Reading Comprehension

PT138 · S1 · P2 · Q10 Plant Evolution

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The characteristic smell or taste of a plant, to insects as well as to humans, depends on its chemical composition.

Topic

How plants’ distinctive smells and tastes (from secondary chemical substances) have evolved, especially through interactions with insects.

Framework

Old / New and Answer / Explain

Main Point

Plants contain unique secondary chemical substances—mainly for attracting or defending against insects—and this chemical arms race has driven the evolutionary specialization between plants and plant-eating insects. The "most valuable sentence" for the main point is near the end of P2:

P1: Basic Plant Chemistry—Primary vs. Secondary Substances

Plants all have “primary substances” (things like proteins and carbs) they need for survival, but the interesting “secondary substances”—unique to different plants and responsible for their taste and smell—don’t impact their basic function.

P2: Why Plants Have Secondary Substances—Insects and Natural Selection

These secondary substances evolved because of genetic mutations, but only stuck around if they provided an evolutionary advantage, mostly by either attracting helpful insects (like pollinators with appealing scents) or defending against harmful ones (with bad tastes, toxins, or warning smells).

P3: The Ongoing Evolutionary Battle between Plants and Insects

Plants and insects have been locked in a chemical arms race for ages: as plants develop new defenses, insects are forced to adapt or find new food. Over time, this has led to insect species specializing in eating just a few types of plants—basically, both sides have become more and more choosy!

10.

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

  1. provide information about how plants

    Detail-Sentence Trap

    The author doesn't provide much of any information about how plants grow or how they metabolize nutrients (other than listing a few required primary substances), so it doesn't make sense to say the author had this purpose. Our author's goal is to talk about secondary substances, but she needs to set up that topic for the reader, who might not know what is meant by secondary substances. This answer is just grabbing words from inside the Detail Sentence we're asked about, which is a classic trap on Local Purpose. The correct answer is almost always reinforcing language from surrounding sentences.

    11% picked this

  2. Correct

    help explain what secondary substances

    Why this is right

    Our author's goal is to talk about secondary substances, but she needs to set up that topic for the reader, who might not know what is meant by secondary substances. So she tells us that there are two categories of substances in plants, primary and secondary, and differentiates the vital and universal primary substances from the more idiosyncratic secondary substances. It's a little weird to say that the sentence of about primary substances somehow explains what secondary substances are, because really it explains what secondary substances aren't. But this question stem isn't asking us, "what is the effect of the sentence with primary substances in it". It's asking us, "why is the author bothering to talk about primary substances". And the overall goal in that first paragraph is to brief the reader on what secondary substances are. The discussion of primary substances serves this bigger goal. The fact that surrounding sentences are dealing with secondary substances should make us feel better about picking an answer about "secondary substances", since that's generally how correct answers on Local Purpose work.

    Skill tested: Local Purpose · how this choice captures the passage's function is the move to repeat next time.

    79% picked this

  3. help distinguish between two ways

    Outside of Support Window: insects

    There's no way we're reaching into future paragraphs to answer a Local Purpose question (once in a blue moon, we might reach into a previous paragraph). Also, insects have only affected plant evolution when it comes to secondary substances (as far as we know). So "primary substances" are not a second way that insects have affected plant evolution.

    2% picked this

  4. indicate the great diversity of

    Opposite, if anything

    With primary substances, there isn't a great diversity. They're "found in all plants".

    7% picked this

  5. provide evidence of plants' adaptation

    Outside of Support Window: insects

    Just like (C), there's no reason to be bringing "insects" into this conversation yet. The author hasn't gotten there in the passage, so this is not her Local Purpose. Also, insects have only affected plant evolution when it comes to secondary substances (as far as we know). So "primary substances" are not a way in which plants have adapted to insects.

    1% picked this

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