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

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