Existence of Plankton 



37 



of droplets of oil. Some pelagic diatoms completely counterbalance 

 the weight of their siliceous shells by means of a cell sap which is 

 lighter than water so that these organisms have no tendency to sink. 



In addition to the truly floating forms many other kinds of plants 

 and animals sink so slowly that they are able to lead a planktonic 



Bigelow, 1926 



Fig. 2.5. Marine phytoplankton. Photomicrograph of several common genera of 

 oceanic diatoms. The largest cell is approximately 0.05 mm in diameter. 



life. In the air environment the sinking rate of the smallest organ- 

 isms, such as bacterial spores, is extremely slow, but eventually all 

 particles settle out. In the water, however, the sinking rate of some 

 organisms, including many multicellular animals, is so retarded that 

 a small amount of swimming allows the organisms to maintain their 

 position. In other instances the amount of sinking may be inconse- 

 quential before the organism is brought up to the surface again by 

 vertical currents. 



