阅读理解

Directions: This part consists of two sections. In Section A, there are three passages followed by a total of 20 multiple-choice questions. In Section B, there are two passages followed by a total of 10 short-answer questions. Read the passages and write your answers on the Answer Sheet.

Passage 5

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 specific; that is, each has more than one effect on the growth and development of plants. The five has 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, 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 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. 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: 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.

问答题 According to the passage, why are the five well-known plant hormones not useful in artificially controlling thegrowth of crops?

【正确答案】Because the five well-known plant hormones have so many simultaneous effects.

【答案解析】第二段第二句指出“The five has so many simultaneous effects that they are not very useful in artificially controlling the growth of crops.”，这五种植物激素同时产生的影响如此之多，以至于它们在人为控制作物生长方面作用不大。

问答题 In terms of their place in hormonal hierarchies, which hormones in animals are similar to the five well-knownplant hormones?

【正确答案】Hypothalamic hormones.

【答案解析】根据关键词“hormones in animals”和“are similar to”定位到第三段第一句“The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animal.”。其中，“is analogous to”与“are similar to”意思相近。第二句指出“For example, hormones from the hypothalamus in the brain...”。根据关联词“For example”可知，第二句是对第一句的举例说明。所以第二句中提到的“hormones from the hypothalamus”与植物激素的层次结构相似。

问答题 What does the author intend to illustrate by mentioning specific effects that auxin has on plant development?

【正确答案】To illustrate the concept of pleiotropy as it is exhibited by plant hormones.

【答案解析】根据关键词“auxin”定位到第二段倒数第一、二句“Auxin, for instance, stimulates the rate of cell elongation, causes shoots...to develop a vascular system, to form lateral roots, and to produce ethylene.”，意思是“以生长素为例，生长素刺激细胞的伸长速度，使芽长起来，根扎下去，抑制侧枝的生长。生长素还使植物形成维管系统，形成侧根，并产生乙烯。”这两句话是在描述以植物生长素为代表的植物激素的多种作用。

问答题 Why does the author discuss animal hormones?

【正确答案】To introduce the idea of a hierarchy of hormones.

【答案解析】介绍动物激素的内容集中在第三段中间部分“For example, hormones from the hypothalamus in the brain stimulate...another the ovarian follicle cells, and so forth. In other words, there is a hierarchy of hormones.”。根据其中的“In other words”可知，后面的内容应该是对前面内容的概括，即“there is a hierarchy of hormones.”

问答题 How do plant hormones work in the process of cells differentiating and forming different structures of plants?

【正确答案】Only a small subset of the genes in a particular kind of cell are expressed, or turned on, at a given time.

【答案解析】题干中的问题出现在第一段第二句“How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits?”第三句“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.”是对第二句问题的回答。