CHAPTER XII 



THE ALGJE 



The algae are the simplest of the green plants and constitute, 

 with the fungi, the group known as thallophytes, -or plants with- 

 out true roots, stems, and leaves. The fresh- water algae inhabit 

 fresh-water streams, lakes, and ponds, as well as wet banks and 

 the bark of trees. They usually occur in simple colonies in. which 

 the cells unite to form cell chains, nets, or spherical aggregates. 

 Like the higher plants, the form which the colony assumes is 

 usually closely related to its needs and to its mode of life. 



The immense growth of algae in ponds and streams, due to 

 their rapid methods of reproduction, is often of the greatest im- 

 portance in producing food for fish and minute aquatic animals. 

 The term plankton is applied to the great mass of minute living 

 algae and other organisms floating on the surface of our lakes and 

 ponds in summer. This plankton is of vital importance to the 

 great fisheries which supply European countries and our own coun- 

 try with fish food. The Illinois State Survey, under Dr. Forbes, 

 estimated that the Illinois River plankton produces annually 

 about 150,000,000 pounds of fish food, and it has been estimated 

 that the plankton of the Rhone River comprises 8000 different 

 species of microscopic plants and 800 species of microscopic 

 animals. This immense number of forms is largely due to then- 

 rapid methods of asexual reproduction during the summer months. 



Water supplies are also affected by the very rapid multiplica- 

 tion of algse during the warm season. In this case the diffi- 

 culty is due not so much to the dangerous nature of the algae 

 themselves as to the fact that their decay furnishes food for 

 bacteria which are inimical to life if taken into the system with 

 drinking water. The presence of algae in any considerable abun- 

 dance in a water supply is therefore a sign, and an indirect cause, 

 of danger to those using the water. 



219 



