GMAT考试阅读资料(五)b

Passage 24
　　All of the cells in a particular plant start out with the　same complement¹ of genes³. How then can these cells　differentiate⁴ and form structures as different as roots，　stems， leaves， and fruits？ The answer is that only a（5） small subset of the genes in a particular kind of cell are　expressed， or turned on， at a given time. This is accom-　plished by a complex system of chemical messengers　that in plants include hormones⁶ and other regulatorymolecules. Five major hormones have been identified：（10） auxin， abscisic acid， cytokinin， ethylene， and gibberel-lin. Studies of plants have now identified a new class ofregulatory molecules⁷ called oligosaccharins.
　　Unlike the oligosaccharins， the five well-known planthormones are pleiotropic rather than specific， that is，（15） each has more than one effect on the growth and devel-opment of plants. The five has so many simultaneouseffects that they are not very useful in artificiallycontrolling the growth of crops. Auxin， for instance，stimulates⁹ the rate of cell elongation， causes shoots to（20） grow up and roots to grow down， and inhibits¹⁰ thegrowth of lateral¹¹ shoots. Auxin also causes the plant todevelop a vascular¹² system， to form lateral roots， and toproduce ethylene.
　　The pleiotropy of the five well-studied plant（25） hormones is somewhat analogous¹³ to that of certain　 hormones in animal. For example， hormones from thehypothalamus in the brain stimulate⁸ the anterior¹⁴ lobeof the pituitary gland¹⁵ to synthesize and release manydifferent hormones， one of which stimulates the release（30） of hormones from the adrenal cortex. These hormones　 have specific effects on target organs all over the body.One hormone⁵ stimulates the thyroid gland， forexample， another the ovarian follicle cells， and so forth¹⁶.In other words， there is a hierarchy¹⁷ of hormones.（35）　Such a hierarchy may also exist in plants. Oligosac-charins are fragments of the cell wall released byenzymes： different enzymes¹⁸ release different oligosac-　 charins. There are indications that pleiotropic plant　 hormones may actually function by activating¹⁹ the（40） enzymes that release these other， more specific chemical　 messengers from the cell wall.

　　1. According to the passage， the five well-known plant　hormones are not useful in controlling the growth of crops because
　　　（A） it is not known exactly what functions the hormones perform
　　　（B） each hormone has various effects on plants
　　　（C） none of the hormones can function without the others
　　　（D） each hormone has different effects on different kinds of plants
　　　（E） each hormone works on only a small subset of a cell‘s genes at any particular time　

　　2. The passage suggests that the place of hypothalamic　hormones in the hormonal²⁰ hierarchies²¹ of animals is　similar to the place of which of the following in plants？
　　　（A） Plant cell walls
　　　（B） The complement of genes in each plant cell
　　　（C） A subset of a plant cell‘s gene² complement
　　　（D） The five major hormones
　　　（E） The oligosaccharins

　　3. The passage suggests that which of the following is a function likely to be performed by an oligosaccharin？
　　　（A） To stimulate a particular plant cell to become part of a plant‘s root system
　　　（B） To stimulate the walls of a particular cell to produce other oligosaccharins
　　　（C） To activate²² enzymes that release specific chemical messengers from plant cell walls
　　　（D） To duplicate the gene complement in a particular plant cell
　　　（E） To produce multiple effects on a particular subsystem of plant cells

　　4. The author mentions specific effects that auxin has on　plant development in order to illustrate²³ the
　　　（A） point that some of the effects of plant hormones can be harmful
　　　（B） way in which hormones are produced by plants
　　　（C） hierarchical nature of the functioning of plant hormones
　　　（D） differences among the best-known plant hormones
　　　（E） concept of pleiotropy as it is exhibited by plant hormones

　　5. According to the passage， which of the following best　describes a function performed by oligosaccharins？
　　　（A） Regulating the daily functioning of a plant‘s cells
　　　（B） Interacting with one another to produce different chemicals
　　　（C） Releasing specific chemical messengers from a plant‘s cell walls
　　　（D） Producing the hormones that cause plant cells to differentiate to perform different functions
　　　（E） Influencing the development of a plant‘s cells by controlling the expression of the cells’genes

　　6. The passage suggests that， unlike the pleiotropic　hormones， oligosaccharins could be used effectively　to
　　　（A） trace the passage of chemicals through the walls of cells
　　　（B） pinpoint²⁴ functions of other plant hormones
　　　（C） artificially control specific aspects of the development of crops
　　　（D） alter the complement of genes in the cells of plants　
　　　（E） alter the effects of the five major hormones on plant development

　　7. The author discusses animal hormones primarily in　order to
　　　（A） introduce the idea of a hierarchy of hormones
　　　（B） explain the effects that auxin has on plant cells
　　　（C） contrast the functioning of plant hormones and animals hormones
　　　（D） illustrate the way in which particular hormones affect animals
　　　（E） explain the distinction between hormones and regulatory molecules