448 BOTANICAL GAZETTE 



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.) 



