CHAPTEK XII. 



HYDROPHYTE ASSOCIATIONS. 



127. General character. — Hydrophytes are related to 

 abundant water, either throughout their whole structure 

 or in part of their structure. It is a well-known fact that 

 hydrophytes are among the most cosmopolitan of plants, 

 and hydrophyte associations in one part of the world look 

 very much like hydrophyte associations in any other 

 region. It is probable that the abundant water makes the 

 conditions more uniform. 



It is evident that for those plants, or plant parts, which 

 are submerged, the water affects the heat factor by dimin- 

 ishing the extremes. It also affects the light factor, in so 

 far as the light must pass through the water to reach the 

 chlorophyll-containing parts, as light is diminished in in- 

 tensity by passing through the water. Before considering 

 a few hydrophyte associations, it is necessary to note the 

 prominent hydrophyte adaptations. 



128. Adaptations. — In order that the illustration may be 

 as simple as possible, a complex plant completely exposed 

 to water is selected, for it is evident that the relations of a 

 swamp plant, with its roots in water and its stem and leaves 

 exposed to air, are complicated. A number of adaptations 

 may be noted in connection with the submerged or floating 

 plant. 



(1) Thin-walled epidermis. — In the case of the soil-re- 

 lated plants, the water supply comes mainly from the soil, 

 and the root system is constructed to absorb it. In the 

 case of the water plant under consideration, however, the 



