220 Nitrogen Fixation— Symbiotic 



tain a small volume of rapidly degenerating bacterial tissue; the 

 quantity of nitrogen fixed per unit volume of bacterial tissue, how- 

 ever, is the same in "effective" and "ineffective" nodules. An "in- 

 effective" or "poor" strain is not one, therefore, which lacks the power 

 of causing nitrogen fixation, but one which in a given host plant 

 evokes the formation of specific substances inhibitory to the devel- 

 opment of the bacterial tissue, as pointed out by Jensen. Certain 

 strains may be effective, however, on some species and not on other, 

 closely related species. 



Nodule Formation by Nonleguminous Plants 



In addition to leguminous plants, certain nonlegumes, such as 

 Ceanothtis (redroot), Elaeagniis (silver berry), Alniis (alder), and 

 Myrica (sweet gale), are also capable of forming nodules on their 

 roots. These nodules are perennial and branch in all directions, 

 finally developing round aggregates of considerable size. 



These nodules were at first believed to be of fungus origin. It 

 was shown later that they are caused by bacteria closely resembling 

 the Rhizobitim group, and that they are capable of causing fixation 

 of nitrogen. Burrill and Hansen emphasized, however, that some 

 of the nodules are not caused by Rhizobiiim. The concept of their 

 ability to fix atmospheric nitrogen was not considered conclusive. 

 In some plants {Myrica) the organism seems to be of the nature 

 of an actinomyces. Coriaria japonica produces nodules similar to 

 those of Ahms, due also to an actinomyces (A. myricae of Peklo). 

 This plant, when it forms nodules, is able to grow vigorously and 

 accumulate nitrogen in a medium free from combined nitrogen. The 

 plants free from such nodules show signs of nitrogen starvation. 



The presence of Rhizohiiim, Azotobocter, and certain algae {Ana- 

 baena, Nostoc) was noted in the roots of Cycas. The ability of most 

 of these plants to fix nitrogen is still questionable, although it is 

 reported that some, like Casuarina, are able to grow readily in poor 

 sandy soil. There is no doubt that some of these bacteria found in 

 the roots of the plants are responsible for symbiotic nitrogen fixa- 

 tion. The production of a growth-promoting substance by the bac- 

 teria has also been suggested. 



There are also certain leguminous plants that do not form nodules. 

 These include various members of the Caesalpinaceae, such as 

 Gymnocladus, Cercis, and Gleditsia. 



