A POND SCUM 



49 



FIG. 18. Branched green algae : 

 A, Cladophora; B, Vaucheria. 



composed, like the 

 bread mold to be 

 described in the 

 next chapter, of a 

 single large cell con- 

 taining many nuclei. 

 Most of the thread- 

 like green algae have 

 some form of sexual 

 reproduction, some 

 of them by means 

 of gametes nearly or 

 quite alike, as are 

 those of Spirogyra ; 



others by means of large, passive female gametes and small, active 

 male gametes. 



75. Algae of Water Supplies. The drinking water of 

 cities, whatever its source, is sure to contain some algae, 

 especially if it is stored in reservoirs. Certain kinds of 

 algae, of which Spirogyra is one, sometimes multiply very 

 rapidly in reservoirs of city water. This leads to unpleasant 

 results, partly because the mains or pipes may become 

 clogged, but chiefly because when the algae die they decay and 

 give an unpleasant taste and odor to the water. It has been 

 found that the growth of algae in drinking water can be 

 checked by the use of small amounts of copper sulphate, a 

 solution of which will kill the algae even though it is much too 

 weak to be poisonous to human beings. 



76. Marine Algae. The salt waters of seas and oceans, 

 like fresh waters, contain an abundance of plant life. Among 

 the inhabitants of salt water are a good many green algae, 

 not, however, including any of the species of Spirogyra. 

 But the most characteristic marine plants (" seaweeds ") 

 belong to the groups of brown and red algae so-called be- 

 cause their cells contain a coloring matter, brown in the one 

 case and red in the other, masking the green color of the 

 chlorophyl which is also present. The brown algae include 



