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arora007
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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 and inducible or active systems. Passive plant defense comprises physical and chemical 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 feeling by insects or may even puncture and kill insect Iarvae. 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 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 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.
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 par a site 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.
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 procuce toxic chemicals
(D) the principles of the hypersensitive response
SOURCE : retired GMAT tests-1996 "RC QUESTIONS GMAT(1995-2000).pdf"
[spoiler] My question here is, on the GMATPrep or real GMAT has anybody seen such a question, i.e questions which asks about what will be the next paragraph about? Is it worth putting the time in such questions? [/spoiler]
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 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.
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 par a site 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.
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 procuce toxic chemicals
(D) the principles of the hypersensitive response
SOURCE : retired GMAT tests-1996 "RC QUESTIONS GMAT(1995-2000).pdf"
[spoiler] My question here is, on the GMATPrep or real GMAT has anybody seen such a question, i.e questions which asks about what will be the next paragraph about? Is it worth putting the time in such questions? [/spoiler]
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https://twitter.com/skiponemeal
Few things are impossible to diligence & skill.Great works are performed not by strength,but by perseverance
pm me if you find junk/spam/abusive language, Lets keep our community clean!!
