I04 



BACTERIA IN RELATION TO PLANT DISEASES. 



mung's bactcrioiden," and (3) bladder bacteroids. The third form occurs where the bacteria have 

 multipHed enormously. The hcmmung's bacterioiden occur outside of the bacteroid tissue in nearly 

 all the outer cells of the nodule, and not rarely in the normal bark of the root. 



Beyerinck found that the nodules did not develop on the roots of plants grown in sterihzed soil. 

 Frank reached the same conclusion in 1879, Hellriegel and Wilfarth in 1886, and Ward in 1887. 

 Plants in soil rich in humus are sometimes free or nearly free from nodules. 



According to Beyerinck there is only one bacterial species, but not all the forms are identical. 

 There are varieties. There is, for instance, a distinct difference between the bacteria occurring in 

 Vicia, Erimm, Trifolium, and Pisiim, on one hand, and in Lotus, Lupinus, Oniithopus, and Pliascolus 

 on the other hand. In the large rapidly growing colonies one is most apt to find B. radicicola like 



ordinary bacteria; in the small 

 slow growing ones there are more 

 branched bacteroids. 



He obtained the strongest 

 growing colonies out of the very 

 young nodules, or out of the outer 

 meristematic zone of the older 

 ones in Vicia faba, this being the 

 plant he studied most carefully. 

 The inner zone of the meristem 

 yielded more bacteroid elements 

 and slower growing colonies. 

 The same result was obtained with 

 Lupinus polyphyllus. This he 

 says is the lupin in which 

 Woronirte first saw the bacteria. 

 The nodules of this plant are 

 very large and the swarmers in 

 them are very minute. There 

 are no slime threads and there 

 is no meristem. 



The large watery colonies 

 consist of a mixture of rods 

 and swarmers, many motile. 

 The rods exclusive of some long 

 forms are about 4X n'. The 

 bacteroids of Vicia faba are some- 

 what larger and average 5X1,". 

 The swarmers are very small: 

 Taken from Vicia faba the}^ are 

 o.gXo.i8,". They are so small 

 that granting them some plasticity 

 they might easily penetrate the 

 pericambium cells without leav- 

 ing any visible wound. They 

 possess one polar flagellum. This 

 was inferred from behavior during slow motility rather than actually seen. Motility ceases almost 

 immediately in hydrogen or carbon dioxide. The little slow-growing colonies also contain swarmers. 

 B. radicicola has no special powers of fermentation, oxidation, or reduction. It does not produce 

 spores. It is not harmed by freezing or drying (see fig. 35). Neither diastase nor invertase are 

 produced. Cellulose and starch are not converted. Nitrates are not reduced. Oxygen is liberated 

 from hydrogen peroxide. It is aerobic. It does not liquefy gelatin. Meat-water peptone gelatin is 

 too concentrated for the first cultures (isolations). The addition of 0.25 per cent asparagin is useful 

 in agar cultures. Cohn's solution is too acid for B. radicicola. It will not grow in it even after 

 neutralization. Aklaline and neutral solutions are also injurious. For B. radicicola from Trifolium 

 repens 0.6 per cent '^ malic acid is useful. 



Fig. 34.* 



*FiG. 34. — Planar cularijement of stained section from a small root-nodule on soy-bean. Great bulk of section 

 consists of cells much enlarged and occupied by enormous numbers of Bad. leguniinosanini. Colorless spaces between 

 are occupied by smaller (non-distended) cells free from infection and Ijearing normal nuclei. Surrounding this central 

 mass is vascular tissue and beyond that cortex (both free from bacteria). 



