174 PLANT RELATIONS. 



like bodies (see Figs. 153, 154). These floats are very 

 common among certain of the seaweeds, and are found 

 among higher plants, as the utricularias or bladderworts, 

 which have received their name from the numerous blad- 

 ders developed in connection with their bodies (see Fig. 

 155). 



Such adaptations as the above may be regarded as the 

 most prominent general adaptations. There are many 

 other special ones in connection with certain groups of 

 water plants which will be mentioned in considering the 

 societies. 



A. Free-swimming societies. 



129. 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. In such plants all of those mate- 

 rials which land plants obtain from the water of the soil are 

 obtained from the water in which they are submerged ; and 

 in the case of completely submerged plants the materials 

 usually obtained from the air are also supplied by the 

 water. Free swimming plants 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. 



130. 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 



