280 PRACTICAL COURSE IN BOTANY 
very thin. In some of the pipeworts (Eriocaulon), the ells 
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- 
Tia. 417.— A pioneer 
swamp colony of cattails. 
(From a photograph by 
Harry B. Shaw, U.S. Dept. 
Agr.) 
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, Fic. 416. Seaweed 
like those of the water (sa7gesswm) with blad- 
derlike floats. 
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. 
