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 aérated 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 
