224 Nitrogen Fixation— Symbiotic 



zobia. When a large number of legumes were planted on an alfalfa 

 field suffering from "fatigue," believed to be due to the presence of 

 the bacteriophage, the nodules of all species contained abnormal, 

 vacuolated forms of the organisms as well as the specific phage. The 

 importance of the phage in the phenomenon of fatigue may still be 

 considered questionable, however. According to Katznelson, a phage 

 for alfalfa was absorbed from suspensions only by members of the 

 alfalfa-sweet-clover cross-inoculation group, thus suggesting a tech- 

 nique which may be useful in distinguishing members of the differ- 

 ent species of Rhizobium. 



Excretion of Nitrogen by Legumes 



The first demonstration that nonleguminous plants are able to 

 benefit from association with a legume was presented by Lipman. 

 He suggested that the beneficial effect was due to nitrogenous com- 

 pounds excreted by the leguminous plants and consumed by the non- 

 legumes. Lipman used a half-and-half mixture of peas and oats 

 grown in a medium consisting of soil plus sand. The total nitrogen 

 and dry weight of the oats associated with the peas exceeded those 

 of the crop of oats grown alone. The yields of oats and peas in the 

 mixture were greater than those of the crops grown alone. 



Lipman further demonstrated in a series of experiments that the 

 benefit to the associated nonlegume arises from excretion of nitro- 

 gen by the leguminous plant. He used as the medium a pure quartz 

 sand treated with all the necessary plant nutrients except nitrogen 

 and placed a small pot within a larger one so that the two plants 

 grew in separate containers. Whenever the inner pot was porous, 

 or would allow the passage of substances in solution, the nonlegume 

 developed normally and, on analysis, showed considerable quantities 

 of nitrogen. If the inner pot was glazed, the nonlegume grew poorly 

 and contained little nitrogen. On the basis of these results, Lipman 

 suggested that nonlegumes benefit from association with legumes as 

 a result of the excretion of nitrogen by the latter. This work was 

 repeated by many others, some quite independently of the above 

 experiments. 



Virtanen observed in 1927 that oats grown in association with peas 

 on a nitrogen-free quartz sand developed as though combined nitro- 

 gen had been supplied. Similar results were later obtained with 

 other combinations of legumes and nonlegumes. The fact that 



