320 ECOLOGY. 



more rapidly, leaving the central pond at the east end. Over 

 a portion of the west end there is an extensive cassandra forma- 

 tion, with some ledum (Iabrador tea), but separated from the 

 circular cassandra zone by an intermediate zone. In this end- 

 cassandra formation other shrubs, and white pines five to fifteen 

 years old, are gaining a foothold, and in a quarter of a century 

 or more, if left undisturbed, one may expect considerable 

 changes in the flora of this atoll. It is possible that a rise of 

 the water for a number of years when the earlier zones were 

 floating accounts for the circular elevation and atoll forma- 

 tion, or that the dense shade from forest trees years ago may 

 have checked the growth of plants in the margin, thus leaving 

 a marginal depression. 



485. A black-spruce moor. — A somewhat similar but more 

 advanced plant formation occurs east of Freeville, N. Y., and 

 about nine miles distant from Ithaca. The centre of the basin, 

 which was perhaps shallower than the former one, has become 

 completely filled, and all of the central formation is more 

 elevated than the margin by the shore of the basin. All around 

 the margin in wet weather the ground is more or less sub- 

 merged, while all the central portion is so elevated that the 

 numerous stools or hummocks of grasses like eriophorum, with 

 its white tufts sparkling in the sunlight like a firmament of 

 stars, shrubs like cassandra, pyrus, nemopanthes, etc., support 

 one in walking above the water which rises in the intervening 

 spaces. Sphagnum, polytrichum, and other mosses grow, 

 especially in the stools of the other plants, where they now are 

 shaded by the larger growth, and in drier seasons catch the 

 water which trickles down during rain. 



Years ago the forest encroached on this formation, and trees 

 of the hemlock-spruce, black spruce, larch, etc., of consider- 

 able size gained a footing, first along the margin, then along 

 the more elevated zone a short distance within. The black 

 spruce trees spread all over the centre of the formation, attain- 

 ing a height of one to six or eight meters, while the trees of the 



