Mycorrhizal fungi infect more plants than do any other fungi and are necessary for many plants to thrive, but they have escaped widespread investigation until recently for two reasons. First, the symbiotic association is so well-balanced that the roots of host plants show no damage even when densely infected. Second, the fungi cannot as yet be cultivated in the absence of a living root. Despite these difficulties, there has been important new work that suggests that this symbiotic association can be harnessed to achieve more economical use of costly superphosphate fertilizer and to permit better exploitation of cheaper, less soluble rock phosphate. Mycorrhizal benefits are not limited to improved phosphate uptake in host plants. In legumes, mycorrhizal inoculation has increased nitrogen fixation beyond levels achieved by adding phosphate fertilizer alone. Certain symbiotic associations also increase the host plant's resistance to harmful root fungi. Whether this resistance results from exclusion of harmful fungi through competition for sites, from metabolic change involving antibiotic production, or from increased vigor is undetermined.

17. Which of the following most accurately describes the passage?

(A) A description of a replicable experiment
(B) A summary report of new findings
(C) A recommendation for abandoning a difficult area of research
(D) A refutation of an earlier hypothesis
(E) A confirmation of earlier research


18. The level of information in the passage above is suited to the needs of all of the following people EXCEPT

(A) a researcher whose job is to identify potentially profitable areas for research and product development
(B) a state official whose position requires her to alert farmers about possible innovations in farming
(C) an official of a research foundation who identifies research projects for potential funding
(D) a biologist attempting to keep up with scientific developments in an area outside of his immediate area of specialization
(E) a botanist conducting experiments to determine the relationship between degree of mycorrhizal infection and expected uptake of phosphate


19. It can be inferred from the passage that which of the following has been a factor influencing the extent to which research on mycorrhizal fungi has progressed?

(A) Lack of funding for such research
(B) Lack of immediate application of such research
(C) Lack of a method for identifying mycorrhizal fungi
(D) Difficulties surrounding laboratory production of specimens for study
(E) Difficulties ensuing from the high cost and scarcity of superphosphate fertilizers


20. The passage suggests which of the following about the increased resistance to harmful root fungi that some plants infected with mycorrhizal fungi seem to exhibit?

(A) There are at least three hypotheses that might account for the increase.
(B) An explanation lies in the fact that mycorrhizal fungi increase more rapidly in number than harmful root fungi do.
(C) The plants that show increased resistance also exhibit improved nitrogen fixation.
(D) Such increases may be independent of mycorrhizal infection.
(E) It is unlikely that a satisfactory explanation can be found to account for the increase.