52 INDEPENDENT MOVEMENT IN BACTERIA. 



ages of the cell, consisting of a membrane enveloping the proto- 

 plasmic contents, which have an immediate connection with the 

 substance of the bacterial cell. 



Adverse influences stop the movement, and the cilia become 

 motionless and torpid. According to the cause, this condition is 

 said to be one of torpidity through cold, heat, darkness, light, 

 hunger, desiccation, or poison. Bearing this in mind, it must not 

 be concluded that any species of bacteria which may not exhibit 

 movement under ordinary microscopic examination is therefore 

 necessarily non-motile ; but it should be further examined under 

 various conditions, and, in extreme cases, tested for the presence 

 of cilia by staining, since it may be in the torpid condition. 



41. Chemotaxis. 



The extended researches of ENGELMANN (II. and III.) teach us 

 that certain roving bacteria (i.e. those endowed with spontaneous 

 movement), and, in particular, various putrefactive bacteria, have 



a great need for 



a B C jj - , p oxygen, while other 



species do not require 

 it. If a drop of liquid 

 containing a mixture 

 of these two kinds 

 be brought under the 

 microscope, it will 

 quickly be seen that 

 the one species has- 

 tens to the edges of 

 the cover-glass, where 

 oxygen penetrates by 

 diffusion and is most 

 abundant, whilst the 

 . individuals of the 



. io. Oxygen-loving bacteria infesting a thread of alga , -, . , 



lying in the micro-spectrum. The chlorophyll granules Other Species gradu- 

 contained in the alga cells are not shown, but the 

 spectrum lines are given to denote the position of the 

 spectrum. Magn. 200. (After Engelmann.) 



FIG 



ally retreat, and col- 

 lect at the centre, 

 where the (to them) 



unwelcome or obnoxious gas does not penetrate. Repeating 

 Engelmann's experiment, by inserting a thread of green (i.e. 

 oxygen-excreting) alga in the drop, and directing a small solar 

 spectrum thereon, then the oxygen-loving bacteria will be seen 

 collected around these alga threads, and surrounding those spots in 

 the micro-spectrum (Fig. io) where the maximum evolution of 

 oxygen is taking place ; that is to say, between the spectrum lines 

 B and C in the red, and also at F. Oxygen, therefore, exerts an 

 attractive and stimulating action on many bacteria, and may thus 

 be employed as an isolating and separating agent therefor. Con- 



