WATER PLANTS AND ANIMALS 



proportions vary in different waters and under diverse condi- 

 tions. Water absorbs oxygen from the air so that near the 

 surface it is more abundant than in the lower layers. Proof 

 of this can be seen in any bowl of goldfish. Whenever their 

 water supply becomes low in oxygen the fish swim about 

 near the top where there is more of it to be found than in the 

 lower levels. 



In their respiration, green plant cells take in oxygen and 

 give off carbon dioxide, just as animals do. But in photosyn- 

 thesis or food making they take in carbon dioxide and give 

 off oxygen. Photosynthesis can go on only in the light and 

 during the daytime this process is so vigorous that its results 

 overshadow those of respiration. Thus during the day the 

 net result of both processes is that plants give off some car- 

 bon dioxide but much more oxygen than they take in, while 

 at night, when respiration alone continues, they give off 

 carbon dioxide only. At night the little crustaceans which 

 live below the surface waters come up to the top to breathe 

 the abundant oxygen which they find there as a result of the 

 day's plant activity, and at the same time they give off carbon 

 dioxide which will be used by the green plant cells next day. 



The amount of the carbon dioxide in water is greater in 

 winter than it is in summer, because plants are at a lower 

 ebb of life and do not use so much of it. But spring sunshine 

 starts them into greater activity, which results not only in 

 their increased demand for carbon dioxid-e, but in the pro- 

 duction of oxygen at a period when the animals will need it. 



Light in the water. — Water is transparent, but it is well known 

 that even the clearest water is far less transparent than air. 

 It lets the sunshine in upon green plants which can prepare 

 their own food from non-living materials only when they 

 have enough of it. The vast hosts of diatoms and desmids, 

 other simple plants, and even many of the higher ones, could 

 not live on pond and stream bottoms as they do if light did 

 not penetrate through many feet of water. Studies of the 



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