﻿196 BOTANICAL GAZETTE [march 



lichen societies varies with surrounding ecologic conditions. The 



most apparent of determining factors are the age of the talus, 



amount of light, shade, and moisture, nature of surrounding vege- j 



tatlon, and edaphic conditions respecting presence or absence of 



humus upon the lichen-bearing rocks. The age of a given talus is 



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by no means an easy matter to determine. The tali bearing the 

 plant societies under consideration are all surely postglacial, 

 for the old shore lines of Lake Superior extend to or above their 

 bases. So much for a maximum age; now for a minimum. The 

 writer does not know of any igneous or metamorphic rocks, such 

 as those bearing the lichen societies now under consideration, 

 exposed within the last quarter of a century and bearing any con- 

 siderable number of foliose or fruticose lichens, if indeed any 

 kind whatever. Lichens do take possession of burned-over 

 ground and old stumps to some extent in half the time indicated 

 above, but not the more resistant rocks. Relative age of tali may 

 of course be ascertained to some extent by the age of the trees 

 found growing upon them ; but lichens do not gain a footho 

 on the hard Algonkian rocks rapidly, and the changes in lichen 

 population go on so slowly that little can usually be learned as 

 to exact time involved in the establishment of the lichen popula- 

 tion of a given talus. However, tali may be found that are 

 comparatively very young and totally devoid even of the pioneers 

 among rock-inhabiting plants, the crustose lichens ; and every 

 gradation of course may be found between this condition and 

 old tali so overgrown with trees and covered with mosses, lichens, 

 and humus that the talus blocks are in places not easily detected. 

 These old tali, for reasons stated elsewhere in this paper, sup- 

 port lichen societies composed for the most part of foliose lichens 

 or fruticose forms, especially the Cladonias, and more com- 

 monly these fruticose species in ombrophytic associations. Thus 

 relative age, really of greater interest in ecologic consideration 

 of such slow-growing plants as lichens than absolute age, may be 

 easily ascertained for lichen societies growing upon tali whose 

 conditions as to light or shade, moisture, and surrounding vege- 

 tation are similar. 



In the field work it was not possible, in the limited time, to 



