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, 



Diatoms have various forms aFid may !><• 

 colonial as well as unicellular. riicrc arc 

 probably I. "),()()(» species known. 



