A POND SCUM 



51 



them are good-sized plants ; some of them are also used for 

 food in various parts of the world. 



77. The Plankton. — The surface layers of any body of 

 water, salt or fresh, contain vast numbers of small organisms. 

 All of these floating plants and animals are spoken of together 



Fig. 20. — One of the giant kelps (Nereocystis). 



as the plankton. The number of minute living organisms 

 present in and near the surface of a lake or river, and the 

 rapidity with which such organisms grow and multiply, are 

 almost inconceivable to one who has not made a microscopic 

 study of such waters. It has been estimated, for example, 

 that more than 149,000,000 pounds of small plankton organ- 

 isms are produced each year in the Illinois River; this is 



