Hill : Penohseot Flants. 297 



habitat unfavorable to most pbmts. In their northward jour- 

 ney two alternatives were open to these plants : either to 

 follow the ice to the Far ISTorth, where many of them are 

 widespread at the present time ; or to remain behind in 

 favorable situations, perhaps following the cold zone up the 

 mountains or lingering in cold bogs or on exposed shores. 

 The sub-arctic species in the eastern Penobscot Bay region 

 belong to this latter group of relic types, which were left 

 behind in the northward march and have been able to main- 

 tain themselves on the outer islands where conditions closely 

 approach those in more arctic regions. 



The second wave was composed of the species of the Hud- 

 sonian and Canadian zones, the plants of the great coniferous 

 forest areas. Although these plants may occur locally in 

 southern Xew England as relic species, in the Penobscot Bay 

 region they have an environment suited to their needs and 

 here they are widespread. 



The third wave to go northward consisted of austral rather 

 than boreal types. In all prol)ability this southern element 

 formed a conspicuous part of the pre-glacial flora and like 

 the northern forms returned to its old localities upon the 

 withdrawal of the ice. This third invasion (if it may be 

 so termed) oi the glaciated region was probably augmented 

 l)y other soutliern species Avhich pressed northward from the 

 original centers of distribution for this flora in the southeast. 

 This austral flora consisted of two elements which followed 

 separate channels in their northward migration : the Al- 

 leghanian phints going along the Appalachians and the 

 Mississippi and Ohio Valleys, the Carolinian plants follow- 

 ing the Atlantic Coastal Plain. 



The first of these elements — the Alleghanian — reaches its 

 best development on the basic soils of the more temperate 

 parts of the eastern United States^ a zone which in general 

 barely reaches Xew England. We have alreadv noted, how- 



