CLASSES OF PLANTS 431 



In same cities the sewage is collected in large tanks, and the decaying 

 mass of organic matter is converted into harmless or less offensive forms 

 through the action of bacteria. After the fermentation the sludge, or 

 solid deposit, may be used as fertilizer. 



Pure cultures of harmless bacteria have also been used medicinally to 

 replace those that are already present in the intestines, interfering with 

 digestion or absorption or producing objectionable substances. 



It is impossible to say that bacteria, on the whole, are more 

 beneficial to man than harmful. This illustrates, then, a gen- 

 eral fact about the groups of living things. 



318. Algae. For centuries a number of the larger seaweeds 

 have been used in Japan as sources of food. Agar-agar, a sub- 

 stance like gelatin, obtained from some of these seaweeds, has 

 been for many years imported into Europe and America for use 

 in laboratories as a convenient medium in which to grow bac- 

 teria. Because it can absorb large quantities of water and 

 swell up it has come to be increasingly used as a help in consti- 

 pation. Other seaweeds, the kelps, are used as a source of iodin, 

 and experiments with kelp are now being made in the hope of 

 extracting potassium on a commercial scale. " Irish moss " yields 

 a gelatinous material sometimes used in puddings. 



The diatoms, which are the most common plants in the vast 

 floating life of the ocean, are microscopic one-celled organisms 

 with strange, sandy shells that are often beautifully marked. 

 In ages past accumulations of such shells have built up vast de- 

 posits of fuller's earth, which is used in scouring soaps, for pol- 

 ishing wood, and for other purposes which require a very fine, 

 hard powder. 



319. Fungi. The three main groups of fungi all have mem- 

 bers that are of value to man, as well as others that are injurious. 



I. Phy corny cetes (algalike fungi). A few of the molds in 

 this group are used for curing cheeses. Many are parasitic and 

 cause injury by attacking the potato (potato rot) or useful 

 fishes or the grape (see Fig. 177). But some of them destroy 

 flies and other insects of which we are glad to be rid. The ring- 

 worm parasite probably belongs here (see page 335). 



