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 (N ereocystis) . 



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 



