The characteristic smell or taste of a plant, to insects as well as to

The characteristic smell or taste of a plant, to insects as well as to humans, depends on its chemical composition. Broadly speaking, plants contain two categories of chemical substances primary and secondary. The primary substances, such as proteins, carbohydrates, vitamins, and hormones, are required for growth and proper functioning and are found in all plants. The secondary substances are a diverse and multitudinous array of chemicals that have no known role in the internal chemical processes of plants’ growth or metabolism. Only a few of these substances occur in any one species of plant, but the same or similar ones tend to occur in related plants such as the various species that constitute a single family. It is these secondary substances that give plants their distinctive tastes and smells.

Insects appear to have played a major role in many plants’ having the secondary substances they have today. Such substances undoubtedly first appeared, and new ones continue to appear, as the result of genetic mutations in individual plants. But if a mutation is to survive and be passed on to subsequent generations, it must pass the muster of natural selection—that is, it must increase the likelihood of the organism’s surviving and reproducing. Some secondary substances are favored by natural selection because they are scents that attract pollinating insects to blossoms. Such scents signal the presence of nectar, which nourishes the insects without damage to the plants. Other secondary substances that arose by mutation were conserved by natural selection because they proved to be biochemical defenses against the enemies of plants, the majority of which are insects. Some of these defensive substances cause insects to suffer unpleasant symptoms or even to die. Still other secondary substances are not in themselves harmful to insects, but are characteristic smells or tastes that dissuade the insect from feeding by warning it of the presence of some other substance that is harmful.

For hundreds of millions of years there has been an evolutionary competition for advantage between plants and plant-eating insects. If insects are to survive as the plants they eat develop defenses against them, they must switch to other foods or evolve ways to circumvent the plants’ defenses. They may evolve a way to detoxify a harmful substance, to store it in their bodies out of harm’s way, or to avoid its effects in some other manner. Insects quickly come to prefer the plants whose defenses they can circumvent, and they eventually evolve the ability to identify them by their characteristic flavors or odors, or both. As the competition has progressed, fewer and fewer plants have remained as suitable food sources for any one species of insects species of insects have thus tended to become associated with narrowly defined and often botanically restricted groups of plants.

1. Which one of the following most accurately expresses the main point of the passage?

(A) Although the secondary substances in plants do not take part in the plants’ basic biological processes, these substances operate as natural defenses against damage and destruction by insects.
(B) Long-term competition between plants and insects has led to a narrowing of the range of secondary substances present in plants and, thus, also to a narrowing of the range of insect species that eat each species of plant.
(C) The particular secondary substances possessed by different plants, and thus the distinctive tastes and smells that present-day plants have, result
in large part from an evolutionary process of interaction between plants and insects.
(D) Due to long-term evolutionary pressures exerted by insects, the secondary substances in plants have become numerous and diverse but tend to be similar among closely related species.
(E) Because plant mutations have led to the development of secondary substances, plants have come to participate in a continuing process of competition with plant-eating insects.


2. Which one of the following is mentioned in the passage as a way in which insects can adapt when a plant develops defenses against them?

(A) to start eating something else instead
(B) to avoid plants with certain distinctive leaf or flower structures
(C) to increase their rate of reproduction
(D) to pollinate other species of plants
(E) to avoid contact with the dangerous parts of the plant


3. In the passage, the author discusses primary substances mainly in order to

(A) provide information about how plants grow and metabolize nutrients
(B) help explain what secondary substances are
(C) help distinguish between two ways that insects have affected plant evolution
(D) indicate the great diversity of chemicals that occur in various species of plants
(E) provide evidence of plants’ adaptation to insects


4. The passage provides the most support for inferring which one of the following?

(A) Some chemicals that are not known to be directly involved in the growth or metabolism of any species of plant play vital roles in the lives of various kinds of plants.
(B) Most plants that have evolved chemical defense systems against certain insect species are nevertheless used as food by a wide variety of insects that have evolved ways of circumventing those defenses.
(C) Most insects that feed exclusively on certain botanically restricted groups of plants are able to identify these plants by means other than their characteristic taste or smell.
(D) Many secondary substances that are toxic to insects are thought by scientists to have evolved independently in various unrelated species
of plants but to have survived in only a few
species.
(E) Some toxic substances that are produced by plants evolved in correlation with secondary substances but are not themselves secondary substances.


5. Which one of the following describes a set of relationships that is most closely analogous to the relationships between plants and their primary and secondary substances?

(A) Electrical power for the operation of devices such as lights and medical instruments is essential to the proper functioning of hospitals; generators are often used in hospitals to provide electricity in case their usual source of power is temporarily unavailable.
(B) Mechanical components such as engines and transmissions are necessary for automobiles to run; features such as paint and taillights give a car its distinctive look and serve functions such as preventing rust and improving safety, but automobiles can run without them.
(C) Mechanical components such as gears and rotors are required for the operation of clothing factories; electrical components such as wires and transformers supply the power needed to run the mechanical components, but they do not participate directly in the manufacturing process.
(D) Some type of braking system is necessary for trains to be able to decelerate and stop; such systems comprise both friction components that directly contact the trains’ wheels and pneumatic components that exert pressure on the friction components.
(E) Specially designed word processing programs are necessary for computers to be able to function as word processors; such programs can be stored either in the computers’ internal memory system or on external disks that are inserted temporarily into the computers.


6. The passage most strongly suggests that which one of the following is true of secondary substances in plants?

(A) Some of them are the results of recent natural mutations in plants.
(B) They typically contribute to a plant’s taste or smell, but not both.
(C) Some of them undergo chemical reactions with substances produced by insects, thus altering the plants’ chemical composition.
(D) Some species of plants produce only one such substance.
(E) A few of them act as regulators of plants’ production of primary substances


7. Based on the passage, the author would be most likely to agree with which one of the following statements about the relationship between plants and insects?

(A) The diversity of secondary substances that develop in a plant population is proportional to the number of insects with which that plant population has interacted throughout its evolutionary history.
(B) Although few species of plants have benefited from evolutionary interaction with insects, many species of insects use plants without either harming the plants or increasing the plants’ chances of survival.
(C) Throughout the process of evolutionary change, the number of plant species within each family has generally increased while the number of families of plants has decreased.
(D) No particular secondary substance has appeared in plants in direct response to insects, though in many instances insects have influenced which particular secondary substances are present in a plant species.
(E) While many species of insects have evolved ways of circumventing plants’ chemical defenses, none has done this through outright immunity to plants’ secondary substances.