600 TEXTBOOK OF BOTANY 



of 150 meters, although a few Hving green plants have been found at 

 300 meters. Beebe reports the disappearance of all but the blue and 

 violet rays at depths of 250 meters in the clear water near the Bermuda 

 Islands. Below 500 meters almost complete darkness prevails, except 

 for the feeble light emitted by certain deep-sea fishes and other animals. 



The gases of the air are absorbed directly from the atmosphere, al- 

 though carbon dioxide and oxygen are subject to local variation because 

 of photosynthesis and various oxidations. Gases are carried to depths 

 far below the surface through the action of storm waves and ocean 

 currents. 



All chemical elements are probably present in sea water, although 

 only about 32 have been reported. Some occur in marine organisms 

 but have not as yet been recognized in the water itself because of their 

 extremely low concentrations. The relative amount of an element may 

 be far greater in a plant or an animal than in the medium in which it 

 lives. Large quantities of potassium and small amounts of sodium, for 

 example, accumulate in some marine plants, even though sodium salts 

 are very abundant in the ocean and potassium salts relatively scarce. 

 The relative amount of any element that accumulates in the different 

 species of plants may vary greatly even though the plants are growing 

 in the same environment. 



The relative proportions of salts in the main body of the ocean are 

 fairly constant at all latitudes. The relative percentages of the different 

 elements in the salt after the water has evaporated do not vary much 

 from the following: chlorine, 54 per cent; sodium, 31 per cent; mag- 

 nesium, 4 per cent; calcium, 1 per cent; bromine, 0.2 of 1 per cent; 

 sulfate radicals, 8 per cent; and carbonate radicals, 0.2 of 1 per cent. 



Surface temperatures of the ocean vary greatly with latitude and 

 with the time of year. However, surface variations between winter and 

 summer are much less than those encountered on land. Sea water be- 

 comes heavier as it is cooled until the freezing point is reached. As a 

 result, temperatures below freezing may occur, in contrast to the rather 

 constant 4° C. of most deep-lake bottoms. The bottom temperatures 

 of the oceans in temperate regions are around 2° C. Seasonal variations 

 in temperature are unusual below 500 feet. At depths of a mile or more 

 the temperature is near the freezing point at all times. 



The effects of temperature are so closely tied up with those of other 

 factors that its influence has been frequently exaggerated. Temperature 

 apparently often determines the range of plant distribution both verti- 

 cally and latitudinally. Some species of plants grow and reproduce at or 



