SUCCESSIONS IN BOGS 27 



an increase in the oxygen supply, in available water, and a change 

 in the character of soil organisms and their reactions. The vege- 

 tation passes to the mesophytic type of plant formations, which 

 may be regarded as the culminating succession in the progres- 

 sive series occurring on the same habitat under present climatic 

 conditions. 



It is an intricate task to discover a guiding principle and to 

 arrange an otherwise confusing diversity into a genetic and dyna- 

 mic system. But the recognition that the nature of a lake and 

 bog environment is constantly selective, and that associations of 

 plants succeeding one another are each characterized by a definite 

 physiognomy in response to their dependence upon the supply 

 of available water under environmental conditions essentially sim- 

 ilar otherwise, renders this classification more significant. 



In separating associations only such have been indicated for 

 each succession as were recognized with sufficient clearness to 

 be the controlling vegetation in a habitat comprising elements 

 of stability in the sum of habitat life-relations. 



I. OPEN WATER SUCCESSION 



1. Plankton associations. Water basins begin with the plank- 

 ton association. Bacteria, diatoms, single-celled algae and vari- 

 ous other forms are the organisms floating in the open water of 

 lakes and partly filled peat bogs. In vertical range they are con- 

 fined largely to the surface of the water. Saprophytic forms 

 occur at the bottom. The correlations of light, temperature and 

 aeration with distribution, vertical zonation, and seasonal changes 

 in quantity and quality of organisms, and the process of "self- 

 purification" of shallow areas need further investigation. 



II. MARGINAL SUCCESSION 



1. Submerged associations. In lakes overlying the limestone 

 area of the western half o( Ohio Cham takes a prominent place. 

 Where Chara is not present there is but little change from the 

 innermost microphytic region to the belt of submerged plants 

 nearer the margin. Almost all lakes and peat-depositing water 



