132 BOTANICAL GAZETTE [august 



Careful collections were made during the entire growing 

 season, and included practically all the species occurring at the 

 lake. They represent fort3-three families ; 5 per cent, of them 

 are natives of arctic regions; 65 per cent, are natives of North 

 America. 



3. Interzonal relations. — -A study of the different zones and 



■ 



their relations to one another shows that their positions are not 

 permanent, but that they are slowly encroaching upon the lake, 

 and as a result are filling it with the soil they produce. Each 

 society of plants is a more or less active soil-forming agency, 

 and accordingly as the vegetation progressively changes the 

 advancing zones leave a different soil from the one they found. 

 The vigorous growth of Potamogetoii zosteraefolms adds by its death 

 and decay a very small amount of humus to the fine clay soil 

 upon which it grows. Nuphar is a much more active soil- 

 forming agent; its strong leaves and petioles projecting above 

 the surface of the water {^fig. 2) catch and hold most of the twigs, 

 plants, and leaves which are blown into the margin of the lake, 

 until they become water-soaked and sink. The debris resulting 

 from the decay of the water lilies, added to that which they have 

 captured, all goes to building up the bottom of the lake. On 

 account of the limitation of Chara to the Nuphar zone, and the 

 consequent absence of any extensive beds, it is not an active soil- 

 forming agent by the production of marl, as it is in some of the 

 glacial lakes of Michigan.s 



In the intense competition among the plants of this crowded 

 zone there is a constant tendency to move out in the direction 

 of least resistance. Limited as it is on the landward side by less 

 favorable conditions, the zone must make its advance, if it makes 

 any, into deeper water, /. ^., into the Potamogeton zone, which it 

 appears to do just in so far as Nuphar is able to adapt itself to 

 the greater depth of water, or as the Potamogeton builds up the 

 bottom. At the other side of this zone there is a tension line 

 between the water lilies and the sedges. Whenever in any place 

 the bottom is not more than three or four inches below the sur- 



5 Davis C. A., Jour. Geol. 8 : 485 and 498. 



