would, if an atoll 

 were to be formed, have to be of such a nature that this force, 

 when acting, should be distributed regularly and should remain 

 at a minimum. 



Eapid anchoring of the atoll. It cannot be conceived that 

 the condition of a floating bog should persist for a long term 

 of years in the atoll-forming turf. Even if all the disturbing 

 causes acted at the minimum demanded by the hypothesis, and 

 under the favorable conditions outlined above, the persistence 

 through many decades of the floating position of the turf would 

 give opportunity for cumulative effects, slight in themselves, 

 but in the aggregate sufficient to distort or disrupt the ring. A 

 necessary condition of permanence in the atoll must be looked 

 for, then, in the comparatively rapid substitution of a grounded 

 or anchored position for the plastic, mobile, easily disturbed 

 position of the floating bog. It could have been for but a few 

 years at the most that the bog remained as a buoyant formation. 

 Through increase in mass and weight it must rapidly have 

 sunken and anchored itself firmly upon the bottom. Here again 

 a definite and not too great size and depth of the parent pond 

 appears as one of the essentials of atoll formation. 



Summary and conclusion. A consideration of all the facts 

 catalogued upon the preceding pages permits a brief summary 

 as follows: 



1. Atolls of sphagnum with various adventitious plants 

 established upon them have been observed in central Minnesota. 



