Narrator: Listen to part of a talk in a botany class.
Professor: OK. We’ve been discussing the various kinds of roots systems that plants have. We’ve even seen slides of some incredibly extensive roots that, I have to admit, are pretty cool. Now I want to talk about a particular type of root—the taproot. There have been some studies that are pretty eye opening. In the past, scientists thought that taproots—those long, thick roots that some plants have that grow straight down, you know—like beets or… or what else?
Male Student: Carrots. Maybe also, um, turnips and radishes, right?
Professor: Right. So the thought was that these taproots were there just to support other roots and store nutrients. A thick taproot would grow, then small branches would grow out of it, and smaller branches would come from those, and so on. The taproots were there to give the branches a base—the branches were what the plant really needed to get water and nutrients. So these scientists in North Carolina decided to investigate—to actually see what role taproots play. Normally, when we want to observe roots, where do we need to grow them?
Female Student: Um… in water so we can see them, right?
Professor: That’s right. But these scientists had a problem. They wanted to watch the root system absorb water, and that’s really hard to do if the plant’s completely immersed in water. By the way, what’s it called when a plant takes in water from its roots?
Male Student: Transpiration?
Professor: Transpiration… they transpire… basically, they drink. All living things need water, so transpiration is a crucial process. To study it, we need to limit the amount of water, so we can see the areas… the parts of the roots… that attract and absorb water. So this group decided to grow plants in sand. Any ideas why the scientists thought sand would be helpful?
Female Student: Well, sand doesn’t hold on to water, right? That’s why there aren’t that many plants that can grow in deserts.
Professor: Exactly. So this group of scientists had complete control over how much water they added and how much water they removed. OK. So this group—this lab—started growing loblolly pine trees… these trees grow taproots when they’re young… they started growing these pine trees in sand. They added water, waited for some of the water to be absorbed by the roots, then drained the excess water. They used magnetic resonance imaging—MRI—technology to image where in the plant the water was. I was so surprised by their results! Remember, taproots supposedly just add structure and store nutrients. They’re thick—too thick, we thought, to help with transpiration. It’s the branches that grow from them that were supposed bring water to the rest of the plant. I say supposedly, because… well, can you guess?
Male Student: Were the taproots holding the water?
Professor: Yes! At least it was mostly the taproots. Not only that, but it looked like there were particular compartments that were made to store water. Some areas of the taproot had more water and some had less… it’s like it was made for this. But that’s just water storage. We want to focus on transpiration… on the uptake of water from the environment. Since we’ve always thought that taproots were just there for structure, maybe the transpiration actually occurs at the branches, and the water moves to the taproots where it gets stored. So if you removed all the branches, the plant should stop transpiring, right?
Female Student: Nope!
Professor: I think I heard the right answer. I can’t fool you guys. The answer is a definitive no. This group removed the branches, and still saw the normal amount of transpiration and water storage occurring. So what can we conclude from this study?
Male Student: That the taproot is doing the transpiration!
Professor: Exactly. So taproots are far more important than we used to think. They help the plant get the water it needs, and maybe the nutrition as well.
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What is the main topic of the lecture? |
Gist-content. The professor starts by introducing and describing taproots. She then discusses an experiment that showed that taproots are involved in transpiration. |
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| ✗ | A Variations in types of roots |
Although two types of roots are discussed—taproots and branching roots—the focus of the lecture is on taproots, not on the differences between the types of roots. |
| ✗ | B Methods of transpiration |
Transpiration is discussed in the lecture, but is a supporting detail to the main topic—the function of taproots. |
| ✓ | C The function of taproots |
Correct. The experiment discussed at length in the lecture supports the idea that taproots are used for transpiration. |
| ✗ | D Improving the water intake of plants |
This idea is not mentioned in the lecture. |
Narrator: Listen again to part of the lecture. Then answer the question.
Professor: In the past, scientists thought that taproots—those long, thick roots that some plants have that grow straight down, you know—like beets or… or what else?
Narrator: Why does the professor say this:
Professor:…like beets or… or what else?
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Function of What Is Said. The professor pauses in the middle of listing examples of taproots to see whether the students can provide additional examples of taproots. |
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| ✓ | A She wants to elicit examples from the students. |
Correct. A student later continues the list of examples that the professor began. |
| ✗ | B She does not believe that taproots are common. |
The professor does not believe that taproots are uncommon. |
| ✗ | C She is trying to trick her students by talking about other kinds of roots. |
The professor is only discussing taproots. There is no indication that she is trying to trick her students. |
| ✗ | D She forgot what she was about to say. |
There’s no evidence to suggest that the professor forgot what she was about to say. |
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The professor mentions that plant root systems are typically observed in water. Why does she mention this? |
Organization. The professor mentions that it would be difficult to track the water uptake in a root system if the roots were immersed in water. |
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| ✗ | A To demonstrate that it doesn’t matter what material plants are grown in |
The opposite is true. For the purposes of the experiment, immersing the taproots in water would make it difficult to study the water uptake of the plant’s roots. |
| ✓ | B To explain why the typical setup for this type of experiment would not work in this case |
Correct. The experimenters decided to grow their plants in sand rather than water, because immersing the plants in water would make it difficult to study the water uptake of the plant’s roots. |
| ✗ | C To remind students of the importance of using the scientific method |
This not the purpose of the professor mentioning that plant root systems are typically observed in water. |
| ✗ | D To reinforce the point that taproots are generally used to store nutrients. |
This fact has nothing to do with whether the experiment should feature plants immersed in water. |
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According to the professor, why did the researchers grow a species of pine trees in sand? |
Detail. In this experiment, researchers wanted to observe water uptake in different parts of the root system. The advantage of using sand is that it does not hold on to water. |
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| ✗ | A To add specific fertilizer types |
The professor does not mention fertilizers in the lecture. |
| ✗ | B To remove the branching roots |
While this procedure was performed later in the experiment, it is not why the researchers grew the pine trees in sand. |
| ✓ | C To monitor the intake of water |
Correct. Sand allowed the scientists to control the level of water present outside the root, making it easier to observe water uptake in different parts of the root system. |
| ✗ | D To control the size of the taproot |
This idea is not mentioned in the lecture. |
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According to the professor, what similarity exists between loblolly pines and carrot plants? |
Detail. Loblolly pines were used in the main experiment discussed in the lecture. Carrots were given as an example of a taproot. |
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| ✗ | A Both can be grown in sand. |
The professor doesn’t claim that carrots can be grown in sand. |
| ✗ | B Both have been extensively studied. |
The professor doesn’t make this claim about either plant. |
| ✗ | C Both rely exclusively on branching roots for transpiration. |
The loblolly pines are shown to rely on taproots for transpiration, not branching roots. |
| ✓ | D Both use taproots to absorb water. |
Correct. Both plants have taproots, and according to the professor, the experiment demonstrated that taproots are used for transpiration. |
Narrator: Listen again to part of the lecture. Then answer the question.
Professor: OK. So this group—this lab—started growing loblolly pine trees… these trees grow taproots when they’re young… they started growing these pine trees in sand. They added water, waited for some of the water to be absorbed by the roots, then drained the excess water. They used magnetic resonance imaging—MRI—technology to image where in the plant the water was. I was so surprised by their results! Remember, taproots supposedly just add structure and store nutrients.
Narrator: Why does the professor say this:
Professor: Remember, taproots supposedly just add structure and store nutrients.
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Function of What Is Said. In this quote, the professor is describing the experiment that determined that taproots are involved in transpiration. By emphasizing that taproots were believed to be limited in function, she is implying that the experiment produced an unexpected result. |
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| ✗ | A To imply that the conclusions reached by the lab were incorrect |
The professor agrees with the conclusions of the lab. She just finds them surprising. |
| ✗ | B To emphasize that structure and storage are the most important roles of a taproot |
The professor never talks about which roles of a taproot are most important. |
| ✓ | C To indicate that the researchers discovered that taproots play a broader role than previously thought |
Correct. In this quote, the professor is implying that the experiment discovered a previously unknown function of taproots. |
| ✗ | D To suggest that MRI technology was not the best tool for this lab to use |
This idea is not mentioned in the lecture. |