182 OUTLINES OF BACTERIOLOGY 



distorted forms such as these are common in many bacterial cultures 

 when the conditions of growth are not favourable. They are involution 

 structures, and are either already devoid of life or are approaching 



o 

 *0\ ft 



~ U 

 FIG. 112. Bac. radicicola. Showing involution-forms. (After Lafar.) 



that state. Formerly all the bacteria inside the bacteroid tissue were 

 denominated 'bacteroids,' but it is now customary to restrict this 

 term only to the involution structures that are abundantly found in 

 this tissue. 



7. RELATION OF THE BACTERIA TO LEGUMINOUS 



PLANTS. 



It was formerly held that the relation was of a symbiotic nature, 

 in which the plant supplied the necessary carbonaceous material, 

 which was repaid by the microbe by a delivery of nitrogenous 

 substances. That doctrine, however, in the light of the results 

 obtained by Hiltner and other recent workers, cannot be regarded 

 as established. The real nature of the relationship may be seen by 

 a consideration of the following facts. In the first place, bacteria, 

 taken from, say, pea nodules and used to inoculate a bean plant do 

 not produce nodules, whereas if inoculated into a pea plant, they do. 

 Now it is comparatively easy by gradual adaptation, to enable, for 

 example, bacteria from pea nodules to grow in bean plants. The 

 relation, therefore, is not a deep-seated one, and either organism can 

 exist without the other. Again, the behaviour of these microbes is 

 not what would be expected from a symbiont. If the inoculating 

 material be weak the bacteria enter the roots, but remove themselves 



