126 THE SEAS 



certain nutrient salts dissolved in the water on which the 

 drifting plants depend for their growth. In localities where 

 these bodies are richest the plants will be most abundant, 

 and, consequently, the animals which depend on them for 

 their food supply. A discussion giving the basic principles 

 that underlie this plankton production will be found in 

 Chapter XI ; it would be out of place to enlarge on the 

 problem here until the reader has made himself acquainted 

 with some of the properties of sea water that are outlined 

 in Chapter X. 



In Plate 47 is given a chart of the North Atlantic Ocean 

 and surrounding seas, in which the density of plankton life 

 is shown diagrammatically. The richest areas are shown by 

 the green colouration, the regions of greatest density of life 

 being the deepest green. The green colour gradually shades 

 off into blue which can be taken to represent a poverty of 

 plankton organisms. 



Having dealt with the geographical distribution of the 

 plankton, let us turn now to consider at what depths this 

 drifting life is to be found and where it is most abundant. 

 Taking the plants first, consisting of the Diatoms and the 

 Peridinians, collections made by research vessels from 

 different depths show that it is only the upper water layers 

 of the sea, from the surface down to about one hundred 

 fathoms, that contain drifting plant life in any quantity. 



A moment's thought will satisfy anyone that this must 

 of necessity be the case, since all plants are dependent on 

 the sun's light for their life and the deeper we go into the 

 water the less light is there present. In fact it has been 

 shown that at little more than ten fathoms in the English 

 Channel off Plymouth the amount of light present is already 

 similar to that in the heart of an English wood. In the 

 clearer open waters of the ocean the light can perforce 

 penetrate deeper, but it is certain that at a depth of one 



