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 small subset of the genes in a particular kind of cell are expressed,or turned on,at a given time. This is accomplished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid ,cytokinin ,ethylene ,and gibberellin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins. Unlike the oligosaccharins,the five well-known plant hormones are pleiotropic rather than specifie, that is, each has more than one effect on the growth and development of plants. The five have so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Auxin, for instance, stimulates the rate of cell elon-gation(使伸长),causes shoots to grow up and roots to grow down, and inhibits the growth of lateral shoots. Auxin also causes the plant to develop a vascular system, to form. lateral roots,and to produce ethylene. The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animal. For example, hormones from the hypothalamus(下丘脑)in the brain stimulate the anterior lobe of the pituitary gland to synthesize and release many different hormones , one of which stimulates the release of hormones from the adrenal cor-tex(肾上腺皮质). These hormones have specific effects on target organs all over the body.One hormone stimulates the thyroid gland, for example,another the ovarian follicle cells,and so forth. In other words, there is a hierarchy of hormones. Such a hierarchy may also exist in plants. Oligosaccharins are fragments of the cell wall released by enzymes(酶);different enzymes release different oligosaccharins.There are indications that pleiotropic plant hormones may actually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall. The five well-known plant hormones are not useful in controlling the growth 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 The passage that the place of hypothalamic hormones in the hormonal hierarchies of animals is similar to the place of____.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 Which of the follow is a function like to be performed by an oligosaccharinA.To stimulate a particular plant cell to become part of a plant’s root system, B.To activate enzymes that release specific chemical messengers from plan cell walls. C.To duplicate the Kcnc complement in a particular pUnt coll. D.To produce multiple effects on a particular subsystem of plant cells. The author mentions specific effects that auxin on plam development in order to iilustrate the ______.A.way in which hormones arc produced by plant B.hierarchical nature of the functioning of plant hormones C.differenecs between the best-known plant hormones D.concept of pleiotropy as it is exhibited by plant hormones