136 GROWTH AND WORK OF PLANTS 



division, each cell dividing into two equal parts. This is called 

 multiplication by fission, which means a cutting in two. For this 

 reason they are sometimes called fission fungi. In reproduction 

 many of the bacteria form spores, the protoplasm condensing 

 inside the cell into a small, rounded, shining body which is much 

 more resistant to dessication, heat, cold, and the action of poison- 

 ous substances than are the vegetative cells, some being able to 

 resist the heat of boiling water for several minutes. 



The bacteria live within, or upon the surface of, the substance 

 upon which they feed. Their method of nutrition is similar to 

 that of the fungi. They absorb solutions of food substances 

 through their cell walls. The bacteria are " omnipresent," and 

 being very minute and capable of rapid multiplication they exist 

 in marvellous numbers. Some species multiply so rapidly as to 

 produce new individuals in a half hour. They and their spores 

 are easily carried about on floating particles of dust, on the cloth- 

 ing, the hands and other parts of the body, and by insects and 

 other animals. They exist in the mouth, in the stomach and 

 throughout the intestinal canal. Fortunately the larger number 

 of bacteria are harmless and many are beneficial, as we have 

 seen in the study of decay, nitrification, fermentation, fixation of 

 nitrogen, etc. (Chapter XIV). 



231. Diseases caused by bacteria. There are, however, a 

 number of diseases of both plants and animals caused by bac- 

 teria. Among plant diseases caused by bacteria are the follow- 

 ing: pear blight, or fire blight of the pear, apple and other fruit 

 trees (killing flowers, leaves, twigs and branches); black rots of 

 cabbage, rots of turnip, potatoes, etc., bacteria also attack the leaves 

 and fruit of beans, cotton; some cause the plants to wilt, as in 

 melon and cucumber wilt; and some produce galls or tubercular 

 swellings on the affected parts, as on the olive tree. 



222. The most serious diseases caused by bacteria are those 

 of man and other animals. Because of the small size of the 

 bacteria they are often spoken of as germs, and the diseases they 

 cause, as germ diseases. Some of these diseases are as follows: 

 Typhoid fever caused by Bacillus typhosus in the alimentary canal, 



