Conflicting Viewpoints · Lesson 4

What each view assumes and predicts

Every viewpoint rests on ideas it takes for granted. An assumption is something a view needs to be true for its explanation to work, whether or not the scientist says it out loud. Questions ask “Scientist 1’s explanation assumes that…” or “Which statement would both scientists accept?”

For each view, write one line in three parts: claims (the cause it blames), because (the link that makes the cause work), and so it predicts (what should happen in a new case). The because part is where the assumptions live: if that link fails, the whole view fails.

For a prediction question, run the new case through one view’s because line at a time. Each view gets its own prediction, and two views can predict the same result for different reasons.

Then look for the overlap. An idea that appears in every view, often a shared mechanism or a fact from the introduction, is something the views agree on. An assumption that every view shares can never be the point where they differ.

Claims, because, predicts: the candle demonstration from Lesson 2
LineStudent 1Student 2
ClaimsThe flame goes out when oxygen runs lowThe flame goes out when carbon dioxide builds up
BecauseA flame needs oxygen above a certain levelEnough carbon dioxide smothers a flame
So it predictsMore oxygen in the jar means a longer burnRemoving carbon dioxide means a longer burn
Both acceptLarger jars burn longer; the candle makes carbon dioxideThe same two ideas

The Leaning Seedlings

A class grew bean seedlings in trays on a bench in the middle of a garden shed. The shed’s east wall is made of glass. Its west wall and its roof are made of wood, and a hot-water heating pipe runs along the bottom of the west wall. After two weeks, nearly every seedling’s stem was curved so that its top pointed toward the east wall. Table 1 gives the seedlings’ average height and average angle of lean from vertical, measured every few days. Two scientists proposed explanations.

Scientist 1: The seedlings lean toward the light. A seedling stem bends toward the direction from which most of its light comes. Cells on the shaded side of the stem grow longer than cells on the lit side, so the stem curves toward the lit side. In the shed, light can enter only through the glass wall, so the seedlings curve toward the east. Temperature does not affect which way a stem bends. A seedling that gets equal light from every side, or no light at all, grows straight up, whatever the temperature around it.

Scientist 2: The seedlings lean away from the heat. The hot-water pipe warms the air on the west side of the bench, so the west side of each stem is warmer than its east side. Cells on the warmer side of a stem grow longer than cells on the cooler side, so the stem curves away from the warmth, toward the cooler side. The direction of the light makes no difference to which way a stem bends. If the pipe were turned off and the shed kept at one even temperature, the seedlings would grow straight up.

Table 1
DayAverage height (cm)Average lean toward east (°)
435
8614
11923
141130
Average seedling height and average angle of lean toward the east wall, measured from vertical: day 4, 3 cm and 5°; day 8, 6 cm and 14°; day 11, 9 cm and 23°; day 14, 11 cm and 30°. Both height and lean increase over time.

Worked example Easy

Show the whole passage

The Leaning Seedlings

A class grew bean seedlings in trays on a bench in the middle of a garden shed. The shed’s east wall is made of glass. Its west wall and its roof are made of wood, and a hot-water heating pipe runs along the bottom of the west wall. After two weeks, nearly every seedling’s stem was curved so that its top pointed toward the east wall. Table 1 gives the seedlings’ average height and average angle of lean from vertical, measured every few days. Two scientists proposed explanations.

Scientist 1: The seedlings lean toward the light. A seedling stem bends toward the direction from which most of its light comes. Cells on the shaded side of the stem grow longer than cells on the lit side, so the stem curves toward the lit side. In the shed, light can enter only through the glass wall, so the seedlings curve toward the east. Temperature does not affect which way a stem bends. A seedling that gets equal light from every side, or no light at all, grows straight up, whatever the temperature around it.

Scientist 2: The seedlings lean away from the heat. The hot-water pipe warms the air on the west side of the bench, so the west side of each stem is warmer than its east side. Cells on the warmer side of a stem grow longer than cells on the cooler side, so the stem curves away from the warmth, toward the cooler side. The direction of the light makes no difference to which way a stem bends. If the pipe were turned off and the shed kept at one even temperature, the seedlings would grow straight up.

Table 1
DayAverage height (cm)Average lean toward east (°)
435
8614
11923
141130
Average seedling height and average angle of lean toward the east wall, measured from vertical: day 4, 3 cm and 5°; day 8, 6 cm and 14°; day 11, 9 cm and 23°; day 14, 11 cm and 30°. Both height and lean increase over time.

Scientist 1’s explanation assumes that the seedlings on the bench receive:

  1. more warmth on their west sides than on their east sides.
  2. about the same amount of light from every side.
  3. more light from the east side than from the west side.Answer
  4. most of their light through the roof above them.

How to solve it

  1. Write Scientist 1 as one line. Claims: the stems lean toward the light. Because: a stem bends toward where most of its light comes from. So it predicts: even light, or none, means a straight stem.
  2. The because line works in the shed only if one side gets more light. Scientist 1 names that side: “light can enter only through the glass wall,” the east wall.
  3. Choice C states that assumption. Choice A is Scientist 2’s assumption, and choices B and D contradict the passage.

Why each choice is right or wrong

  • A. Incorrect. Warmer west sides is Scientist 2’s assumption: “the west side of each stem is warmer than its east side.” Scientist 1 says “Temperature does not affect which way a stem bends.”
  • B. Incorrect. Equal light from every side is the case in which Scientist 1 predicts no lean at all: a seedling “that gets equal light from every side, or no light at all, grows straight up”. The seedlings did lean.
  • C. Correct. Scientist 1’s view works only if the light is uneven, and Scientist 1 says “light can enter only through the glass wall, so the seedlings curve toward the east.”
  • D. Incorrect. The introduction says the roof is “made of wood,” and Scientist 1 says light enters “only through the glass wall”.

Worked example Medium

Show the whole passage

The Leaning Seedlings

A class grew bean seedlings in trays on a bench in the middle of a garden shed. The shed’s east wall is made of glass. Its west wall and its roof are made of wood, and a hot-water heating pipe runs along the bottom of the west wall. After two weeks, nearly every seedling’s stem was curved so that its top pointed toward the east wall. Table 1 gives the seedlings’ average height and average angle of lean from vertical, measured every few days. Two scientists proposed explanations.

Scientist 1: The seedlings lean toward the light. A seedling stem bends toward the direction from which most of its light comes. Cells on the shaded side of the stem grow longer than cells on the lit side, so the stem curves toward the lit side. In the shed, light can enter only through the glass wall, so the seedlings curve toward the east. Temperature does not affect which way a stem bends. A seedling that gets equal light from every side, or no light at all, grows straight up, whatever the temperature around it.

Scientist 2: The seedlings lean away from the heat. The hot-water pipe warms the air on the west side of the bench, so the west side of each stem is warmer than its east side. Cells on the warmer side of a stem grow longer than cells on the cooler side, so the stem curves away from the warmth, toward the cooler side. The direction of the light makes no difference to which way a stem bends. If the pipe were turned off and the shed kept at one even temperature, the seedlings would grow straight up.

Table 1
DayAverage height (cm)Average lean toward east (°)
435
8614
11923
141130
Average seedling height and average angle of lean toward the east wall, measured from vertical: day 4, 3 cm and 5°; day 8, 6 cm and 14°; day 11, 9 cm and 23°; day 14, 11 cm and 30°. Both height and lean increase over time.

Suppose the glass wall is covered so that a new set of seedlings grows in total darkness, with the heating pipe running as usual. Which predictions would the two scientists most likely make about the new seedlings?

  1. Scientist 1 would predict straight stems, and Scientist 2 would predict stems leaning east.Answer
  2. Scientist 1 would predict stems leaning east, and Scientist 2 would predict straight stems.
  3. Both scientists would predict stems leaning east, as in the original trays.
  4. Both scientists would predict straight stems, because no light reaches them.

How to solve it

  1. Run the new case through each view’s because line, one view at a time.
  2. Scientist 1: a stem bends toward its light. With no light, there is no direction to bend toward, and Scientist 1 says such a seedling “grows straight up”.
  3. Scientist 2: a stem bends away from the warm side. The pipe still runs, so the west side is still warmer, and light “makes no difference”. The stems still curve east.
  4. Different predictions from the two views: choice A. Choice B is the same pair, swapped.

Why each choice is right or wrong

  • A. Correct. Scientist 1 says a seedling that gets “no light at all, grows straight up, whatever the temperature around it.” Scientist 2 says “The direction of the light makes no difference”, and the pipe still warms the west side, so the stems still curve east.
  • B. Incorrect. This swaps the two views. Straight stems in the dark is Scientist 1’s prediction, and a lean caused by the warm pipe is Scientist 2’s.
  • C. Incorrect. Scientist 2 would predict a lean east, but Scientist 1 would not: with “no light at all,” Scientist 1 says the seedling “grows straight up”.
  • D. Incorrect. Darkness changes nothing in Scientist 2’s view, because “The direction of the light makes no difference to which way a stem bends.” Only turning off the pipe would give straight stems.

Trap A shared assumption is not a difference

When a question asks what one view assumes but the other does not, the most tempting wrong answer is an assumption that both views need. In the candle demonstration, both students say a larger jar lets the candle burn longer, so a finding that jar size makes no difference would hurt both views. Before you call an idea one view’s alone, check the other view’s paragraph for the same idea in different words.