Listening audioscript

 
Questions 1 through 3.
Listen to a lecture in a biology class. 
(Professor)
Here’s a question for you: if you throw a seed on the ground, how do the 
roots of the seed know to grow down into the ground and the shoot, 
where the leaves will grow, know to grow up? The answer is tropism. 
Tropism is the preferential growth of one part of a plant toward or away 
from a stimulus. Positive tropism is growth toward the stimulus, and 
negative tropism is, of course, growth away from the stimulus. I’m going 
to get into the mechanisms of exactly how this happens on a cellular 
level in a moment, but let me just begin with some kinds of tropism to 
illustrate the general concept. 
To start with, let me make it clear that there are many different kinds of 
tropism, and, as often seems to be the case in science, sometimes 
different names for the same thing. To simplify the introduction of the 
concept though, I’m going to restrict myself initially to three kinds of 
tropism: phototropism, hydrotropism, and geotropism. 
OK, let’s start with phototropism. “Photo” means light, so this is when the 
growth of a plant responds to a light stimulus. For instance, the shoot 
and leaves of a plant would exhibit positive phototropism, orienting 
themselves in the direction of a light source. On the other hand, the root 
emerging from a seed might show negative phototropism, growing away 
from any light. 
OK, so you can imagine this, let’s take a houseplant as an example. 
When you get it, say as a present for your dorm room from your family, 
it’s nice and even, with leaves on all sides. After a few months though, 
you start to notice that the side of the plant near the window seems to be 
fuller than the other side. It’s got more leaves and your once balanced 
houseplant looks lopsided, or uneven. So what do you do? You rotate 
the plant, and after a few weeks it looks even again. But if you leave it, 
again it starts to be fuller on the side toward the window. Over the course 
of a few weeks, the leaves orient themselves to the light source, and you 
have to keep rotating it if you want it to look balanced, right? 
OK. Now, what about geotropism? What do you think this is? 
(Student)
Well, “Geo” means Earth, so geotropism must be movement toward the 
Earth.
(Professor)
Yes, growth toward the earth. It’s actually uneven growth on different 
sides of the plant structure that changes the direction of growth, not 
movement per se. So where would we observe geotropism? 
(Student)
I guess that roots would show geotropism when they grow down. 

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LPREP IBT 3 E AudioScript 
47
(Professor)
Right. Or a shoot might exhibit negative geotropism, growing away from 
gravity. The stimulus in this case is actually gravity, not just the Earth. 
I’m going to show you some pictures of these different kinds of tropism, 
and in many cases you’ll be looking at pea plants. They’re popular to 
experiment on because they germinate quickly and these different kinds 
of tropism are easy to see. You’ll be seeing some experiments with pea 
shoots to show negative geotropism and positive phototropism. All you 
have to do is turn some pots full of pea plants on their sides and you can 
watch the pea plants bend and turn upward. In fact, they’ve been taken 
into space to see the effects of zero-gravity and hydrotropism on plant 
germination and growth.
And so this brings me to hydrotropism. This is the preferential growth 
toward water. If it’s positive hydrotropism, that is. This is again something 
most associated with roots and easily seen in the pea plant. Yes, is there 
a question? 
(Student)
Is this how tree roots can tell if there are water pipes nearby and can 
grow into them and tear them up? 
(Professor)
Well, not exactly. In spite of what many people believe, plants can’t 
sense water from a few feet away or through the walls of a pipe. Plant 
roots don’t grow toward pipes and tear them apart because they 
somehow know there’s water inside. They might grow along pipes 
because they present less resistance than the surrounding soil. Or if 
there’s any kind of leak that moistens the surrounding soil…then yes, 
hydrotropism can act to draw the roots of a plant…but only within a few 
millimeters of the plant roots.
OK, so before we go on to chemicals and cellular mechanisms, are there 
any questions?