GRE阅读练习每日一篇(三十四)_GRE阅读

GRE阅读练习每日一篇(三十四)

　　Concentrated on the complex relations between the technical elements in Afro-American poetry and its political content.

　　Two relatively recent independent developments stand behind the current major research effort on nitrogen fixation, the process by which bacteria symbiotically render leguminous plants independent of nitrogen fertilizer. The one development has been the rapid, sustained increase in the price of nitrogen fertilizer ). The other development has been the rapid growth of knowledge of and technical sophistication in genetic engineering. Fertilizer prices, largely tied to the price of natural gas, huge amounts of which go into the manufacture of fertilizer, will continue to represent an enormous and escalating economic burden on modern agriculture, spurring the search for alternatives to synthetic fertilizers. And genetic engineering is just the sort of fundamental breakthrough that opens up prospects of wholly novel alternatives. One such novel idea is that of inserting into the chromosomes of plants discrete genes that are not a part of the plants natural constitution: specifically, the idea of inserting into nonleguminous plants the genes, if they can be identified and isolated, that fit the leguminous plants to be hosts for nitrogen-fixing bacteria. Hence, the intensified research on legumes.

　　Nitrogen fixation is a process in which certain bacteria use atmospheric nitrogen gas, which green plants cannot directly utilize, to produce ammonia, a nitrogen compound plants can use. 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; as a matter of fact , there is a specific strain of Rhizobium for each species of legume. The host plant supplies the bacteria with food and a protected habitat and receives surplus ammonia in exchange. Hence, legumes can thrive in nitrogen-depleted soil.