Allthe cells in aparticular plant start outwith the same complement of genes. How then can these cells differentiate and form structures asdifferent 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 isaccomplished by a complex system of chemical messengers that in plantsinclude hormones and other regulatory molecules. Five major hormones have been identified: auxin,abscisic acid, cytokinin, ethylene, and gibberellin. Studies of plants have nowidentified a new class of regulatory molecules calledoligosaccharins.（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五大荷尔蒙和OL物质被表达出来。) //

Unlike（出现比较词，注意对比的双方）theoligosaccharins, the fivewell-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 ofplants. // (五大荷尔蒙与OL物质对比，具有多效性、不专一性。) The fivehave so many simultaneous effects that they are not very useful（负评价词） in artificially controllingthe 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 upand roots to grow down, and inhibits the growth of lateral shoots. Auxin alsocauses the plant to develop a vascular system, to form lateral roots, and toproduce ethylene.（五大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。）//

Thepleiotropy of the fivewell-studied planthormones is somewhatanalogous to that of certain hormones in animals. For example, （举例略读）hormones from the hypothalamus in thebrain stimulate the anterior lobe of the pituitary gland to synthesize andrelease many different hormones, one of which stimulates the release ofhormones from the adrenal cortex. // (植物中五大荷尔蒙的多效性和动物相似。) Thesehormones 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. Inother words, there is a hierarchy ofhormones.(注意第一次出现hierarchy, 与下段开头照应。) //（五大荷尔蒙的专一性体现在动物目标器官上。）

Such a hierarchy may also existin plants. // (分级系统也出现在植物中。) Oligosaccharins arefragments of the cell wall releasedby enzymes: different enzymes release different oligosaccharins. Thereare indications that pleiotropic plant hormones may actually function by activating the enzymes thatrelease 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，并且是通过五大荷尔蒙和OL物质表达的。

第二段：

第一层：五大荷尔蒙与OL物质不同，具有多效性。

第二层：五大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：五大荷尔蒙在植物中的多效性和动物中相似。

第二层：五大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 hormones are not useful incontrolling the growth of crops because （细节题：定位到第二段的第二层）

(A)it is not known exactly what functions the hormones perform 错误表述

(B) each hormone has various effects on plants 同义转换，正确

(C)none of the hormones can function without the others 新说法

(D)each hormone has different effects on different kinds of plants 新说法

(E)each hormone works on only a small subset of a cell’s genes at any particulartime

  段落定位混淆

2.The passage suggests that the place of hypothalamic hormones in the hormonal hierarchies of animals is similar to the place of which of thefollowing in plants?（细节题之推论题：定位到第三段的第一层）

(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 followingis a function likely to be performed by an oligosaccharin?  （细节题：注意用OL物质定位，原文中分别有三处提到了OL物质-第一段（第二层）；第二段（第一层）；第四段（第二层））

(A)To stimulate a particular plant cell to become part of a plant’s root system 对应第一段的信息

(B)To stimulate the walls of a particular cell to produce otheroligosaccharins  新说法

(C)To activate enzymes that release specific chemical messengers from plant cellwalls. 错误表述

(D)To duplicate the gene complement in a particular plant cell. 新说法

(E)To produce multiple effects on a particular subsystem of plant cells. 错误表述

4.The author mentions specific effects that auxinhas on plant development in order to illustratethe（细节题：定位到第二段的第二层；举例中的信息，注意例子前的总结句。）

(A)point that some of the effects of plant hormones can be harmful. 新说法

(B)way in which hormones are produced by plants. 新说法

(C)hierarchical nature of the functioning of plant hormones. 段落定位混淆

(D)differences among the best-known plant hormones. 新说法

(E) concept of pleiotropy as it is exhibited by plant hormones. 例子前的总结句

5.According to the passage, which of the following best describes a functionperformed by oligosaccharins?  （细节题：注意用OL物质定位，原文中分别有三处提到了OL物质-第一段（第二层）；第二段（第一层）；第四段（第二层））

(A)Regulating the daily functioning of a plant’s cells 新说法

(B)Interacting with one another to produce different chemicals 新说法

(C)Releasing specific chemical messengers from a plant’s cell walls 错误表述

(D)Producing the hormones that cause plant cells to differentiate to performdifferent functions 错误表述

(E) Influencing the development of a plant’s cells by controlling theexpression of the cells’ genes 对应第一段信息

6.The passage suggests that, unlike the pleiotropichormones, oligosaccharins could be usedeffectively to （细节题：定位到第二段的第一层。）

(A)trace the passage of chemicals through the walls of cells 新说法

(B)pinpoint functions of other plant hormones 新说法

(C) artificially control specific aspects of the development of crops 对应第二段的信息

(D)alter the complement of genes in the cells of plants 新说法

(E)alter the effects of the five major hormones on plant development 新说法