单选题 Passage Six
Plants are subject to attach and infection by a remarkable variety of symbiotically 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 and inducible or active systems. Passive plant defense comprises physical and chemical barriers that prevent entry of pathogens(病原体), 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(表皮层) and a waxy cuticle, often carry spiky hairs known as trichomes(毛状物), 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.
If the physical barriers of the plant are breached, then preformed chemicals may inhibit or kill the intrude, 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 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(糖蛋白) in plant cell walls may inactivate enzymes that degrade cell walls. These enzymes are often produced by bacteria and fungi.
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 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 basis of hypersensitive resistance.
单选题 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.
【正确答案】 C
【答案解析】从全文来看，主要讲述了植物的防御系统怎样发挥作用，并拥有两个方面，即physical和chemical barriers。显然答案是C。
单选题 Which of the following substances does the author mention as NOT necessary being toxic to the Colorado beetle?
  • A. Resins
  • B. Tannins
  • C. Glycosides
  • D. Alkaloids
【正确答案】 D
【答案解析】依据文章第二段，“The success of the Colorado beetle in infesting potatoes, for example, seems to be correlated with its high tolerance to alkaloids”那种侵扰西红柿的害虫，对alkaloids有很强的忍耐性，D项不一定对它有毒。
单选题 Why does the author mention "glycoproteins" in Line 16?
  • 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 show the advantages of plant defense mechanisms.
【正确答案】 C
【答案解析】文中第二段阐明了植物如何通过化学物质抵御外来入侵，举例后使意思更具体。选项C是正确的。
单选题 The word "dramatic" in Line 22 could best be replaced by ______.
  • A. striking
  • B. accurate
  • C. consistent
  • D. appealing
【正确答案】 A
【答案解析】“dramatic”是戏剧性的，给人留下深刻印象的意思，根据文中出处的大意可以推断选项A的意思最接近。
单选题 The passage most probably continues with a discussion of theories on ______.
  • A. the basis of passive plant defense
  • B. how chemicals inhibit a parasitic relationship
  • C. how plants produce toxic chemicals
  • D. the principles of the hypersensitive response
【正确答案】 D
【答案解析】文章最后一段讲到，“Several theories have been put forward to explain the basis of hypersensitive resistance.”，因此答案是D。