﻿50 BOTANICAL GAZETTE [January 



were probably under water long after the ice itself had disap- 

 peared. It is probable that the pine-heath ^° group, which today 

 reaches farthest north, reached well toward the glacier front dur- 

 ing the ice age. And at the end of that age, the ice in its retreat 

 was probably followed northward by vegetation, the pine-heath 

 society leading the way. Near the ice margin the soils were 

 probably raw, absolutely without humus, subject to great drought 

 in summer and to extreme cold in winter. These are just the 

 conditions in which the pine-heath group is found today in north- 

 ern Michigan. It is probable that at one time they occupied all 

 of Kent county, but the climate became warmer and more equa- 

 ble with the farther retreat of the ice, and the growth of the 

 hardy pines, etc., produced a little humus. Their roots fixed the 

 soil so that erosion was less rapid and perhaps the sassafras and 

 the white and red oaks and the whole of our society V gradually 

 crept in, occupying the better part of the ground along with the 

 pines and heaths. Then, as the soil improved, the oaks became 

 more and more numerous, and the pine seedlings could not 

 develop on account of the shade. The pines thus became fewer in 

 the south and the oaks at last predominant. This would be the 

 stage of society IV. But the process of working over the soil 

 continued, though perhaps the ice-sheet had shrunken by this 

 time nearly to its present size, and humus continued to 

 accumulate in favored places; the hickories, maples, and beeches 

 of Ohio and Indiana spread continually northward over every 

 suitable stretch of soil, as fast as it was made fit for them. When 

 the maples and beeches reached maturity in the richest parts of 

 the oak and hickory forest, the oaks and hickories probably 

 ceased to mature. Seedlings of these trees fail to develop well 

 under maples and beeches, possibly on account of the dense 

 shade. Thus the stage of society I might be reached. 



In such a northward advance, the plant societies would not 

 progress in uniform lines. On the contrary, there would be 

 many mixed areas, and the advance would often be almost 



'°This group comprises, besides several pines, two species of juniper, bearberry, 

 hairbell, bracken, and several of the other forms found in our society V, Cf. WHlT- 

 FORD, H. N. The genetic development of the forests of northern Michigan; a study 

 in physiographic ecology. BOT. Gaz. 3I : 289-325. 1901. 



I 





