40 POWKK OF iNnFFiAniAT MONFMFNT 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. 



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

 bacteria were subjected to plasinolysis, 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 tlie 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, wliich have an immediate connection 

 with the substance of the bacterial cell. 



Adver>e 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. Hearing 

 thi- 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- 

 dition.-, 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 EXCKLM ANN (II. and III.) teach us that certain 

 roving bacteria (i.e. those endowed with spontaneous movement), and, in pai- 



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 rt-treat. 

 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- 

 excreting) alga in the drop, and 

 directing a small solar spectrum 

 thereon, then the oxygen-loving 

 ' M Collected around these al-a threads, ai.d surrounding those 

 spot* in the micro spectrum ( Kig. 10) where the maximum evolution of oxygen ia 

 taking place ; that i> to >ay, between the spectrum lines H and C in the r d.and 

 n, therefore, exerts an attractive and stimulating action on 

 many bact-ria, and may thus be employed as an isolating and separating 

 ft S ei1 ( '"ivei>ely, motile bacteria may also be employed as a delicate 



<Mt for oxygen. BEYEHINCK (I.), to whom we are indebted for a very 



lOTing l.anni;, iiiiotiim :i llir;i<l nt 



I.viii- in the niicro->pcrtrmn. The chlorophyll 



gnnalM contained in the :ili,'a cells art- not shown, lint 



tin- -piTtrim, lini- :irc i;i\i-li to ill not.- tin- position of 



tlie-piTtriiui. Mau-i. Jou. i. (>/<, /:/, i,,wi,n.) 



