REED ON ECOLOGY OF A GLACIAL LAKE. 45 



sedgos have exterminated the -walci- lilies and now the willows have in 

 tnrn all bnt externiinaled the sedges. 



My stndy as far as it has been cairied shows at this lake the fol- 

 lowing fads. 



1. The coniitai-ative scarcity of terrestrial plants, already explained. 



2. A striking predominance of northein foinis, undoubtedly the 

 result of the glacial in\asiou of recent geological time which forced 

 northern forms far to the southward. 



3. An interaction of organic and inorganic agencies that has caused 

 and is now causing an unmistakable advance of plants into the water. 



4. A struggle in each zone, severer on the landward side than on 

 the lake ward side of that zone. 



5. A marked tendency for plants that are engaged in this severe 

 struggle to mass themselves together in solid ranks. 



Botanical Laboratory, University of Michigan. 



