Although a ton of dry weed yields only four pounds of iodine, the 

 kelps long remained the only source of this important element. Then 

 iodine was found in the niter of Chile. So the kelps were once again 

 neglected - except in places like Japan where they are still valued as 

 food and processed for the extraction of agar. 



The phy toplankton is made up of four main constituents : the 

 diatoms, peridinians, coccolithophores, and smaller members of 

 these and other algal classes which together make up the so-called 

 "nanoplankton." Diatoms are of many shapes, and they range in 

 size from one three thousandths to one thirtieth of an inch. The 

 larger ones are solitary while the smaller ones tend to live in chains. 

 All float in the upper layers of the sea in much the same way as 

 minute particles of dust are suspended in the air, and you can see 

 them dancing in a sunbeam. The protoplasm of their bodies is 

 enclosed in a lovely shell of highly ornamented silica, but the 

 ornamentation is not so much a thing of beauty as a means of 

 helping the organisms remain afloat. In bodies so small, the slightest 

 irregularity of the surface structure provides enough friction with 

 the water to arrest any downward movement due to gravity. 



The peridinians are about the same size as the diatoms, but they 

 have distinctive shapes. Their shells are made of cellulose and they 

 have two flagella, one of which is extended, while the other lies in 

 a groove around the equator of the shell. The flagellum in the 

 groove keeps the peridinian spinning around, while the free 

 flagellum, by a whipping action, drives it through the water. 



Coccolithophores are much smaller than the diatoms and peri- 

 dinians. Seldom more than one five hundredth of an inch in diameter, 

 they have a single flagellum and their surfaces are protected by 

 small disks of lime. Like the diatoms and peridinians, they contain 

 green chlorophyll, multiply rapidly by simple fission, and often 

 contain a small droplet of oil — a food reserve that some biologists 

 believe may represent the source of the worlds's supply of petro- 

 leum. The primary function of the oil droplet is to help buoy up 

 the plant, but when it dies it sinks down to the floor of the ocean. 

 Century after century the slow accumulation of these oil droplets 

 under the sediments may be transformed into our reserves of oil. 



The fourth type of phytoplankton, the nanoplankton, is so 

 small that fine-mesh (600 meshes per inch) tow nets are required to 

 collect the creatures. But most work done on nanoplankton to date 

 has been performed by growing cultures in the laboratory. 



There is one more form of plankton plant life: the bacteria. 

 These differ from the rest in that they are concerned not with the 

 nourishment but with the breakdown and decay of their fellow sea 

 creatures. The bacteria are highly important because they help to 

 reduce all organic remains into phosphates and nitrates, which in 

 turn serve to feed the other four types of phytoplankton and so 

 complete the cycle of life. These dissolved salts accumulate in the 

 water at the bottom of the oceans and are brought up to the surface 

 waters by the currents. Where they upwell, as in the Peru and Ben- 

 guela currents, the phytoplankton flourish. And where the phyto- 

 plankton flourish, so do the animals feeding on them. Thus the 

 rich supplies of guano from seafowls off the Peruvian coasts, and — 

 to a less spectacular degree - the great fishery stocks wherever they 

 occur. Quite simply, the cycle of life is as follows : Dissolved salts 

 and light from the sun provide food for the phytoplankton; the 



The cellular structure of a chain type of 

 diatom that lives in tropical and 

 subtropical seas is shown in the engraving 

 above. At the end of each individual 

 cell in the chain is a pair of horns which 

 help give the chain buoyancy. Below: a 

 photograph of a variety of diatoms 

 (enlarged twenty-five times) shows some 

 in chain form, others as individuals. 



109 



