DIATOMS 25 



Diatoms 



In the long run marine oroanisms, including the fishes and 

 even such mammals as whales and seals, depend for their food 

 very largely on certain minute brown plants called Diatoms, 

 which play the greatest part in producing organic substances 

 in the sea. They have been called the " pasture of the sea," and 

 compared to the grass of our fields. Diatoms are found every- 

 where where there is water and where sunlight penetrates, at 

 the surface of the sea and at the bottom of the shallow fresh- 

 water pools, in rivers and in lakes. They secrete skeletons of 

 flint which, when they die in the ocean, drop to the bottom and 

 cover some millions of square miles of the depths of the ocean. 

 Their protoplasmic body with its nucleus is enclosed in a 

 flinty shell shaped like a pill-box, and in it is found a cJiloro- 

 plast, or that unit of protoplasm which is set apart to contain 

 the green colouring part of a plant, the chlorophyll. After 

 a time of growth the body splits in two and each half of the 

 pill-box then forms a new shell to replace the one that is 

 separated off with the sister cell. As this process involves 

 a diminution in size of the diatom, it would ultimately go out 

 like Alice very nearly went out when nibbling cake in the 

 "Wonderland." To avoid this humiliating experience, at 

 certain times two diatoms come together and their proto- 

 plasmic contents leave the shells and fuse, and from tliis 

 fusion emerges a diatom of typical size and form. 



Their flinty skeletons are pitted or scored into wonderful 

 symmetrical patterns, and so fine are these that they are 

 used to test the powers of the highest lenses of our micro- 

 scopes. Although diatoms contain but 10 per cent, of protein, 

 2-8 per cent, of fat, 22 per cent, of carbohydrate, and 

 65-2 per cent, of ash, they are so abundant that the higher 

 life of the sea is mainly dependent on them as an ultimate 

 source of food. Thej^ in their turn are, of course, dependent 

 on the action of sunlight and cannot live below the level to 

 which the rays of the sun penetrate. It has been shown that 

 1,200,000 diatoms of three species (and the number of species 

 is enormous) exist beneath each square metre of surface o\'er 

 considerable areas of the North Sea. The incalculable number 



