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All thecells in a particular plant start out with the same complement of genes. How thencan these cells differentiate and form structures as different as roots, stems,leaves, and fruits? The answer is thatonly a small subset of the genes in a particular kind of cell are expressed, orturned on, at a given time. This is accomplished by a complex system ofchemical messengers that in plants include hormones and other regulatorymolecules. Five major hormones have been identified:auxin, abscisic acid, cytokinin, ethylene, and gibberellin. Studies of plantshave now identified a new class of regulatory molecules called oligosaccharins.

Unlike the oligosaccharins, the five well-known plant hormones arepleiotropic rather than specific; that is, each has more than one effect on thegrowth and development of plants. The five have so manysimultaneous effects that they are not very useful in artificially controllingthe growth of crops. Auxin, for instance, stimulates the rate of cell elongation, causesshoots 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 lateralroots, and to produce ethylene.

The pleiotropy of the five well-studied plant hormones is somewhatanalogous to that of certain hormones in animals. For example, hormones from thehypothalamus in the brain stimulate the anterior lobe of the pituitary gland tosynthesize and release many different hormones, one of which stimulates therelease of hormones from the adrenal cortex. These hormones have specificeffects on target organs all over the body. One hormone stimulates the thyroidgland, for example, another the ovarian follicle cells, and so forth. In otherwords,there is a hierarchy of hormones.

Such a hierarchy mayalso exist in plants. Oligosaccharins are fragments ofthe cell wall released by enzymes: different enzymes release different oligosaccharins.There are indications that pleiotropic plant hormones may actually function byactivating the enzymes that release these other, more specific chemical messengersfrom the cell wall.

132.According to the passage, which of the following best describes a functionperformed by oligosaccharins?

(A)     Regulating the dailyfunctioning of a plant's cells

(B)     Interacting with one another toproduce different chemicals

(C)     Releasing specific chemicalmessengers from a plant's cell walls

(D)     Producing the hormones thatcause plant cells to differentiate to perform different functions

(E)      Influencing the development ofa plant's cells by controlling the expression of the cells' genes

133. Thepassage suggests that, unlike the pleiotropic hormones, oligosaccharins couldbe used effectively to

(A)     trace thepassage of chemicals through the walls of cells（为什么错？）

(B)     pinpoint functions of otherplant hormones（原文没提）

(C)     artificiallycontrol specific aspects of the development of crops（正确，原文：The five have so many simultaneous effects thatthey are not very useful in artificially controlling the growth of crops.）

(D)     alter the complement of genesin the cells of plants（原文没提）

(E)      alter the effects of the fivemajor hormones on plant development（原文没提）

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