A Hierarchy of Hormones All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and from 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 (生长素), abscisic acid(脱落酸) ,cytokinin(细胞激动素), hetylene(乙烯), and gibberellin(赤霉素). 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 (多向性的) rather than specific, 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 elongation(拉长), causes shoots to grow up and roots to grow down, and inhibits the growth of lateral(侧面的) shoots. Auxin also causes the plant to develop a vascular(维管的) system, to from lateral roots, and to produce ethylene. The pleiotropy of the five well-studied plant hormones is somewhat ogous (类似的) to that of certain hormones in animal. For example, hormones from the hypothalamus (下五脑) in the brain stimulate the anterior(前面的) lobe of the pituitary gland to synthesize and release many different hormones, one of which stimulates the release of hormones from the adrenal(肾上腺的) cortex(皮质|). These hormones have specific effects on target organs all over the body. One hormone stimulates the thyroid (甲状腺的) gland, for example, another the ovarian (子房)follicle cells, and so forth. In other words, there is a hierarchy(统治集团) 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. According to the passage, the five well-known plant hormones are not useful in controlling the growth of crops because______.