EARLY OBSERVATIONS ON ROOT TUBERCLES 291 



pointed in the other direction. Pot and field experiments carried 

 out in England, France, Germany, and the United States during the 

 early eighties furnished unmistakable evidence that the legumes 

 possessed the power of utilizing atmospheric nitrogen. Atwater's 

 experiments (1883-84) fully demonstrated this. In some of his 

 trials the nitrogen gained was 50 per cent, or more of the total 

 quantity harvested. However, the mystery was not solved until 

 1886 when Hellriegel and Wilfarth announced that the fixation of 

 free nitrogen is a property possessed by the legumes and is due to 

 the bacteria associated with them in the root tubercles. 



Early Observations on Root Tubercles. The presence of tubercle 

 on the roots of leguminous plants had long before been noted by 

 Malpighi. He regarded them as root galls. Later they were 

 regarded as buds of incomplete plants, or as rudimentary roots. 

 In 1866 Woronin found in them numerous minute bodies which bore 

 some resemblance to bacteria. They were rod-shaped but often 

 slightly forked to "T"- or "Y"-shaped bodies. On account of this 

 irregularity in shape the discoverer was unable to say whether they 

 were true bacteria or not. He, therefore, called them bacteroids, 

 and regarded them as the cause of the tubercles. In 1874 Erickson 

 found that in the early stages of the development of the tubercle it 

 was filled with long, branching threads resembling the mycelium of 

 fungi, and to these hyphse he attributed the formation of the tuber- 

 cles. In later stages of the growth of the tubercles he found bac- 

 teroids, but was unable to determine whether they had any connec- 

 tion with the hyphse or not. 



Frank (1879) not only showed that tubercles are almost invariably 

 present on the roots of legumes but that their formation may be 

 prevented by the sterilization of the soil. He was thus in possession 

 of facts which might have revealed to him the true nature of the root 

 tubercles. However, he accepted the interpretation of his pupil, 

 Brunchhorst, who claimed the bacteria-like bodies, were merely 

 reserve food materials. 



Marshall Ward not only proved that tubercle formation is due to 

 outside infection but that such infection may be brought about by 

 placing pieces of old tubercles in contact with the roots of growing 

 leguminous plants. 



Hellriegel found, as the result of a long series of experiments, that 

 when pea plants were grown in sterilized soils as a rule no tubercles 

 were formed, but when the plants were watered with soil infusions 

 made by allowing water to act upon soil in which peas had been 

 grown, the tubercles appeared in abundance. If the soil infusion 

 was sterilized by boiling before it was put upon the plants no tuber- 

 cles appeared. These experiments were thought to prove that the 

 tubercles were really caused by living organisms in the soil infusion, 

 which were killed by heat. The tubercles could not, therefore, be 



