268 GENERAL MORPHOLOGY OF PLANTS 



others they hold together for some time, making a long and fine 

 thread which later separates into the separate rods. Beggiatoa 

 forms long filaments of short cells similar to those of Oscillatoria 

 but lacking the blue-green color. The screw or spiral forms are 

 found in the genera Spirillium, Vibrio, etc. They also multiply 

 by cross division of the cells. The round, or spherical, forms are 

 found in a number of different genera according to the method of 

 association of the individuals. In Micrococcus (minute berry) the 

 rounded cells divide into two which quickly separate. In 

 Streptococcus with rounded cells, the cells after dividing remain in 

 chains similar to the bead-like chains of Nostoc. In Sarcina the 

 rounded cells divide in two or three directions, the cells remain- 

 ing held together in small cubical groups for a time. 



407. 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 tubercu- 

 lar swellings on the affected parts, as on the olive tree, crown 

 gall, etc. 



408. 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 ali- 

 mentary canal, consumption or tuberculosis (the " great white 

 plague") caused by B. tuberculosus in the lungs and other parts 

 of the body, diphtheria caused by B. diphtheria, lockjaw or 

 tetanus caused by B. tetani. This last organism is abundant 

 in cultivated soil and on old rusty nails. When deep wounds 

 in the flesh are caused by punctures with objects carrying the 

 germs, because of the character of the wound the air is ex- 

 cluded. Bacillus tetani is an anaerobe and can only grow 



