托福og1听力According to the professor, why did one scientist grow a ryeplant in water? 答案解析

[00:00.00]Narrator: Listen to part of a talk in a botany class.
[00:03.11]Professor: OK, so we've talked about some different types of root systems of plants, and I’ve shown you some pretty cool slides, but now I want to talk about the extent of the root system-the overall size of the root system ... the depth. [00:17.52]I want to tell you about one particular experiment. [00:20.67]I think you're going to find this pretty amazing. [00:23.00]OK, so there was this scientist...this very meticulous scientist decided that the best place to see a whole root system-to actually see how big the entire system got-the best place would be to grow it... where?
[00:35.41]Female Student: Um, water?
[00:37.75]Professor: In water. [00:38.97]So he took rye plants-it was rye plants-and he started growing them in water. [00:43.82]Now, you’ve all heard of growing stuff in water before, right?
[00:47.30]Male Student: It's done commercially, right? [00:48.99]Uh, like to grow vegetables and flowers?
[00:51.59]Professor: Right.They grow all kinds of commercial crops in water. [00:54.75]So if you're growing things in water, you can add the fertilizer. [00:58.33]What do you need to do to that water besides put fertilizer in it? [01:02.01]Anyone ever actually tried to grow plants in water? [01:05.09]You must bubble water through it. [01:07.00]Bubble gas through it. [01:08.04]I’m sorry, you must bubble gas through it. [01:10.02]So, gas, you have to bubble through. [01:12.32]Think about the soil we talked about last week, about growing plants in soil. [01:16.87]Think about some of you who have killed your favorite houseplants, 'cause you loved them too much. [01:21.65]If you overwater, why do your favorite houseplants die?
[01:25.07]Female Student: Oh, no oxygen.
[01:27.23]Professor: Not enough oxygen for the roots ... which do what twenty-four hours a day in all seasons?
[01:33.51]Female Student: Respiration?
[01:34.85]Professor: Respire ... respiration ... they breathe. [01:37.65]So if you just stick rye plants in water, it doesn't make a difference how much fertilizer you add, you also need to bubble gas through the water, so they have access to that oxygen. [01:49.27]If they don’t have that, they're in big trouble. [01:51.82]OK. So this guy this scientist-grew a rye plant in water so he could see the root system, how big it got-its surface area. [02:00.20]I read about this and the book said one thousand kilometers of roots. [02:05.67]I kept thinking: this has to be a mistake. [02:08.03]It just doesn’t make any sense to me that... that …that could be right. [02:12.04]But that’s what all the books have, and no one's ever corrected it. [02:15.33]So let me explain to you about this rye plant. [02:18.37]If you take a little seed of many grasses-and remember rye is a grass; if you take a tiny little seed and you germinate it - actually, take one of my least favorite grasses that starts growing about May. [02:31.97]What's my least favorite grass that starts growing about May?
[02:35.16]Male Student: Crabgrass.
[02:36.17]Professor: Crabgrass.
[02:37.04]Remember how I showed you in the lab, one little seed starts out producing one little shoot. [02:43.63]Then at a week or so later you’ve got about six shoots, and then, three weeks later you’ve got about fifteen shoots coming out all directions like this- all those little shoots up there? [02:55.33]Well, that’s what they did with the rye. [02:57.85]And the little seedling started and pretty soon there were several shoots, and then more shoots. [03:03.57]In the end, that one single seed produced eighty shoots, with an average of fifty centimeters of height ... from one seed. [03:12.30]Eighty shoots coming out, average fifty centimeters high. [03:16.52]When they looked at the shoot versus the root surface, they found that the shoot surface, with all of its leaves, had a total surface area of about five square meters. [03:27.40]Now, here’s the biggie: when they looked at the root surface area, you would expect that the root and the shoot would be in balance, right? [03:35.87]So they should be pretty close in terms of surface area, right?
[03:39.93]Male Student: Uh-un.
[03:40.63]Professor: What’s that? [03:41.16]Did somebody say "no"? [03:42.56]Well, you're absolutely correct. [03:44.80]Instead of five square meters, the root system was found to have more than two hundred square meters of surface area. [03:52.45]Where did all of that extra surface area come from? [03:55.48]Who did it? [03:56.76]Who was responsible for all those extra square meters of surface area? [04:00.66]What did roots do to increase their surface area?
[04:04.53]Female Student: Root hairs.
[04:05.84]Professor: Root hairs, that’s exactly it. [04:08.07]So those root hairs were responsible for an incredible chunk of surface area. [04:12.83]They constantly have to be spread out in the water so they can absorb minerals from the fertilizer, and of course they need oxygen access as well.