lTJr PLANT RELATIONS. 



like bodies (see Figs. L53, 154). These floats are very com- 

 mon among certain of the seaweeds, and are found among 

 higher plants, as the utricularias or bladderworts, which 

 have received their name from the numerous bladders de- 

 veloped in connection with their bodies (see Fig. 155). 



129. The two groups of societies. — The hydrophyte so- 

 cieties may be put into two great divisions. True hydro- 

 phytes are those in which the contents and temperature of 

 the water are favorable to plant activity ; while xeroplnjtic 

 hydrophytes are those in which the contents and tempera- 

 ture of the water are unfavorable to plant activity, and the 

 structures of the plants are adapted to reduce transpiration, 

 resembling in this feature the structures displayed by the 

 true xerophytes (see §155). 



I. Tkue hydrophytes. 

 A. Free-sivimming societies. 



130. Definition. — In these societies there is the largest 

 exposure to water, and no relation at all to the nutrient or 

 mechanical support of the soil, the plants being completely 

 supported by the water. They may be either submerged 

 or floating, and they are free to move either by locomo- 

 tion or by water currents. Two prominent societies are 

 selected as types. 



131. The plankton.— This term is used to designate the 

 minute organisms, both plants and animals, which are 

 found in the water. The plankton is composed of indi- 

 viduals invisible to the naked eye, but taken together they 

 represent an enormous organic mass. The plankton socie- 

 ties are especially well represented in the colder oceanic 

 waters, but they are not absent from any waters. Among 

 the most prominent plants in these societies are the dia- 

 toms. Diatoms are minute plants of various forms, and all 

 have a wall very full of silica. This makes their bodies 



