CHEMOTAX1S. 47 



through exhaustion of free oxygen by the actively 

 germinating aerobic varieties, the conditions are sup- 

 plied that enable the anaerobic species to develop and 

 exercise their biological activities. Again, through the 

 proteolytic activity of enzymes produced by certain 

 species of bacteria, other species are supplied with nu- 

 trition that would otherwise be unassimilable or only 

 imperfectly so. Similar symbiotic relations betAveen 

 bacteria and higher plants are also noticed, notably that 

 between certain bacteria of the soil and the group of 

 leguminous plants, whereby the latter are enabled, 

 through the assistance of the former, to make up their 

 nitrogen deficit in large part from the free nitrogen of 

 the atmosphere. (See page 36.) 



CHEMOTAXIS. Another interesting biological pecu- 

 liarity of bacteria is that discovered by Engelmann and 

 by Pfeffer, known as chemotaxis. This term applies to 

 the peculiar phenomena of attraction and of repulsion 

 that are exhibited by motile bacteria when in the pres- 

 ence of solutions of bodies of various chemical compo- 

 sition. Engelmann demonstrated that the bacteria in 

 decomposing infusions accumulate in great numbers in 

 the neighborhood of the sources of oxygen. In a hang- 

 ing-drop of such an infusion the bacteria will be seen to 

 accumulate in a dense mass along the edge or around the 

 edge of small bubbles of air in the fluid. Even plant 

 cells in the infusion, whose chlorophyll sets free oxygen 

 in the light, are surrounded by large numbers of bacteria. 

 The positive chemotactic affinity between oxygen and bac- 

 teria was employed by Engelmann as a basis for the dem- 

 onstration of small quantities of oxygen in studying the 

 assimilative action of various kinds of light upon the 

 green plant-cell. Pfeffer showed that when a neu- 



