PLANT LIFE 155 



plants are alone able with the aid of sunlight 

 to assimilate the carbon dioxide of the atmos- 

 phere and to build up carbohydrates. 



It follows from what has been stated above 

 that in the ocean bacteria are most abundant 

 near shore and in shallow water, where there 

 is a large supply of dead organic matter, and 

 become less and less abundant far out at sea 

 and in deep and cold water. They seem to be 

 especially abundant at what has been called 

 the " mud-line," that is at the position where 

 all minute organic and inorganic particles 

 settle on the bottom, and form mud — the 

 humus of the ocean — in place of sand or 

 gravel. In the surface waters again they are 

 extremely abundant and active where cold 

 and warm currents meet. Here, owing to 

 the rise of temperature, the bacteria which 

 had been dormant or lethargic in the cold 

 water become active and break down into 

 nitrites and nitrates the albuminoid ammonia 

 which had been protected by the low tempera- 

 ture from decomposition in polar currents 

 or cold upwelling waters from the deep sea ; 

 thus abundant food for plants is formed. 

 It has frequently been remarked that the 

 pelagic algae are especially numerous in such 

 areas, for instance throughout the whole 

 Southern Ocean and to the south-west of 

 Iceland in the North Atlantic. 



