MORPHOLOGY, REPRODUCTION, ETC. 15 



The flagella are long filaments, averaging in thickness from one- 

 tenth to one-thirtieth that of the bacterial body, which often are 

 delicately waved and undulating, and, judging from the positions in 

 which they become fixed in preparations, move by a wavy or screw- 

 like motion. In length they are subject to much variation, but are 

 supposed to be generally longer in old than in young cultures. Very 

 short flagella have been described only on nitrosomonas, one of the 

 nitrifying bacteria discovered by Winogradsky. 15 



As to the finer structures of flagella, little can be made out except 

 that they possess a higher refractive index than the cell body itself, 

 and that they can be stained only with those dyes which bring clearly 

 into view the supposedly true cytoplasm of the cell. Whether they 

 penetrate this cytoplasmic membrane or whether they are a direct 

 continuation of this peripheral zone of the bacterial body, can not be 

 decided. 



The manner in which bacteria move is naturally subject to some 



FIG. 3. ARRANGEMENT OF BACTERIAL FLAGELLA. 



variation depending upon the number and position of the flagella 

 possessed by them. Whether bacteria exercise or not the power of 

 motility depends to a large extent upon their present or previous 

 environment. They are usually most motile in vigorous young cul- 

 tures of from twenty-four to forty-eight hours' growth in favorable 

 media. In old cultures motility may be diminished or even inhibited 

 by acid formation or by other deleterious products of the bacterial 

 metabolism. 



At the optimum growth-temperature motility is most active, and 

 a diminution or increase of the temperature to any considerable de- 

 gree diminishes or inhibits it. Thus actively motile organisms, in the 

 fluid drop, may be seen to diminish distinctly in activity when left 

 for any prolonged time in a cold room, or when the preparation is 

 chilled. Any influence, in other words, chemical or physical, which 



15 Winogradsky, Arch. <lcs sci. biologiques, St. Petersburg, 1892, I, 1 and 2. 



