596 TEXTBOOK OF BOTANY 



up mud; and with the increased turbidity of the water resulting in 

 decreased hght penetration, the algal population may decrease. 



Plant population. Most flowering plants cannot survive complete sub- 

 mergence, and only a limited number can thrive with their roots and 

 lower stems permanently covered with water. Besides the rooted-seed 

 plants along the shore, the aquatic plant population consists of the 

 countless numbers of minute floating and submerged algae which are 

 described in the next chapter. 



All bodies of fresh water contain bacteria and lesser numbers of 

 parasitic and saprophytic fungi. Bacteria are known to occur in the 

 open water of lakes and ponds at all depths, and are often exceedingly 

 abundant on the bottom deposits. They are usually not harmful to man. 

 There are bacteria of decay, of nitrification, and of nitrogen-fixation as 

 well as those truly parasitic within other plants and animals. Their 

 abundance is influenced by light, by sudden changes in temperature, 

 by the amount of sedimentation, and by certain animals, such as the 

 protozoa and rotifers, that feed on them. Direct counts have shown that 

 there may be from a few thousand to several million bacteria in a cubic 

 centimeter of lake water. By way of comparison, a cubic centimeter of 

 a moist, fertile soil may contain several billion bacteria. 



RIVERS AND STREAMS 



The temperature of the water in rivers and streams is fairly uniform 

 from top to bottom. The greatest extremes of turbidity occur, varying 

 from the muddy Missouri and Yangtze rivers to the nearly transparent 

 water of "spring-fed" mountain brooks. Light penetration is diminished 

 greatly in muddy waters. Suspended silt may decrease the light in the 

 first inch of water to 10 per cent of that at the surface. Oxygen deficiency 

 does not usually limit the growth of aquatic plants in unobstructed 

 streams. The oxygen content varies between day and night and may 

 at times even exceed the saturation point in streams with abundant 

 submerged plants. 



Stream water is usually moderately alkaline or neutral; only rarely 

 is it as acid as the water in bog lakes, unless there is seepage from coal 

 mines or other mineral deposits. Free carbon dioxide does not accumu- 

 late in running water but may increase in deep quiet stream pools. 



In swift streams seed plants are not conspicuous, except for a few 

 well-rooted species on the margins. Sluggish waters may, however, 

 become completely choked by the luxuriant growth of submerged and 



