274 MICROBIOLOGY OF SOIL. 



ing the fall of Rome the use of legumes for soil improvement persisted to 

 some extent in Italy, France and other countries; yet the practice was 

 not followed consistently and the fertility of European soils was declining 

 for lack of available nitrogen, and, to a large extent, also of phosphoric 

 acid. The more general introduction of clover into Germany and 

 England in the eighteenth century helped to restore the fertility of many 

 farms, and led, ultimately, to the recognition of the peculiar place held 

 by legumes in the maintenance of soil fertility. But while practical 

 farmers knew of the soil-enriching power of legumes, and while they 

 retained their belief in it even when it seemed contrary to scientific 

 authority, they did not know the secret of this power. It remained for 

 Hellriegel and Wilfarth to demonstrate in 1886, and more fully in 1888, 

 that this power, already hinted at by the investigations of others, is the 

 resultant of the combined activities of the plants and of bacteria that 

 enter their roots, and produce there the well known nodules or tubercles. 

 They showed in no uncertain manner that legumes can improve the soil 

 only in so far as they add nitrogen to it with the aid of the bacteria in the 

 tubercles; in other words, legumes were shown to enter into a symbiotic 

 relationship with certain bacteria and to acquire, thereby, the ability to 

 fix atmospheric nitrogen. 



The presence of tubercles on the roots of leguminous plants was first 

 recorded by Malpighi in 1687. He regarded them as root galls. The 

 botanists who studied them in the first half of the nineteenth century clas- 

 sified them as modifications of normal roots or as pathological processes. 

 In 1866 the Russian botanist Woronin found that the tubercles were filled 

 with minute bodies resembling bacteria and concluded that they were 

 pathological outgrowths. Some years later Frank not only showed that 

 tubercles are almost invariably present on the roots of legumes, but 

 that their formation may be prevented by sterilizing the soil. Frank 

 was thus in possession of facts that might have revealed to him the true 

 nature of the root-tubercles. However, he later modified his belief in 

 the origin of tubercles as due to outside infection, and accepted the 

 interpretation of his pupil Brunchhorst who claimed that the bac- 

 teria-like bodies in the tubercles were merely reserve food materials. 

 Because of their resemblance to bacteria Brunchhorst named them 

 bacteroids. 



The studies of Marshall Ward, published in 1887, proved not merely 

 that tubercle formation is due to outside infection, but that such infection 



