458 A TEXTBOOK OF GENERAL BOTANY 
Marine vegetation. Inseas, floating microscopic alge are numer- 
ous, and among them diatoms are prominent. ‘These microscopic 
floating plants are the chief source of food for marine animals. 
Macroscopic marine plants are either alge or angiosperms, 
the algze predominating. The angiosperms are grasslike plants 
but not true grasses, and grow rooted in sandy or muddy bottoms 
of calm, shallow bays. Macroscopic alg are most abundant on 
rocky coasts; for the most part they are attached to the rocks, 
but they may be attached to other objects, such as shells or other 
alow. Algze occur not only below the level of low tide but be- 
tween tide levels, and in cold temperate regions on rocky coasts 
the development of algz between tide levels is very luxuriant. 
Macroscopic alge range in size from small plants which are hardly 
visible to large ones many meters in length. 
Fresh-water vegetation. In fresh water, as in salt water, micro- 
scopic floating algve are very abundant, and among them diatoms 
are numerous. The macroscopic vegetation differs very greatly 
from that of salt water, as flowering plants predominate over the 
algve, while the alge are small as compared with those of salt 
water. The macroscopic plants may be divided into three gen- 
eral classes: submerged plants, rooted plants with floating leaves, 
and floating plants. 
_ Ingeneral, plants which are entirely submerged grow in deeper 
water than do those with floating leaves. The conducting and 
strengthening tissues are very poorly developed in submerged 
plants. As they float in the water, they do not need strengthen- 
ing tissue to the same extent that land plants do; and as they 
do not transpire, there is no necessity for any considerable develop- 
ment of water-conducting tissue. The leaves are thin and in 
many cases are finely divided, thus exposing large surfaces for 
the absorption of materials. 
Plants with floating leaves have a conspicuous development 
of air spaces (Fig. 40). These serve as an aérating system for 
the diffusion of oxygen from the leaves to the roots. Moreover, 
the air spaces in the leaves are of assistance in enabling the leaves 
to float on the surface of the water. 
