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TOEFL Readings 37
Plants are subject to attack and infection by a remarkable variety of symbiotic
species and have evolved a diverse array of mechanisms designed to frustrate the
potential colonists. These can be divided into preformed or passive defense mechanisms
Line and inducible or active systems. Passive plant defense comprises physical and chemical
(5) barriers that prevent entry of pathogens, such as bacteria, or render tissues unpalatable
or toxic to the invader. The external surfaces of plants, in addition to being covered by
an epidermis and a waxy cuticle, often carry spiky hairs known as trichomes, which
either prevent feeding by insects or may even puncture and kill insect larvae. Other
trichomes are sticky and glandular and effectively trap and immobilize insects.
(10) If the physical barriers of the plant are breached, then preformed chemicals may
inhibit or kill the intruder, and plant tissues contain a diverse array of toxic or
potentially toxic substances, such as resins, tannins, glycosides, and alkaloids, many of
which are highly effective deterrents to insects that feed on plants. The success of the
Colorado beetle in infesting potatoes, for example, seems to be correlated with its high
(15) tolerance to alkaloids that normally repel potential pests. Other possible chemical
defenses, while not directly toxic to the parasite, may inhibit some essential step in the
establishment of a parasitic relationship. For example, glycoproteins in plant cell walls
may inactivate enzymes that degrade cell walls. These enzymes are often produced by
bacteria and fungi.
(20) Active plant defense mechanisms are comparable to the immune system of
vertebrate animals, although the cellular and molecular bases are fundamentally
different. Both, however, are triggered in reaction to intrusion, implying that the host
has some means of recognizing the presence of a foreign organism. The most dramatic
example of an inducible plant defense reaction is the hypersensitive response. In the
(25) hypersensitive response, cells undergo rapid necrosis ― that is, they become diseased
and die ― after being penetrated by a parasite ; the parasite itself subsequently ceases to
grow and is therefore restricted to one or a few cells around the entry site. Several
theories have been put forward to explain the bases of hypersensitive resistance.
43. What does the passage mainly discuss?
(A) The success of parasites in resisting plant defense mechanisms
(B) Theories on active plant defense mechanisms
(C) How plant defense mechanisms function
(D) How the immune system of animals and the defense mechanisms
of plants differ
44. The phrase "subject to" in line 1 is closest in meaning to
(A) susceptible to
(B) classified by
(C) attractive to
(D) strengthened by
45. The word "puncture" in line 8 is closest in meaning to
(A) pierce
(B) pinch
(C) surround
(D) cover
46. The word "which" in line 13 refers to
(A) tissues
(B) substances
(C) barriers
(D) insects
47. Which of the following substances does the author mention as NOT
necessarily being toxic to the Colorado beetle?
(A) Resins
(B) Tannins
(C) Glycosides
(D) Alkaloids
48. Why does the author mention "glycoproteins" in line 17 ?
(A) To compare plant defense mechanisms to the immune system of animals
(B) To introduce the discussion of active defense mechanisms in plants
(C) To illustrate how chemicals function in plant defense
(D) To emphasize the importance of physical barriers in plant defense
49. The word "dramatic" in line 23 could best be replaced by
(A) striking
(B) accurate
(C) consistent
(D) appealing
50. Where in the passage dose the author describe an active plant-defense
reaction?
(A) Lines 1-3
(B) Lines 4-6
(C) Lines 15-17
(D) Lines 24-27