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Auxins and tropisms
Teaching notes
Working in pairs students read the article 'Sensitive plants'.
Their task is to use the information they have read to annotate each of the five diagrams
provided.
The challenge is to construct sentences which do not appear in the text. You can set a
target such as 'annotations must not contain more than five words taken from the text e.g.
'plants respond to light, water, touch, gravity and various chemicals' would not be
acceptable, but 'plants respond to water, chemicals, gravity, light and touch' meets the
challenge.
To differentiate the task, individual students could be challenged to have no more than
four/three/two consecutive words from the text.
The idea is for students to draw meaning from the text, to summarise and to express
themselves in their own words.
When they have completed their annotations they can check each other’s work. They can
show surprising dedication when trying to find phrases which do not meet the criteria!
The text also shows that plants move in response to auxins even when they are not
exposed to light – something which is not made clear in many text books.
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Auxins and tropisms
Task
Read the article below. Use the information to annotate the five diagrams. The challenge
is to construct sentences which do not appear in the text.
Sensitive plants
For a long time gardeners and growers have known that plants are sensitive creatures
which respond to a wide range of things such as light, water, touch, gravity and various
chemicals. They also realised that different parts of a plant would respond in different
ways to these stimuli. Scientists called these responses tropisms. Perhaps the most
obvious tropisms are phototropism and geotropism (also known as gravitropism).
It wasn't until Charles Darwin (yes, THE Charles Darwin of evolutionary fame) and
others devised a set of fiendishly clever experiments involving bright lights and
amputation, that scientists began to work out what was going on. But... hold on... don't get
too excited because even today this is an area of HOT research. Nobody is precisely
certain how these mechanisms function. Basically however, plant hormones are at the
heart of it.
The group of hormones we are interested in are known as auxins. Auxins are
produced in the very newest tips of the plant's shoots and roots. They are made in
minuscule amounts but they have a
big effect.
Auxins move in solution to older parts of the shoots and roots, just below the tip and
here they cause cells to elongate and to divide. Cell elongation happens because auxin
alters the cell wall making it more springy or elastic. The cells absorb more water and
stretch – et voilà, longer cells.
The plants' shoots and roots bend because scientists think, see they're not certain,
light and gravity affect the transport of auxins meaning the hormone is unevenly
distributed, causing only SOME of the plant cells in the shoot or root to elongate.
But this is not all! We've all tortured cress seedlings, putting them in dark boxes
with only one tiny window; or seen video of the same. We know plants grow towards the
light. They have to, to maximise photosynthesis. It's a tough botanical world out there.
But a seed is underground, in the dark. How does a germinating seed know which way to
send its roots and shoots with no light to guide it?
Roots and shoots BOTH respond to gravity. Roots are positively geotrophic and
shoots are (you guessed it) negatively geotrophic. This is because root cells and shoot
cells respond differently to auxins. High auxin concentrations in roots cause less cell
elongation than in shoots.
Scientists think gravity causes auxins to collect on the lower side of a root or shoot.
In shoots, this causes cells on the lower side to elongate making the shoot bend and grow
upwards away from the pull of gravity. In roots, cell elongation and growth on the lower
side is inhibited by the presence of auxins. Cells on the opposite side, where there are
no auxins, grow more and the root bends down. Isn't nature cunning?
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Auxins and tropisms
Diagrams to annotate
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2.
3.
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Auxins and tropisms
4.
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