44 BAOTERIOLOOY. 



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 between 

 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.) 



Another interesting biological peculiarity of bacteria 

 is that discovered by Pfeffer, known as chemotaxis. 

 This term applies to the peculiar phenomena of attrac- 

 tion and of repulsion that are exhibited by motile bac- 

 teria when in the presence of solutions of bodies of 

 various chemical composition. Thus, for instance, when 

 a neutral fluid (a drop of water) containing motile bac- 

 teria is brought in contact with a weak solution of either 

 peptone, sodium chloride, or dextrin, the bacteria are at 

 once attracted toward the solution ; this reaction is 

 designated " positive chemotaxis." On the other hand, 

 if brought in contact with an acid, an alkaline, or an 

 alcoholic solution, the bacteria are repelled or driven 

 from the point at which the two fluids are diff'using; 

 that is, they exhibit " negative chemotactic " affinities. 

 The significance of these reactions is not understood, 

 but it has been aptly suggested that they may be funda- 

 mentally analogous to the specific positive and negative 

 affinities exhibited by the ions (see page 81) resulting 

 from the dissociation of electrolytes, and that they may 

 " have their explanation in the forces of ionic attraction 

 and repulsion."^ In this connection it is important to 



' Eead Sewall on "Some Eolations of Osmosis and Ionic Action in 

 Clinical Medicine," International Clinics, vol. xi., Eleventh Series. 



