30 BEHAVIOR OF THE LOWER ORGANISMS 



tive reaction it is the change from water containing oxygen to water con- 

 taining none. 





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A B 



Fig. 25. — Collections of bacteria about algae, due to the oxygen produced by the latter. 

 A, Spirilla collected about a diatom. After Verworn. B, Bacteria gathered about a spherical 

 green alga cell in the light, a shows the condition immediately after placing the bacteria and 

 alga on a slide; no collection has yet formed, b, Condition two minutes later; part of the bac- 

 teria have gathered closely about the cell. After Engelmann (1894). 



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Spirilla collect in the way above described about any source of oxy- 

 gen. Green plants give off oxygen in the light, so that the bacteria col- 

 .._ . . lect about desmids, diatoms, and other microscopic 

 plants, in a lighted preparation, in the same way 

 as about air bubbles (Fig. 25). Many other bac- 

 teria react in the same way to oxygen; notably 

 the ordinary bacterium of decaying vegetable 

 infusions, Bacterium termo. Bacteria react to ex- 

 ceedingly minute quantities of oxygen, so that it 

 is possible to use them as tests for the presence of 

 small amounts of this substance. Engelmann 

 calculates that a bacterium may react to one one- 

 hundred-billionth of a milligram of oxygen. By 

 means of such reactions he has carried on investi- 

 gations to determine whether various green or col- 

 orless organisms do or do not give off oxygen ; 

 results may be attained in this way that could 

 scarcely be reached otherwise (Fig. 26). Spirillum 

 (especially S. tenue) is so remarkably sensitive to 

 oxygen that many individuals may react to the 

 oxygen produced by a single specimen of another 

 smaller bacterium (Engelmann). 



When bacteria collect about bubbles or near 

 the edge of the cover-glass as a reaction to oxy- 



Fig. 26. — An experi- 

 ment of Engelm ann ( 1 894), 

 showing that when a di- 

 atom is partly lighted, only 

 the part exposed to the 

 light produces oxygen. 

 The upper half of the di- 

 atom was in -the shade, the 

 lower half in the light. 

 The bacteria have gathered 

 only about the lighted half 

 of the diatom. 



