SUCCESSIONAL RELATIONS OF VEGETATION. 



275 



florus, Aster novi-belgii, Matricaria inodora, Festuca ovina, 

 Achillea Millefolium, Agrostis alba and Rumex acetosella. 

 The piesence of several alders points to the succession of the alder 

 thicket. 



Near the northern end of the island upon which the light- 

 house is situated, is a small pond — the last stage of a larger water 

 area now occupied by bog. Here all the stages from freshwater 

 pond through bog to forest are well represented. 



The Potamogeton association occupies the deeper water. 

 It consists of a single species, P. epihydius. The marginal 

 shallow water supports a dense growth of Hippuris vulgaris 

 (Hippuris association). The water level fluctuates markedly 

 and the zone of Hippuris terminates abruptly at the low water 

 level. 



The area immediately surrounding the pond is occupied by 

 the Carex-Juncus association, in which the dominant species are 

 Carex hormathodes and Juncus articulatus, the sedge being very 

 much more abundant than the rush. North of the pond the 

 area has been pastured and the secondary species show a tendency 

 toward the development of the Pasture Formation, described 

 later. They are Juncus effusus, Ca rex la nuginosa, Lysimachia 

 terrestiis, Mentha arvensis canadensis, Rhinanthus Crista-galli, 

 Equisetum pratense, Melampyrum lineare, Rumex Brittanica, 

 Epilobium densum, Agrostis hyemalis, Hypericum virginicum, 

 Iris versicolor and Ranunculus acris. 



On the south side where undisturbed conditions prevail, 

 the substratum is of the quaking bog type. The Carex-Juncu9 

 association consists of an almost pure growth of Carex horma- 

 thodes. 



The Calamagiostis-Sphagnum association of the Bog For- 

 mation follows. The principal species are Calamagrostis cana- 

 densis, Sphagnum, Vaccinium macrocarpon, and Drosera ro- 

 tundifolia. The secondary species are Viola blanda, Juncus 

 effusus, Hypericum virginicum, Osmunda cinnamomea, Smila- 

 cina trifolia, Galium trifidum, Carex Oederi pumila, Microstylis 

 unifolia, Eriophorum gracile, Solidago uniligulata, and Impatiens 

 biflora. Between this and the next association there is a very 

 gradual change, the substratum rising as development proceeds. 



The Alnus-Myrica association which is so important in the 



