1902] SURVEY OF THE HURON RIVER VALLEY 137 



plants on the landward side of the zone are overshadowed by 

 the taller Salix and Betula plants, while on the other side there 

 is almost nothing to diminish the supply of light. In general 

 the plants of a zone which is successful in displacing another 

 zone have a greater capacity for the absorption of light than 

 those plants which they displace. One of the chief advantages 

 of Nuphar over Potamogeton is to be found in the fact that it 

 sends its leaves up to the surface of the water where they 

 receive the full intensity of the light instead of the diminished 

 amount which is able to penetrate the cloudy water to the sub- 

 mersed species. The lack of light caused by the cloudiness of 

 the water seems to be the only reason for the absence of vegeta- 

 tion from the central deep portion of the lake. 



The amount of heat which the different aquatic plants receive 

 also varies with their distance from the surface of the water; 

 however, they seem perfectly able to adapt themselves to these 

 differences. 



{d^ Biotic factors. — Fully as important as any of the fore 

 gomg factors are the conditions imposed upon plants by the 

 presence of other plants or animals; conditions to which they 

 must adapt themselves if they survive. The competition 

 between different societies of plants is mainly directed toward 

 obtaining light and food. The former has been briefly consid- 

 ered ; there are certain biological characteristics which are 

 favorable for obtaining the other. Perennial plants are the most 

 successful in the interzonal struggle; they are usually adapted 

 to xerophytic conditions by their rhizomes, waxed epidermis, 

 and hairy surfaces. There are also special advantages in their 

 manner of reproduction. The possession of underground root- 

 I stocks is one of the best adaptations to enable a plant to hold 



Its own against its competitors; young plants are thus sustained 

 by the parent plant until they are fully capable of maintaining 

 an independent existence, and prompt occupation of a given 

 area is easier. The advantage of the willow lies in the great 

 number of seeds produced and the good means of disseminating 

 them. The advantage of the sedges is in the thick mat of 



