280 



PRACTICAL COURSE IN BOTANY 



FIG. 410. Seaweed 

 (sargassum) with blad- 

 derlike floats. 



very thin. In some of the pipeworts (Eriocaulori) , the cells 

 are so large as to be easily seen with the unaided eye. If 

 you can obtain one of these, examine it 

 with a lens and notice how very thin the 

 walls are. Water plants also contain nu- 

 merous air cavities, and often develop 

 bladders and floats, as in the common blad- 

 derwort and many 

 seaweeds. The leaves 

 of submerged plants 

 are usually either 

 greatly reduced in size 

 or very much cut and 

 divided, while the ones 

 that rise above water, 

 like those of the water 

 lily, are apt to be large 

 and entire, to facilitate floating, and have 

 stomata on their upper surface. Float- 

 ing plants sometimes form such large 

 colonies as to be a serious menace to 

 navigation. Well-known instances of 

 this are the water hyacinths in the St. 

 John's River, Florida, and the vast 

 formations of swimming gulfweed from 

 which the Sargasso Sea takes its name. 

 319. Swamp societies. -- These in- 

 clude what may be regarded as the am- 

 phibious portion of the hydrophyte 

 group. They compose the sedge and 

 cattail bogs, reed jungles, moss and fern 

 thickets, forests of cypress, magnolia, 

 black gum, pine, tamarack, balsam, and 

 the like. The sedges and cattails are the pioneers of these 

 societies, which tend constantly to encroach upon the water 

 and so prepare the way for the advance of other colonists. 



FIG. 417. A pioneer 

 swamp colony of cattails. 

 (From a photograph by 

 Harry B. Shaw, U.S. Dept. 

 Agr.) 



