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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
(5) small subset of the genes in a particular kind of cell are
expressed, or turned on, at a given time. This is accom-
plished by a complex system of chemical messengers
that in plants include hormones and other regulatory
molecules. Five major hormones have been identified:
(10) auxin, abscisic acid, cytokinin, ethylene, and gibberel-
lin. 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,
(15) each has more than one effect on the growth and devel-
opment 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
(20) 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
(25) 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
(30) 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.
(35) Such a hierarchy may also exist in plants. Oligosac-
charins are fragments of the cell wall released by
enzymes: different enzymes release different oligosac-
charins. There are indications that pleiotropic plant
hormones may actually function by activating the
(40) enzymes that release these other, more specific chemical
messengers from the cell wall.
68.The passage suggests that the place of hypotha-
lamic hormones in the hormonal hierarchies of
animals is similara to the place of
which of the followeing in plants?
(A) Plant cell walls
(B)The complement of genes in each plant cell
(C)A subsetof a plant cell'sgene complement
(D)The five major hormones
(E)The oligosaccharins
The best answer is D.Acording to the passage ,"The pleiotropy ofthe five well-studied plant hormones is somewhat analogous to that ofcertain hormones in animals"(lines 24-26).The example given involves certain hypothalamic hormones that "stimulate the anterir love of the pituitary gland to synthesize and release many different hormones from the adrenal cortex"(lines27-30)……
请教：题目问的是 hypothalamic hormones 的place,为什么回答是D,难道是hormones 在The five major hormones里，请大虾们解释一下 | 
