626 RELATION TO ENVIRONMENT. 



zone of bulrushes (3); 2d, mid zone, zone of pond-lilies (4); 

 3d, submerged zone, zone of pondweeds (5), zone of bass- 

 weed, Chara (6). We see then that in different localities these 

 three different zones may be represented by different species, 

 but the vegetation types in the different zones are the same. 



1114b. The free-floating forms, like the duckweed, riccias, 

 Salvinia, etc., are found on the surface in the first and second 

 zones, floating between the stems and leaves. These are buoyed 

 up by large intercellular air-spaces. Here occur also floating 

 mats of algae, like Spirogyra, Vaucheria, (Edogonium, etc., 

 which are buoyed up by the bubbles of gas entangled in the 

 meshes of the mat, or algaelike Cladophora, which are attached 

 to the reeds or bulrushes, or Coleochaete, which forms sessile 

 disks on the same supports. Then there are the numerous 

 free-swimming green algae floating in the meshes of thread- 

 like alga mats, or swimming on the surface, or resting on larger 

 plants, or on the bottom in shallow places, as well as numerous 

 blue-green algae, diatoms, bacteria, and the water-mold fungi. 



1115. River, or fluvial, plant societies. The plants which 

 grow in the running water of streams constitute a rather distinct 

 type from those in quiet water. Because of the continued 

 movement of the water, sometimes rapid or violent, the plants 

 are attached to the rock or rooted well in the ground, and have 

 flexile stems, with only slight leaf development, or the leaf may 

 not be well differentiated from the stem. This type of stem 

 is suited to the waving movements produced by the flowing 

 water. The river-weed (Podostemon) is a good example of 

 one of the seed-plants which has become so changed in its adapta- 

 tion to an aquatic life that it strongly resembles certain algae ; the 

 mosses are represented by Fontinalis, and the algae by Clado- 

 phora and Lemanea. 



Where the water is shallow or quiet, as in pools or ponds 

 having an open connection with the streams, the societies are 

 of the pool or pond type. Semi-aquatics (bulrushes, reed- 

 grasses, arrow-leaves, pickerel-weeds, etc.) often develop along 

 the shore of streams or in shallow water in midstreams, where 



