334 ROOT-NODULE BACTERIA 



also present. By means of this system of vascular tubes the plant supplies the bac- 

 teria with sugars and other food substances, and in turn takes away the nitrogenous 

 compounds prepared by the bacteria. 



The shape, size, and position of the nodules vary with the dififerent leguminous 

 plants, e.g., alfalfa nodules are small, finger-shaped swellings, single or in bunches or 

 clusters; soy-bean nodules are larger, usually spherical, and rarely in clusters. Fig- 

 ures I and 2 show the nodules of pea and soy bean. 



Of the ten thousand or more species of leguminous plants described by botanists, 

 all except a very few, like Kentucky coffee tree and wild senna, show root nodules. 

 The bacteria, however, from these various plants are not all alike. In general, plants 

 closely related harbor the same kind of bacteria.' Some idea of the relationship among 

 the bacteria and higher plants is shown in the groups of the more commonly cultivated 

 plants. Below is a list of common leguminous plants divided into groups on the basis 

 of cross-inoculation. The plants of each group have their own specific organism which 

 can inoculate any member of the group but will not interchange with the plants of 

 another group. 



CROSS-INOCULATION GROUPS 



1. Alfalfa, white sweet clover, yellow sweet clover, Hubam, bur clover, yellow trefoil, and 

 fenugreek 



2. Red, mammoth, alsike, crimson, Egyptian, and white Dutch clovers 



3. Garden, canning, and field peas; hairy, spring, and wild vetches; broad bean; lentil; 

 , sweet pea; and perennial pea 



4. Cowpea, peanut, Japan clover, velvet bean, lima bean, partridge pea, wild indigo, and 

 tick trefoil 



5. Garden, field, navy, kidney, wax, and scarlet runner beans 



6. Lupines and serradella 



7. Soy beans 



8. Wood's clover (Dalea) 



9. Sanfoin 

 10. Locust 



The explanation for the grouping of the plants into the so-called "cross-inocula- 

 tion" groups has not been determined. Probably some phase of the physiological 

 complex of the plant is responsible. Support for this assumption may be seen in the 

 results of recent precipitin tests with the seed proteins of leguminous plants.^ These 

 tests show that the seed proteins of those leguminous plants belonging to any one 

 cross-inoculation group are closely related. 



There is some evidence that the bacteria within a given cross-inoculation group 

 are not all alike. The organisms from pea and vetch are interchangeable, and yet the 

 best results are usually secured from pea bacteria on peas and vetch on vetches. All 

 in all, there exists a high degree of specificity. Why this difference should exist is at 

 the present time unknown. 



' Burrill, T. J., and Hansen, R.: Illinois Agr. Exper. Sla. Bull. 202. 1917; Fred, E. B., Whiting, 

 A. L., and Hastings, E. G.: Wisconsin Agr. Exper. Sla. Research Bull. 72. 1926. 

 ^ Baldwin, I. L., Fred, E. B., and Hastings, E. G.: BoL Gaz., 83, 217. 1927. 



