Positive and Negative Geotropism (page 2)
In the “1: Bottom” packet, the stems of the future plant tend to grow upwards, away from gravity. The stems therefore show a negative tropism towards gravity. The roots of the future plants, on the other hand, tend to grow downwards, showing a positive tropism towards gravity. In the “2: Side”.packet, the differing tropisms of roots and stems are likely to be even more evident because of the packet’s vertical orientation.
The seedlings in the “3: Turn” packet might look confused! Each turn reorients the seeds in space, so the stems and roots are likely to show twisting patterns.
The paper towel filled the role of soil, holding water for the sprouting seed. The foil served two purposes: It helped keep the paper towel from drying out while blocking any light stimulus from reaching the seeds. We wanted this experiment to be about geotropism, not phototropism!
How plants perceive and respond to gravity is still a bit of a mystery. It is thought that plants have special cells called statocytes that detect gravity through dense starch grains that tend to settle at the bottom of the root. This settling triggers a chemical message in the plant which in turn causes other chemical messages that make some parts of the plant grow more than others.
If tropisms have piqued your interest, you might try designing experiments on other types of tropisms, like phototropism or thigmotropism, which is a plant’s response to touch. The venus fly trap or mimosa would be good experimental candidates.
Tropisms can even be studied in outer space. After seeing the results of your experiment on geotropism, how do you think a seed would sprout in zero gravity?
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