138 BOTANICAL GAZETTE [august 



rhizomes and the dense covering of long grassy leaves ; the 

 seeds are heavy and tend to fall on the territory already occu- 

 pied by the parent plant. 



Tension lines are the strongest where the environment is 

 most equally favorable for each of two different societies; e, g., 

 the boundar^^ between the Nuphar and Carex zones is the scene 

 of a severe struggle because the conditions are almost equally 

 suitable for both societies, but where the Nuphar zone borders 

 immediately upon the Salix zone the tension line is weak. 



The ability to compete successfully seems to depend largely 

 upon the extent to which the plants are massed in solid ranks ; 

 such a plant society is generally able to force out other plants 

 from the territory available for occupation. The inherent vigor 

 and hardiness of these plant societies due to their northern origin 

 must also be reckoned in attempting to account for the success 

 with which they compete with other forms. 



v. CONCLUSIONS. 



The work as presented in the foregoing pages is the result of 

 an attempt to study the actual operations of known ecological 

 factors and leave a record of existing conditions in the life his- 

 tory of a glacial lake and its flora. Little attempt has been 

 made to discover new factors or to modify accepted ideas of 

 ecological principles. Most of the statements made can be veri- 

 fied by actual observation; some must be inferred from present 

 conditions. If carried farther, the work should consist in veri- 

 fying certain statements either by subsequent observation or by 

 laboratory experiment. The results of my study may 

 summed up as follows: 



be 



1. The comparative scarcity of terrestrial plants is a result 

 of the former exclusively hydrophytic conditions which gave 

 aquatic species an advantage which has since been maintained 

 by all other hydrophil species. 



2. There is a striking predominance of northern species, 

 undoubtedly the result of the glacial invasion of recent geologi- 

 cal times, and of conditions which tend to reproduce boreal 

 environment. 



