92 SECTIONAL ADDRESSES. 



ance are (1) the amount of ligKt which enters the water, an amount 

 which vaiies with the length of the day, the altitude of the sun, and 

 the cleai'ness of the au- and of the water ; (2) the presence in adequate 

 quantity of mineral food substances, especially nitrates and phosphates; 

 and (3) a temperature favourable to the growth of the species which 

 are present in the water at the time. Experiments with cultm-es of 

 diatoms have shown clearly that if the food-salts required are present, 

 and the conditions as to light and temperature are satisfactory, other 

 factors, such as the salinity of the water and the proportions of its 

 constituent salts, can be varied within very wide limits without checking 

 gi'owth. The increased abundance of plankton, especially of diatom 

 and peridinian plankton, in coastal waters and in shallow seas largely 

 surrounded by land, such as the North Sea, is due to the supply of 

 nutrient salts washed dii'ectly from the land by rain or brought down 

 by rivers. An exceptional abundance of plankton in particular localities, 

 which produces an exceptional abundance of all animal life, is also 

 often found where there is an upwelling of water from the bottom layers 

 of the sea. These conditions are met with where a strong current 

 strikes a submerged bank, or where two currents meet. Food-salts 

 which had accumulated in the depths, where they could not be used 

 owing to lack of light, are brought by the upwelling water to the surface 

 and become available for plant growth. The remarkable richness of 

 fish life in such places as the banks of Newfoundland and the Agulhas 

 Banks off the South African coast, each of which is the meeting-place 

 of two great currents, is to be explained in this way. 



Our detailed knowledge of the steps in the food-chain from the 

 diatom and peridinian to the fish is increasing rapidly. The Copepod 

 eats the diatom, but not eveiy Copepod eats every diatom; they make 

 their choice. The young fish eats the Copepod, but again there is 

 selection of kind. Even adult fishes like herring and mackerel, which 

 were fonnerly supposed to swim with open mouth, straining out of the 

 water whatever came in their way, are now thought largely to select 

 their food.'' 



A result of extraordinary interest in connection with the food-chain 

 has recently been brought to light by two sets of investigators working 

 independently. In seeking to explain certain featui-es which he had 

 foimd in connection with the growth of the cod, Hjort^" undertook a 

 study of the distribution in marine organisms of the gi'owth stimulant 

 known as vitamin. Pat-soluble vitamin was already known to be 

 present in large quantities in cod-liver oil, and is what probably gives 

 the oil its medicinal value. Hjort was able to trace the vitamin^ by 

 means of feeding experiments on rats, in the ripe ovaries of the cod, 

 in shrimps and prawns, which resemble the animals on which the cod 

 feeds, and in diatom plankton and green algse. Jameson, Drummond, 

 and Coward-' cultivated the diatom Nitzsckia closterium, and by a 

 similar method to that used by Hjort showed that it was extraordinarily 



" Bullen. Joiinf. Mar. Biol. Axf'OC., 9, 1912, p. 394. 

 " Pror. Boy. Son., May 4, 1922. 

 -■■' Biochemical Journal. 1922. 



