448 BOTANICAL GAZETTE [DECEMBER 
Fraxinus americana, F. pennsylvanica, Acer rubrum, Ostrya vir- 
giniana, Tilia americana, and Liriodendron tulipifera are associated. 
The undergrowth consists of Solidago patula, S. neglecta, Aster 
lateriflorus, Mitella diphylla, Euonymus obovatus, Viola pubescens, 
Agrimonia hirsuta, Cornus florida, C. candidissima, Eupatorium 
perfoliatum, Rosa Carolina, Viburnum Lentago, Juniperus communis, 
and Spiraea salicifolia. The substratum is almost entirely occupied 
by mosses, including Hypnum fluitans, H. Schreberi, H. Blandovii, 
H. roseum, Thuidium recognitum, and Climacium americanum. 
On the farm of James Barton (SW. % Sec. 2, Lyndon Tp.) the 
black ash, red maple, and American elm have replaced a former 
growth of tamaracks and black ash. 
In a previous publication (55: p. 403) the writer called attention 
to the absence of a genetic relationship between the bog plants 
and the surrounding vegetation in southern Michigan. This was 
explained on the basis that the bog vegetation is a relict of former 
climatic conditions; that it has a genetic relationship with the 
conifer forest formation of northeastern North America, as shown by 
studies in northern Michigan and Pennsylvania, and that in this 
region it has been surrounded by a more southern flora whose center 
of distribution is the southeastern United States. Consequently no 
order of succession between the tamarack and the oak floras is to 
be expected. 
When, however, bog areas are cleared or their normal development 
disturbed, such trees as the black ash, white ash, red maple, and elm 
replace the tamarack, and a definite order of succession is established. 
It was also maintained that present bog habitats are continuations 
of similar habitats which came into existence when a colder climate 
prevailed than at present. More recent observations tend to confirm 
and strengthen this statement. : 
The dominance of bog and swamp plants respectively in adjoining 
areas is to be explained largely by the time when the areas came to 
Support their present ground vegetation. If the habitat has existed 
undisturbed since the time when a colder climate prevailed, the bog 
plants will be dominant. If it came into existence in recent times, 
or has been disturbed, it will be dominated by swamp species. 
(To be concluded.) 
