SEA WATER 219 



for it has been estimated that there exists dissolved in the 

 oceans 46,700 times as much silver as has been mined all 

 over the world between 1902 and the discovery of America 

 by Columbus, in 1492. This is, in all, 13,300 million tons, 

 but it would be a very tedious matter to extract it ! 



But the most important bodies discovered in sea water 

 are certain manurial salts which are only present in very 

 minute quantities. These are the phosphates and the 

 nitrates, substances which we know have a very great 

 nutrient value in the soil on land and are among the chief 

 components of all natural or artificial manures. 



We have mentioned in a previous chapter (page 114) 

 that in the sea, as well as on the land, all life is ultimately 

 dependent on the plants for its supply of food. In the 

 off-shore waters of the sea the plant life is represented 

 almost solely by that great drifting community of plankton 

 plants among which the minute, unicellular diatoms are 

 the most numerous. It is on the nutrient salts present in 

 solution in the sea that these diatoms depend for their 

 food. On land the plant extracts phosphates and nitrates 

 from the soil by means of its roots. The fine " root hairs " 

 absorb the nourishing salts from the minute interstices 

 among the soil particles where the moisture holds them in 

 solution. In the case of diatoms, however, there is no need 

 of any specialized structure such as the root to take up these 

 valuable substances, for its whole body is bathed in the sea 

 water from which the food is absorbed. More will be said 

 in the next chapter on this subject and the effects of the 

 presence or absence of the manurial salts on the seasonal 

 cycle of life in the sea. 



Besides these " solid " salts in the sea there are also 

 present certain gases dissolved in the water. Animals 

 cannot live without oxygen. We extract the oxygen that 

 we breathe in with the air into our lungs, and it is carried 



