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.
129. The passage suggests that the place of hypothalamic hormones in the hormonal hierarchies of animals is similar to the place of which of the following in plants?
(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
这题是这样的：hypothalamic hormones in the hormonal hierarchies of animals 出现在第二段。第二段的结构是开头给出一个中心句：The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animal.然后接着就开始举例说明，后面的例子都是要例证这个中心句的。从这个中心句可以看出，就是将那五种主荷尔蒙和动物的某些荷尔蒙类比。这题最重要的是明白第二段的结构，就是第一句和后面那些句子的结构关系.作者: appleontree 时间: 2011-9-15 06:25
这个题目我细细研究了，发现有两个办法来解决。
一，快速解决，适用于考试时。
题目问的是hypothalamus,那么就到文中第一次出现这个词的地方找。定位到29句。29句处于第二段，看第二段的第一句，The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animal. 这句话告诉我们pleiotropy of the five well-studied plant hormones和动物中的一些hormones是相似的。hypothalamus hormones 作为动物的hormones 就和 five plant hormones相似了。
二，是我仔细阅读后发现的答案。
来细看文中各种物质间的关系
28行开始：hormones from the hypothalamus in the brain stimulate the anterior lobe （hypothalamus hormones-->anterior）of the pituitary gland to synthesize and release many different hormones（anterior-->different hormones）, one of which stimulates the release of hormones from the adrenal cortex. These hormones have specific (one of the different hormones have specific effects)effects on target organs all over the body. One hormone stimulates the thyroid gland,
hypothalamus hormones-->anterior-->different hormones have specific effects
其中箭头的层层推导关系，就是文中所谓的 hierarchy吧。
38行开始：Oligosaccharins are fragments of the cell wall released by enzymes（enzymes-->Oligo）: different enzymes release different oligosaccharins. There are indications that pleiotropic plant hormones may actually function by activating the enzymes(pleiotropic plant hormones -->enzymes) that release these other, more specific chemical messengers from the cell wall.
pleiotropic plant hormones -->enzymes-->Oligo
