. Biology; the story of living things. THE STAGE SETTING 21 Life in the Water Plants are adapted for lite in water hy a mucli reduced root system, by leaves which either float, are ribbonlike, or are finely divided with air passages and air spaces. The latter spaces help buoy up the plant and also allow for an accumulation of oxyu-en and carbon (hoxidc Green coloring matter is abundant, such plants being better fitted for vegetative propagation than reproduction by flowers and fruits, as is shown by their numerous horizontal and thickened stems. In general, aquatic plants are restricted to rel
. Biology; the story of living things. THE STAGE SETTING 21 Life in the Water Plants are adapted for lite in water hy a mucli reduced root system, by leaves which either float, are ribbonlike, or are finely divided with air passages and air spaces. The latter spaces help buoy up the plant and also allow for an accumulation of oxyu-en and carbon (hoxidc Green coloring matter is abundant, such plants being better fitted for vegetative propagation than reproduction by flowers and fruits, as is shown by their numerous horizontal and thickened stems. In general, aquatic plants are restricted to relatively shallow water, many species being found floating near the surface. Animals, usually locomotor and having definite adaptations for movement in the water, have a much wider ^•ertical range. The bodies of most fishes are more or less streamlined, and protected by mucus which covers the backward-pointing scales, their fins being placed where they offer the least possible resistance to the medium. In some animals, the limbs are transformed into flii)pers, while in lower types, such as protozoa, threads of living matter, cilia, are used as whiplike organs of locomotion. Since the oxygen content of water is only about 1 per cent as against over 20 per cent in air, we find special adaptations for taking in oxygen. These structures are usually in the form of gills, delicate struc- tures which will be discussed more fully later. The water forms an ideal medium for vast numbers of small, free-swimming, or float- ing organisms, the plankton. Oceans and lakes swarm with them. Every small pool has its plankton, and even rapidly flowing waters will disclose some of these tiny organisms. In certain tested regions in the Atlantic, plants form about 56 per cent and animals 44 jx-r cent of the total plankton. The flora consists mostly of diatoms, bac- teria, and many forms of algae, while the fauna includes numerous dinoflagellates and other one-celled animals, eggs of fish, molluscs,.
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