432 BOTANICAL GAZETTE [DECEMBER 



herbs gradually are superseded by a growth of Larix. This society 

 has been much disturbed by lumbering, and a large part of the 

 original area has been cleared. But there is good evidence to show 

 that the part of the basin filled with peat formerly supported a dense 

 covering of tamaracks. Where best developed and least disturbed, 

 it shows an undergrowth of Vaccinium corymbosum, Aronia nigra, 

 etc. As the other species are practically the same as at the lake to 

 be described next, they need not be enumerated here. In contrast 

 with most of the areas studied, the almost complete absence of 

 sphagnum is worthy of note. It is also important that the absence 

 of any gradation between the forest societies of the upland and of the 

 bog be kept in mind. 



On this lake, then, there are two divergent series of plant societies. 

 Starting with practically the same species, the one series leads us 

 on mineral soil through willows, maples, and elms to the oaks of the 

 surrounding forests ; the other, owing to the development of a floating 

 substratum, involves a very different set of shrubs and ends with the 

 tamarack. The former series therefore more closely approximates 

 the climatic type, while the latter is dependent upon edaphic factors. 



FIRST SISTER LAKE. 



This lake and its accompanying bog are located three miles west 

 of Ann Arbor in a glacial drainage valley. Its origin is probably 

 connected with the melting of a mass of stagnant ice after the 

 abandonment of the valley by glacial drainage. The surrounding 

 and underlying soil is a sandy gravel. At least a part of the western 

 side presents an original tamarack bog vegetation, and it is particu- 

 larly interesting in showing the results of competition between bog 

 plants and those of other habitats (fig. 6). The vegetation in general 

 presents a different phase of the bog societies, as compared with 

 West Lake. Especially to be noted are the dominance of cassandra 

 and sphagnum in the shrub zone, the absence of cattails and swamp 

 loosestrife as important members of the outer margin. The tamarack 

 zone is also raised somewhat more above the water level. 



Aquatics. With the exception of the shallow- water forms, the 

 lake is almost free of higher vegetation. Potamogeton lucens and 

 P. zosteraefolius occur sparingly. About the margin, however, 



