3 



BEHAVIOR OF THE LOWER ORGANISMS 



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

 taining none. 





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). 



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. lenue] 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 Engelmann (1894), 

 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 

 af the diatom. 



