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TEXTBOOK OF BOTANY 



some of which are 70 meters long. Among the smaller plants are the 

 one-celled diatoms ( Fig. 272 ) and bacteria. Animal life probably exists 

 at all depths and over the entire ocean floor; but living green plants 

 are rare below 300 meters because of inadequate light. The plant popu- 

 lation consists of both suspended and bottom species. It reaches its 



Fig. 272. Diatoms. The names of these species and others may be found in 

 papers by P. S. Conger and Albert Mann pubhshed by the Smithsonian Institution, 

 Washington. Photos by P. S. Conger, except the middle upper from Bausch & 

 Lomb Optical Co. 



greatest density and has the greatest number of species in shallower 

 water near shore where inorganic salts are more abundant and there is 

 more adequate light from top to bottom. Mud bottoms fairly teem with 

 organisms, and many plants and animals are attached to rocky shores, 

 whereas sand is a comparatively barren habitat. 



The bacteria of the sea vary greatly in abundance depending upon 

 depth of water, distance from shore, availability of suspended or dis- 

 solved organic matter, and the presence of other plants as well as ani- 

 mals. The sea water itself is not an especially good medium for the 

 growth of most bacteria unless considerable organic matter is present. 

 As might be expected, bacteria are much more numerous near shore, 

 and especially where the water is polluted by large centers of population. 

 The number of surface bacteria, living and dead, may range from a 

 few or none up to some 300 million individuals in one cubic centimeter 

 of water. 



In bodies of water less than 200 meters deep the bacteria are much 

 more numerous in the mud bottoms than in the water above. These 



