Passage 24
¡¡¡¡All of the cells in a particular plant start out with the£ same complement of genes. How then can these cells£ differentiate 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 hormones and other regulatorymolecules. Five major hormones have been identified£º£¨10£© auxin£¬ abscisic acid£¬ cytokinin£¬ ethylene£¬ and gibberel-lin. Studies of plants have now identified a new class ofregulatory molecules 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£¬stimulates the rate of cell elongation£¬ causes shoots to£¨20£© grow up and roots to grow down£¬ and inhibits thegrowth of lateral shoots. Auxin also causes the plant todevelop a vascular system£¬ to form lateral roots£¬ and toproduce ethylene.
¡¡¡¡The pleiotropy of the five well-studied plant£¨25£© hormones is somewhat analogous to that of certain£  hormones in animal. For example£¬ hormones from thehypothalamus in the brain stimulate the anterior lobeof the pituitary gland 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 hormone stimulates the thyroid gland£¬ forexample£¬ another the ovarian follicle cells£¬ and so forth.In other words£¬ there is a hierarchy of hormones.£¨35£©£ Such a hierarchy may also exist in plants. Oligosac-charins are fragments of the cell wall released byenzymes£º different enzymes release different oligosac-£  charins. There are indications that pleiotropic plant£  hormones may actually function by activating 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 hormonal hierarchies 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 gene 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 activate 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 illustrate 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£© pinpoint 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

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