396 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



method of reproduction is for a single cell to divide into two cells. 

 In the unicellular algae the two cells so formed separate at once; 

 in multicellular forms, the cells remain attached to each other in 

 arrangements that give the plants their characteristic spherical, fil- 

 amentous, or leaflike shape. Rapid multiplication of the cells in a 

 filamentous form, as in the blue-green algae, is often accompanied by 

 the breaking up of the filament into short rodlike pieces, each of 

 which forms a new filament. Other means of reproduction, both 

 sexual and asexual, exist in the algae, in the process of which, special 

 reproductive bodies unlike the parent cells are formed. The essen- 

 tial feature of their sexual reproduction consists in the fusion of 

 two cells to form one. This is normally followed by vigorous new 

 growth. In asexual reproduction one cell divides to form several 

 bodies, which may grow into new plants without any fusion of the 

 cells occurring. 



Thick-walled resting spores are frequently formed. These cells 

 can remain at rest for some time, often withstanding cold and dry- 

 ness until favorable conditions arise for their germination. They 

 increase greatly the power of the algae to maintain themselves in the 

 waterworks, for they can be blown with the wind in the dust and 

 be carried on the feet and feathers of birds from one body of water 

 to another, and they can survive the emptying of reservoirs and the 

 drying of reservoirs and filters. 



BACTERIA 



The bacteria (pi. 7, fig, 1; pi. 10, fig. 3), the tiniest of microscopic 

 organisms to be found in the water supply, are of interest in that 

 they may include the BaciUus coli. Pollution of the water introduces 

 large numbers of bacteria of the colon group, usually harmless 

 organisms when confined to the intestinal tract of man, but definitely 

 pathogenic when liberated into the serous cavities of the body. The 

 bacteria of this group are frequently found in the water supply in 

 greater numbers and variety than the bacteria that produce diseases, 

 such as those causing typhoid fever. The bacteria of the colon group 

 are also more hardy than the disease-producing bacteria. Treated 

 water, after filtration, should be examined daily for Bacilhts coli. 

 If the purified water is free from them, it is safe to assume that it 

 will not contain the harmful bacteria, which are less resistant. 



Crerwthrix, one of the higher bacteria, is a microscopic threadlike 

 organism closely related to true bacteria and fungi. It can live 

 in the dark and in the absence of dissolved oxygen but does not 

 thrive in water containing large quantities of dissolved oxygen. 

 It requires iron as an essential food and so will live inside distribu- 

 tion systems that contain iron. It can grow on the inside of water 



