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2.2 Darwin's Experiment

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Understanding Plants - Part I: What a Plant Knows

特拉维夫大学

4.8（666 個評分） | 36K 名學生已註冊

For centuries we have collectively marveled at plant diversity and form—from Charles Darwin’s early fascination with stems and flowers to Seymour Krelborn’s distorted doting in Little Shop of Horrors. This course intends to present an intriguing and scientifically valid look at how plants themselves experience the world—from the colors they see to the sensations they feel. Highlighting the latest research in genetics and more, we will delve into the inner lives of plants and draw parallels with the human senses to reveal that we have much more in common with sunflowers and oak trees than we may realize. We’ll learn how plants know up from down, how they know when a neighbor has been infested by a group of hungry beetles, and whether they appreciate the music you’ve been playing for them or if they’re just deaf to the sounds around them. We’ll explore definitions of memory and consciousness as they relate to plants in asking whether we can say that plants might even be aware of their surroundings. This highly interdisciplinary course meshes historical studies with cutting edge modern research and will be relevant to all humans who seek their place in nature. This class has three main goals: 1. To introduce you to basic plant biology by exploring plant senses (sight, smell, hearing, touch, taste, balance). 2. To introduce you to biological research and the scientific method. 3. To get the student to question life in general and what defines us as humans. Once you've taken this course, if you are interested in a more in-depth study of plants, check out my follow-up course, Fundamentals of Plant Biology (https://www.coursera.org/learn/plant-biology/home/welcome). In order to receive academic credit for this course you must successfully pass the academic exam on campus. For information on how to register for the academic exam – https://tauonline.tau.ac.il/registration Additionally, you can apply to certain degrees using the grades you received on the courses. Read more on this here – https://go.tau.ac.il/b.a/mooc-acceptance Teachers interested in teaching this course in their class rooms are invited to explore our Academic High school program here – https://tauonline.tau.ac.il/online-highschool

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您將學習的技能

Plant Biology, Genetics, Cell Biology, Plant

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4.8（666 個評分）

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JM

Feb 11, 2019

Great course - the professor is enthusiastic, engaging, and teaching complex concepts in a way that makes them easy to understand and remember, even for someone with no biology background.

LD

Sep 18, 2019

I started this course with very little knowledge of biology and finished it with a solid foundation in plant biology. I'd recommend it to anybody wanting to learn more about plants.

從本節課中

What a Plant Sees?

This week we start a systematic review of a plant's sensory systems by starting with plant responses to light. We will cover an overview of human vision, plant responses to light, Darwin's experiments showing plant responses to light, phototropism, phytochrome and flowering, and modern research on phototropism. In other words, this week we get into more advanced concepts in plant sensory biology. The last module is especially advanced, and will be clearer for those of you with a strong biology background. But do not fret, aside from very basic concepts, this module will NOT be included in the exam (you will not be responsible for understanding the intricacies of the experimental methods, etc.). If you have not already, please review the Course Syllabus for general information about this course.

2.1 Introduction5:21

2.2 Darwin's Experiment6:50

2.3 Light and Flowering7:06

2.4 Phytochrome: A Light-Activated Switch7:35

2.5 Arabidopsis Blind Mutant8:42

2.6 What a Plant Sees?5:55

2.7 Molecular Biology of Plant Vision8:53

教學方

Professor Daniel Chamovitz, Ph.D.
Dean, George S. Wise Faculty of Life Sciences

腳本

Alright, now that we've gone through human sight, how do we know that plants see? And actually, I think most of you have seen plants responding to light. Let's take a look at this movie right here. What we have is a movie of bean seedlings. That, every 12 hours, the light is being changed from one side to the other. Now as it's clear, you could see that the seedlings are starting to actually bend and follow where the sun is. Here now they've gone back the other one way. And in a second, they'll go back the other way. So we really see the plants seeing the light, and responding by growing and bending towards the light. [BLANK_AUDIO] Now you might be surprised to learn that one of the first people to study how plants respond to light and how they bend, was Charles Darwin. While you're all familiar with Darwin's work on evolution, what you're probably less familiar with is that the last 20, 30 years of his life, most of Darwin's research had to do with plant biology. And one of his most important works was published in 1880, together with his son, Francis Darwin. It's called The Power of Movement in Plants. And what Darwin noticed, and here he was studying many types of plants. But to take for example, he was studying a, a certain type of grass called canary grass. That if you put the canary grass in a room with only a very dim candle on one side of the room. And I think Darwin wrote about it saying that it was so dim that he couldn't even see the clock on the wall. The plant, in this case the grass, bent towards the candle. Well Darwin did this experiment on tens of types of plants, and he came to the following conclusion, and this is in his own words. Heliotropism, that's what we call the bending towards the light, prevails so extensively among higher plants, that there are extremely few of which some part, either the stem, flower, peduncle, petiole, or leaf, does not bend towards a lateral light. Or, in more modern English, what Darwin is saying here, that almost every type of plant is sensitive to light, and will bend towards the light. But Darwin was not only an observer of nature, he was also an excellent experimentalist. And Darwin asked the following question. Where is the eye of the plant? Which part of the plant is sensing the light in order to allow it to bend towards it? Now, Darwin's hypothesis was that the part of the plant that senses the light, what we would call the eye. Of course we're not talking about human eyes here, is at the tip of the plant. So what experiments could he do in order to test his hypothesis? Here's his observation, that when he puts lateral light from the side, the plant, in this case the seedling, bends towards it. His hypothesis was that it's the tip of the plant. So the first thing he did, was he chopped off the tip. And as you can see here, the plant lost the ability to bend towards the light. So what's the conclusion from such an experiment? Now your conclusion might be that the plant, that that obviously proves that the tip of the plant is what sees the light. But I've got news for you. If I cut off your heads, you also lose the ability to see. So we needed a few more experimental controls. So his first control that he did, was he now covered the top of the plant with a hat, with a cap, that was impermeable to light. And in this case, the plant also didn't bend. So does this prove, or support, Darwin's hypothesis, that the tip of the plant is what's seeing the light? Well it could be that the tip, the that it does support it, but it could also be that this cap was so heavy that it inhibits the plant from responding. So, he did another control. And this time, he covered the tip of the plant with a glass cap, which allowed light signals to go through. And under this conditions, lo and behold, the plant does bend. So we can see here, in a very simple experiment with no huge technology that was published in 1880, Darwin successfully showed that it's the tip of the plant that senses the light signal. And somehow or another, it transfers this light energy, or this light signal, down to the bottom part of the plant. And that's where the bending occurs. So Darwin concluded that a signal moves from the tip to the base. in his own words, we must therefore conclude that when seedlings are freely exposed to lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend. Again, that's very verbose. What's he saying? That we must conclude that there is some way that the tip is sensing the light, is transducing a signal to the lower part, which is causing it to bend. So this isn't quite so different than what happens for us. When I caught the baseball, my eyes sensed the light, and sent the signal to my hand. In phototropism, the ability of the plant to bend to the light, the tip is sensing the signal, and transferring that signal down the length of the plant to the part that's bending. So Darwin showed in his experiment that plants sense light and respond by bending to it, and this is a picture actually taken from his book, in his own drawing The Power of Movement in Plants. But now I want to go back to what I talked about in the beginning of today's lecture, that plants can differentiate between colors. And here's an experiment that was carried out by one of Darwin's colleagues, Jules von Sachs in Germany in the late 19th century. And Sachs asked the question, can plants differentiate between different colors? And what he showed, is that the same seedlings that Darwin studied, will bend to blue light, but not to red light. In other words, plants, like humans, differentiate between different colors of light. Now, some of you may know people who are color blind, and we know how that affects their life. So I want you to imagine for a second, what would be the behavior, or the response, of a plant that's color blind?

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