Studies of Bacillus Radicicola of Canada Field Pea 75 



14. In some media and under certain conditions the organisms died 

 much sooner than in other media. The nitrogenous media did not seem 

 to influence the infecting power of the organism. 



15. It is not difficult to determine whether or not a given culture can 

 produce nodules, but there is no accurate method of measuring the slight 

 variations in infecting power that may exist in the different cultures. 



GENERAL DISCUSSION 



The definition of virulence as given by various authors in the medical 

 texts is not clear, but in general the word is defined as meaning the power 

 of microorganisms to invade and multiply in the tissues of a host and 

 cause some injury or disease to the host. It was found by Peirce (19G2), 

 Fred (1911), and others, that the cells in the nodule are injured by the 

 nodule-forming organism and become abnormal, and that for this reason 

 the relation between the microorganism and the legume is a parasitic 

 one. Furthermore, a nodule on a leguminous plant is a swelling, a hy- 

 pertrophic formation; and morphologically speaking, it is an abnormality, 

 or a form of disease. Nodule formation, therefore, may be considered as 

 of a pathological nature. But, it may be asked, which is the normal root 

 of the legume — the one with the nodules, or the one without the nodules^ 

 or both? Whatever the morphological and cytological evidence may be, 

 one well-established fact stands out; namely, that leguminous plants are 

 benefited by the presence of nodules. No positive evidence has been 

 produced thus far to show that the microorganism which causes nodules 

 is injurious to leguminous plants. The nodule-forming organism pen- 

 etrates the root tissue, multiplies therein, and apparently derives its 

 necessary food therefrom; and in return for this it enables the plant to 

 obtain a certain amount of nitrogen. There is, therefore, a mutual and 

 beneficial exchange and the relation is symbiotic. 



The primary object of the experiments reported m this investigation 

 was to determine whether the power of the nodule-forming organism to 

 cause nodules is easily altered by artificial media. The nodule-forming 

 organism of Canada field pea was chosen for this purpose, and the results 

 apply only to that organism. It is probable that the organisms from 

 other legumes might have given different results. Since the value of the 

 investigation depended so much on the securing of a pure culture, the 

 various precautions, as previously indicated, were adopted for this purpose. 



