246 Science of Plant Life 



more highly prized as food than they are now. When that 

 time comes, the cultivation of algae will probably be under- 

 taken as a first step toward greater fish production. 



But while the pond scums are a source of food for water 

 animals, they are also a source of annoyance in reservoirs in 

 which drinking water is stored. When they accumulate in 

 large quantities and die, they cause the so-called " fishy 

 taste " of water. This trouble has been to some extent con- 

 trolled during recent years by the exclusion of light from 

 small reservoirs, and by the addition of small amounts of 

 copper sulfate to the water in large reservoirs. Copper sul- 

 fate is very poisonous to algae, even in quantities of one part 

 to a million parts of water. Since animals are not injured by 

 such small amounts, the water may be used without harm for 

 drinking purposes. 



The seaweeds. The brown and red algae, commonly known 

 as seaweeds, grow attached to rocks in the shallow water 

 along our coasts. They are generally from a few inches to 

 several feet in length. Some of the brown kelps grow in 

 deep water and attain lengths of 100 feet or more. In tex- 

 ture the seaweeds vary from the most deUcate filaments to 

 broad, leathery expanses, with stalks so tough that they are 

 used for making ropes. 



In China and Japan, seaweeds have long been used for food. 

 They are cooked with fish and take the place of vegetables. 

 Along our northern coasts the " Irish moss " and " dulse " 

 are collected in considerable quantities for food purposes. The 

 agar used in laboratories for growing bacteria and fungi is 

 made from certain kinds of red algae that grow abundantly on 

 the coasts of Asia. 



The brown kelps are an important source of iodin, and also 



