HYDROPHYTIC SOCIETIES 505 



merged, standing in the water, or floating on the surface of the 

 water. As previously stated, they include plants from various 

 phylogenetic groups. Many are Thallophytes, as the Algae 

 illustrate, while some, like the Pond Lilies, Duckweeds, Pond- 

 weeds, Eelgrass, and others, are Angiosperms. Not only repre- 

 sentatives of the lowest and highest divisions' of the Plant 

 Kingdom, but also some Bryophytes and Pteridophytes are 

 included in these societies. 



The hydrophytes are adapted in various ways to living in the 

 water. In the Algae the unicellular and filamentous bodies with 

 all cells thin-walled afford the maximum amount of surface for 

 absorbing gases and minerals from the water and for absorbing 

 the light that reaches them. The more massive Algae are com- 

 monly so anchored that they are aerated through wave action, 

 and many are provided with floats or air chambers whereby they 

 float near the surface where there are more gases and light than 

 at greater depths. The more complex Hydrophytes, such as 

 the Seed Plants, that live chiefly submerged in the water have a 

 thin-walled epidermis, so that all parts of the plant can absorb, 

 and water-conducting tissues are feebly developed. Since they 

 depend upon the buoyant power of the water for support, the 

 root system is commonly reduced or even wanting, and their 

 mechanical tissues are not so well developed as those of Seed 

 Plants that live on land. Usually such plants collapse when 

 taken out of the water. Some, like the Pond Lilies, raise their 

 leaves to the surface of the water where they receive good 

 light, while others, as the Pondweeds and Eelgrass illustrate, are 

 wholly submerged and are able to get along with the little light 

 that reaches them. The submerged forms even bear their flow- 

 ers under water. Among the Hydrophytic societies there are 

 the free-swimming, pondweed, and swamp societies. 



The free- swimming societies are made up of such plants as 

 the Diatoms, Algae, Duckweeds, and other plants which float 

 in stagnant or slow-moving water. 



In the pondweed societies the plants are anchored, but their 

 bodies are submerged or floating (Fig. 455). To this society 

 belong the Water Lilies, Pondweeds, Water Ferns, Marine Algae, 

 some fresh-water Algae, and some species of Mosses. 



Swamp societies consist of water plants which have leaf-bear- 

 ing stems reaching above the surface of the water. Some typical 



