154 NATURE OF BACTEROIDS OF THE LEGUMINOUS NODULE, 



shown, Bac. megatherium occurs in the acid fluid of the nodules 

 similar in size to that originally observed by De Bary. 



In a Scandinavian publication abstracted in the Experimental 

 Station Record, xi , 1013, L. Hiltner, after summarising previous 

 investigations relative to the fixation of atmospheric nitrogen, 

 says that the nodule organisms are true parasites and secrete 

 peculiar substances that cause the root hairs to shrivel up. "The 

 injurious influence of the secretory products disappears when the 

 tubercles attain their final development, but since these products 

 continue to form inside the mature tubercle the supposition is 

 that they are immediately converted into substances harmless to 

 the plant. Such a conversion takes place with the co-operation 

 of the host plant by supplying the organism with a part of the 

 nutritive substances produced by the plant. This is further 

 corroborated by the fact that from legumes and alders bacteria 

 can be grown only in nutrient media containing extracts from 

 the roots of leguminous or alder plants. The exclusive preference 

 which is shown by Bacillus radicicola to leguminous plants tends 

 to prove that the Leguminosc^ alone are capable of producing the 

 substances necessary for bacteria, the nature of which is being 

 investigated." Although infusions of leguminous plants are 

 commonly employed for the culture of the organisms that frequent 

 the root nodules, it must not be assumed from this that the plant 

 extract is absolutely necessary for the growth. The bacteriologist 

 employs media which he considers will be best suited to the 

 growth of the particular organism. For this reason extract of 

 meat is used for the bacteria that are parasitic in animals, whey 

 is used for milk bacteria, beer or yeast extract for the saccharo- 

 mycetes, and so on. But most of the bacteria parasitic in animals 

 will grow in media devoid of meat extract, and milk bacteria in 

 media containing no milk. It is true that the nodule-formers 

 are not found in plants other than the Leguminosce, but we are 

 no more entitled to assume from this that leguminous plants 

 contain substances absolutely necessary for the growth of Rhizo- 

 hium than that the tissues of man alone contain substances that 

 are absolutely necessary for the growth of Bad. typhi, Vibrio 



