Passage 2
By-申友教学部
All 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? // (提出现象一组基因中产生的细胞形成不同的根茎叶结构。) 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.（专有名词第一次出现需引起重视，生词可用首字母代入法表示。） // (解释现象，只有一部分基因通过荷尔蒙和OL物质被表达出来。) //
Unlike（出现比较词，注意对比的双方）the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific ; that is,（解释说明，可以略读） each has more than one effect on the growth and development of plants. // (荷尔蒙与OL物质对比，具有多效性、不专一性。) The five have so many simultaneous effects that they are not very useful（负评价词） in artificially controlling the growth of crops. Auxin, for instance,（举例可以略读，注意看前一句总结句。） stimulates the rate of cell elongation, 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.（荷尔蒙因为具有多效性所以在控制植物生长方面不是很有作用。）//
The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals. For example, （举例略读）hormones from the hypothalamus 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 cortex. // (植物中荷尔蒙的多效性和动物相似。) 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.(注意第一次出现hierarchy, 与下段开头照应。) //（荷尔蒙的专一性体现在动物目标器官上。）
Such a hierarchy may also exist in plants. // (分级系统也出现在植物中。) Oligosaccharins are fragments of the cell wall released by enzymes: 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. // （具体说明植物中的分级系统，荷尔蒙释放酶，酶释放OL物质。）
逻辑简图：
文章属于自然科学类题材，写作结构为现象解释型。
第一段：
第一层：提出现象，一组相同的基因产生出的细胞形成不同的组织结构，如根茎叶。
第二层：解释现象，因为只有一部分的基因被表达了，并且是通过荷尔蒙和OL物质表达的。
第二段：
第一层：荷尔蒙与OL物质不同，具有多效性。
第二层：荷尔蒙因为具有多效性，而在控制植物生长方面不是很有效。
第三段：
第一层：荷尔蒙在植物中的多效性和动物中相似。
第二层：荷尔蒙的专一性体现在动物的目标器官中。
第四段：
第一层：分级系统也出现在植物中。
第二层：植物中的分级系统表现为荷尔蒙释放酶，酶释放OL物质。
1. According to the passage, the five well-known plant hormonesare not useful in controlling the growth of crops because
