174 PLANT RELATIONS. 



like bodies (see Figs. 153, 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 

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



129. The two groups of associations. — The hydrophyte 

 associations may be put into two great divisions. True 

 hydrophytes are those in which the contents and tempera- 

 ture of the water are favorable to plant activity; while 

 xerophytic hydrophytes are those in which the contents 

 and temperature of the water are unfavorable to plant 

 activity, and the structures of the plants are adapted to 

 reduce transpiration, resembling in this feature the struc- 

 tures displayed by the true xerophytes (see §155). 



I. True hydrophytes. 

 A. Free-swimming associations. 



130. Definition. — In these associations 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 locomotion 

 or by water currents. Two prominent associations are 

 selected as types. 



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

 minute organisms, both plants and animals, that are found 

 in the water. The plankton is composed of individuals 

 invisible to the naked eye, but taken together they repre- 

 sent an enormous organic mass. The plankton associa- 

 tions are especially well represented in the colder oceanic 

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

 the most prominent plants in these associations are the 

 diatoms. Diatoms are minute plants of various forms, and 

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



