THE PI.ANT WORI.D 185 



for portions at least of our own country. It is, therefore, very gratifying 

 to turn over the pages of Transeau's recent paper on the geographic dis- 

 tribution of the bog plant societies of North America,* in which a real 

 contribution to our knowledge of the origin of at least one class of our 

 plants is very satisfactorily presented. 



The present distribution of bogs extends from the mouth of the St. 

 I^awrence River, on the east, to the Great L,akes, and northwestward into 

 the Mackenzie basin. The present distribution also of such plants as the 

 larch and birch, and other bog plants, indicates that in preglacial times 

 their distribution was circumpolar. How the bog plants came to occupy 

 their habitats of to-day is accounted for of course by the encroachment 

 of the ice in the glacial period driving before it whatever plants could 

 endure its cold, and killing what could not do so, and in post-glacial 

 times by the retreat northward of whatever plants were most hardy. In 

 this last connection it happened that many plants anciently associated 

 together were again thrown into each other's company, so to speak, but 

 not all, and this forms the center of interest in Transeau's paper. 



In certain regions where there are bogs, as in northern Ontario, which 

 are surrounded by coniferous forests of pine, spruce, and fir, the ancient 

 relations of the bogs to the forests are maintained. In northern Indiana, 

 northern Ohio, and southern Michigan, however, the case is different. 

 The zonal succession of plants in any bog is the same, and the bog plants 

 are for the most part the same as farther north ; but immediately outside 

 the bogs, when conifers are absent, they show no order of succession to 

 forest societies. How may this anomaly be explained? 



Briefly, it is shown that in the southern extension of the coniferous 

 forests, and of the plants which make up the present-day bogs, because 

 of the encroachments of the ice sheet, and in the retreat north of the ice, 

 and the retrogression of these plants, there was a division in the coni- 

 fers. One portion occupying the west and the other the eastern part of 

 the ice sheet, while the bog plants occupied favorable positions along the 

 whole ice front. In the retrogressive movement of the plants, the bogs 

 to the east and the west were therefore surrounded by coniferous forests, 

 as they are found at the present time. But the bogs between these arms, 

 those in northern Ohio, northern Indiana, and southern Michigan, be- 

 came surrounded by the oaks, hickories, maples, ash, and elm forms, 

 which were strangers to the ancient bog plants. 



W. A. Cannon. 



LIVERWORTS IN DRY REGIONS. 



It has become increasingly felt that the very interesting group of 

 plants, the Liverworts, are not properly appreciated bj^ teachers. These 



* Botanical Gazette, 36: 401, 1903. 



