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# Geotropism, One Last Time

Shawn Carlson investigates how plants grow in reduced gravity

 IMAGE: DANIELS AND DANIELS
In my latest device, a single motor drives all 20 cans. The \$24 I paid to a dealer in surplus electronics bought me a 1/8-horsepower electric motor, which runs at 1,600 revolutions per minute. Turning the cans at one revolution per minute required two sets of reduction gears: a worm gear and wheel with a ratio of 80 to 1 and another pair of gears with a ratio of 20 to 1. The cans are driven through a belt and miter gear assembly, as shown in the illustration. I rescued all the gears from various surplus stores and purchased the belts from McMaster-Carr.

I'm so happy with my current setup that I'm already making plans for my next model. It will contain 10 shelves with nine cans on each shelf. Two of the shelves will be adjustable so that they can be set at any angle. This provision will help me explore the threshold of geotropic response.

I plan to analyze all the results in terms of the angularity of the seedlings, the sum of all the bend angles in the stem divided by its length. Seedlings germinated at low effective gravities are often quite contorted and so have unnaturally high angularities. Graphing values of angularity against effective gravity should allow me to identify the range over which a given plant responds to gravity.

In some of my early experiments, I had to kill the seedlings to measure their angularities accurately. No longer. Now I position the plants on a digital scanner, cover them with a sheet of graph paper and gently press them to the glass. The scanned image contains the complete plant atop a reference grid. Of course, this procedure flattens the three-dimensional structure into a two-dimensional representation. But you'll still arrive at a fairly accurate number.

A plant germinated in a low-gravity environment may have more problems than just a crooked stem. If you want to measure floral metabolism, too, the December 1958 Amateur Scientist column explains how. But the scheme described there requires you to know the surface area of the plant¿something that is notoriously hard to compute. Fortunately, once you have scanned the specimen, you'll have several options. Most simply, you can count the squares on the graph paper that are covered by the leaves. You can also print the image, cut out the silhouetted plant and weigh the paper. Or perhaps you can find a way to have your computer tally up the pixels covered by the plant, a number that is proportional to area. This approach seems the most elegant to me, but I haven't found satisfactory means to carry it out with standard software. If you know a simple way, please share your ideas in the online discussion area hosted by the Society for Amateur Scientists.

Armed with these techniques, any ambitious amateur can begin to search out the secret dependencies that plants have on gravity. In this field, it's easy to stand shoulder to shoulder with the professionals. What food plants might one day support human settlements on the moon or Mars? Perhaps your own research may help to develop them.

Further Information:

For more information about this and other projects for amateur scientists, check out the online discussions on the Society for Amateur Scientists's Web site. Link to www.sas.org and click on the "Forum" button. You may write the society at 5600 Post Road, Suite 114-341, East Greenwich, RI 02818 or call 401-823-7800. To purchase a CD-ROM containing every project published in this department through the end of 1999 (more than 1,000 projects in all), call 888-875-4255 or consult www.tinkersguild.com.

This article was originally published with the title Geotropism, One Last Time.

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