184 



CRVP'roGAMS 



Bacteria 



441. The Bacteria (Fig. 301) include the smallest of 

 all living organisms. Even the highest powers of the 

 micrciscope fail to show much of their inner structure ; 

 so that at present very little is known of their relation- 

 shi[) to other groups. Our knowledge is confined to their 

 external forms, methods of multiplication, and modes of 

 life, with their effects, good and bad ; but this knowledge 

 is of the highest practical importance, since the Bacteria 

 affect the lives of other living beings, including man, in 

 very direct ways. 



442. Size. A common spherical form is -g-gVo o^o' ^'^^^^ ^'^ 

 diameter ; the rod-shaped germ of consumption is from 

 three to nine times as long as this ; many species, however, 



are considerably larger. Form. The 

 principal forms are (1) spherical, 

 (2) straight cylindrical, (3) spiral. 

 Movements. Many Bacteria exhibit 

 very lively movements. Locomotion 

 is usually accomplislied by means of 

 extremely fine cilia (Fig. 301). 3Inl- 

 tvplication commonly takes place by 

 fission. Each individual divides into 

 two parts, by transverse division, 

 each part becoming a Jiew individual. 

 Under favorable conditions — abun- 

 dance of food and considerable 

 warmth — the Bacteria may double 

 in numbers about every half hour. 

 In this way enormous multitudes may 

 result even from one original indi- 

 vidual in a comparatively short time. 

 ]vOW tcnnpcratures retard growth and 

 hence the utility of ice in ])reserving foods in 



oOl. Bacteria, highly mag- 

 nified: a, file germ 

 of lyplioid lever, 

 stained to show tlie 

 cilia; h, a spiral 

 ciliated form ; c, a 

 rod-shaped foriii, in 

 chains ; d, a spheri- 

 cal form. — (V, h, 

 from E.NMiLER and 

 Pi:axtl. 



warm weather. Under certain conditions Bacteria pass 

 into a .spore condition, in which they become highly 

 resistant to destruction by heat or drying. In a dry 



