313 BOTANY. 



■water. They occur in solutions of organic matter in im- 

 mense numbers, and are said even to appear in solutions of 

 inorganic salts under proper conditions.* 



278.— Order BacteriacesB. This includes the organisms 

 known as Bacteria, and which are present in. fermenting and 

 putrefying matter ; they also occur in the blood and the air- 

 passages of diseased animals, and the tissues of some dis- 

 eased plants, where they have been shown to be the cause of 

 many kinds of disease. Cohn f defined Bacteria as " chlor- 

 ophyll-less cells of spherical, oblong, or cylindrical form, 

 sometimes twisted or bent, which multiply themselves ex- 

 clusively by transverse division, and occur either isolated or 

 in cell-families." Many forms have since been shown to 

 produce spores, and these are most important agents in their 

 multiplication and reproduction. In the unicellular Bac- 

 teria the cells resulting from division separate at once, while 

 in the filamentous forms they remain in connection, forming 

 elongated strings or threads. Bacteria sometimes form a 

 jelly-like mass by the swelling up of their cell membranes; 

 this is the Zuoglma stage. When they have exhausted the 

 nutriment from the liquid, they form a pulverulent precipi- 

 tate, which may be regarded as a resting state. " Most 

 Bacteria present a motile and a motionless condition; the 

 former is connected with the presence of oxygen." 



It is now known that many Bacteria pass through various 

 stages, e.g.. Coccus, Bacillus, Vibrio, etc., which were for a 

 time supposed to be generic forms, under which species were 

 described, as was done by Oohn. The real limits of genera 

 and species cannot in the present state of our knowledge of 

 these organisms be determined. We may, for the present, 

 make use of Cohn's system, remembering that it is merely 

 a classification of observed forms. 



* See Bastian's " Beginnings of Life," Vol. II., Appendix. 



f "EeBearches on Bacteria" (Untersncli. iiber Bacterien) in " BeitrSge 

 zur Biologie der Pflanzen," Breslau, 1873. See a resume of tliis paper 

 in Quarterly Journal of Mieroseopieal Science, 1873, p. 156. See also 

 English accounts of further researches by Cohn, 1875, 1876; In the 

 journal just cited, 1876, p. 259. and 1877, p. 81. Consult " The Bao- 

 tcria," by Dr. A. Magnin ; translated by Dr. Sternberg. Boston, 1880. 



