1010 MINNESOTA BOTANICAL STUDIES. 



characterize creviced rock and a distribution of plants like that 

 of creviced rock comes to exist where originally there was a 

 pure talus type of distribution. 



VI. Talus and soil formations. Upon the humus masses of 

 the interstices a group of plants become established such as 

 habitually occupy rock crevices. The characteristic species 

 are Spiraea salicifolia, Houstoniapurpurea,Heucheraamericana, 

 Campanula rotundifolia and Ambrosia artemisiae folia. But if 

 the accumulation of soil has progressed to such an extent that 

 larger plants can gain a foothold, one finds upon the talus slope 

 abundant forests of pine and hardwood timber. One of the 

 favorite habitats of Pinus divaricaia throughout the Lake of 

 the Woods region seems to be the talus slopes with soil admix- 

 ture, and they grow almost as vigorously when established as 

 crevice plants on the rock shore, where Pinus strobus is also 

 abundant. 



VII. Talus and sand formations. Where they occur these are 

 most clearly a mixed type of distribution. I have seen on 

 Shoal lake masses of talus-blocks, three or four feet cube 

 covered with Hypnum and Polypodium, lying imbedded, as it 

 were, in a sandy beach tract bearing Elymus and Prunus pumila. 

 No doubt the only proper explanation of such a remarkable 

 juxtaposition of plants is the assumption that the low talus slope 

 was first established and that afterwards the sand was washed 

 in by the waves. Thus a composite formation was developed. 

 Extremely old talus with small, weathered blocks and sand 

 infiltration can not ecologically be distinguished from boulder 

 drift shore. 



A view of talus with formation of PopuJus tremuloides, com- 

 ing under Group V of the classification is shown in Plate 

 LXXVIII, to the right in the background. This is a charac- 

 teristic azonal talus population on an area with considerable 

 soil substances infiltrated by drainage from the higher back 

 country. 



A variety of mixed talus types with coarse talus as the foun- 

 dation might be discussed with profit — for example, the buried 

 talus-blocks of some of the northern islands, where a general 

 coating of soil covers the talus masses but lies deeper over the 

 interstices than over the rock fragments, or the talus-blocks 

 imbedded in morassic shores and the influence of their pres- 

 ence upon the formation of Scirpus or Salix belts — but the 

 problems are of such endless complexity that they can hardly 

 be entered upon in this preliminary sketch. 



