CHEMOTAXIS. 47 



develop and exercise their biological activities. Again, 

 through the proteolytic activity of enzymes produced by 

 certain species of bacteria, other species are supplied 

 with nutrition that would otherwise be uuassimilable or 

 only imperfectly so. Similar symbiotic relations be- 

 tween bacteria and higher plants are also noticed, nota- 

 bly 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. This latter relationship is 

 probably an example of true symbiosis. 



CHEMOTAXIS. Another interesting biological pecu- 

 liarity of bacteria is that discovered by Engelmann and 

 by Pfeifer, known as ehemotaxis. 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. Pfeifer showed that when a neu- 

 trial fluid (a drop of water) containing motile bacteria 

 is brought in contact with a weak solution of either 



