380 MXPERIMKJNTS WITH PLANTS 



typhoid, lockjaw, tuberculosis, diphtheria, pueumouia, 

 cholera, yellow fever, etc. 



Bacteria which live iu living plants or animals are 

 known as parasitic, while those which live in decaying 

 substances are called saprophytic. There are some kinds 

 which live in both ways ; such are capal )le of multiply- 

 ing outside the living body, like typhoid bacilli, and 

 may cause infectious diseases; these may be carried 

 by the air, by streams, etc., and widely distributed. 

 Railway trains are also agents of distribution. Flies 

 are especially dangerous, since they carry disease from 

 the tilth in which they breed directly into the house. 

 Try the experiment of letting a fly crawl over sterilized 

 gelatin and observe the colonies of bacteria which 

 spring up in its footprints. Bacteria which cannot live 

 outside the body can cause contagious diseases only 

 (i. e. diseases communicated only by direct contact 

 with the diseased person) . In some cases such bacteria 

 (or other disease-producing organisms) can be trans- 

 ferred by mosquitoes (malaria, yellow fever) or by 

 fleas (bubonic plague) .^ 



We have now had our attention called to several 

 kinds of bacteria, («) those which pi-oduce decay and 

 putrefaction, e. g., those which flourish in- infusions 

 and which attack and destroy the potato or 

 gelatin; (6) those which cause fermentation, e. g., 



1 See an article by Howard in the Year-Book of the U. S. Department of 

 Agriculture for 1901. 



