40 POWER OF INDEPENDENT MOVEMENT IN BACTERIA 



cellulose reaction with ammoniacal copper oxide. Zopf, on the other hand, 

 explained these organs as contractile plasma-threads, which could be alternately 

 protruded from, and wihdrawn into, the central cell mass through aper- 

 tures in the cell integument, which apertures, however, have hitherto been 

 unobserved. 



This assumption was combated by A. Fischer, who found that when motile 

 bacteria were subjected to plasmolysis, and the cell contents therefore caused to 

 contract, the cilia were not drawn in, as should be the case if they were 

 continuations of the plasma (pseudopodia). For arresting the movement of the 

 bacteria examined by him, the strength of the solution of salt had to be higher 

 than the minimum capable of producing plasmolysis. Fischer's observations 

 favour the view that the cilia are appendages of the cell, consisting of a mem- 

 brane enveloping the protoplasmic 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 con- 

 ditions, 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 par- 

 ticular, various putrefactive bac- 

 teria, have a great need for 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 hastens to the edges of the 

 cover-glass, where oxygen pene- 

 trates by diffusion and is most 

 abundant, whilst the individuals of 

 the other species gradually retreat, 

 and collect at the centre, where the 

 (to them) unwelcome or obnoxious 

 gas does not penetrate. Repeating 

 Engelmann's experiment by insert- 

 ing a thread of green (i.e. oxygen- 



FIG. 10. Oxygen-loving bacteria infesting a thread of 

 alga lying in the micro-spectrum. The chlorophyll 

 granules contained in the alga cells are not shown, but 



me aiga cens lire not shown, but PXPrp fi nc \ a l, rj , T n t v, J,. ~ r i 



the spectrum lines are given to denote the position of excret . 1D gJ al ga in tne diop, and 

 the spectrum. Magn. 200. (After JSnyeimann.) 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. 10) where the maximum evolution of oxygen is 

 taking place ; that is to say, between the spectrum lines B and 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. Conversely, motile bacteria may also be employed as a delicate 

 reagent for oxygen. BEYERINCK (I.), to whom we are indebted for a very 



