HYDROPHYTE SOCIETIES. 



173 



out of water submerged plants which seem to be upright, 

 with all their parts properly spread out. When removed they 

 collapse, not being able to support themselves in any way. 

 (o) Development of air cavities. — The presence of air in 

 the bodies of water plants is necessary for two reasons: (1), 





^i 



:^ 



-"Jv- 





Fig. 155. Bladderwort, Bhowing the numerous bladders which float the plant, the 

 finely divided water leaves, and the erect flowering stems. The bladders are also 

 efilective " insect traps," Utrici'lana being one of the " carnivorous plants." 

 — .\fter Keener. 



to aerate the plant ; (2), to increase its buoyancy. In most 

 complex water plants there must be some arrangement for 

 the distribution of air containing oxygen. This usually 

 takes the form of air chambers and passageways in the 

 body of the plant (see Pigs. 87, 88, 89, 156). Of course 

 such air chambers increase the buoyancy of the body. 

 Sometimes, however, a special buoyancy is provided for 

 by the development of regular floats, which are bladder- 



