76 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



from A. purpurea; Aureolaria Pedicularui (L.) Raf. (Dasy stoma 

 Pedicularia) has given rise to the variety ambigens (Fernald) of the 

 Middle West. In other cases, the parent species, existing through the 

 glacial period south of the ice margin, has followed both routes of 

 migration and in so doing has become segregated into a pair of closely 

 related forms, one of which took the southern route leading northward 

 from the Ohio valley and the other the northern route along the Alle- 

 ghenies and the Great Lakes. Thus Trillium hivale, T. declinatum, t 

 and Cynoglossum virginianum of the southern portion of the Middle 

 West are paired with Trillium undulatum, T. cernuum, and C. boreale 

 respectively of the northern portion. 



Changes in the prairie flora took place at the same time. Distinctly 

 western species withdrew entirely from the eastern extension or, like 

 Cristatella. Jamesii and others already mentioned, left relics behind in 

 especially favorable xerophytic habitats. Relic colonies were isolated 

 throughout the region, many of which still persist. These were sub- 

 ject to slow but persistent succession by the forests ; many must have 

 been obliterated, while others were greatly reduced in size. 



Prairie and Boreal Relics. — The nature and location of these prairie 

 relics and of the boreal relics in the same area may yet give a clue 

 for the better understanding of the period and route of the northern 

 migration of deciduous forests. There is in general a great difference 

 in environment between forested and unforested habitats, such as a 

 prairie and either a coniferous or a deciduous forest. Differences also 

 exist between the two types of forest, but these are of less importance 

 and to many species of little significance. Boreal relics, therefore, 

 have not commonly persisted in the prairie, but remained in definite 

 colonies. Similarly, prairie relics have been confined largely to colonies 

 and have not persisted as scattered plants within the invading deciduous 

 forests. 49 So, wherever the deciduous forests have succeeded the coni- 

 ferous forests directly, numerous boreal species persisting to the present 

 time bear evidence of the fact. These are of common occurrence in 

 southeastern Michigan and indicate that the prairies were not uni- 

 versal there, but are rare or lacking in Illinois and Indiana, showing 

 that the coniferous forests had disappeared before the advent of the 

 deciduous trees from the south. Relic colonies, as has been pointed 

 out before, remain in habitats not necessarily favorable to themselves, 

 but unfavorable to the invaders. Consequently, prairie and boreal 

 relics sometimes occupy the same station, into which they have been 



o Vestal, Arthur G. Local inclusions of prairie within forest. Trans. Illinois 

 Acad. Sci. 11: 122-126. 1918. 



