GRE Reading Comprehension: ETS-GRE阅读ETS - GWW7H3OM46T5YNR38

It is one of nature's great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants' leaves are bathed in a sea of nitrogen gas. The leguminous plants – among them crop plants such as soybeans, peas, alfalfa, and clover – have solved the nitrogen supply problem by entering into a symbiotic relationship with the bacterial genus Rhizobium.