---------------------

by Shawn Carlson
Mar, 2001

---------------------

In June 1970, when I was just 10 years old, a delightful exploration into geotropism (the response of plants to gravity) appeared in the Amateur Scientist. Believe it or not, it was contributed by my grandfather, George Donald Graham. When I was a boy, sitting on his knee and listening to how he created this experiment ignited my interest in science. Thirty years later, now writing this column myself (for the last time, I regret to say), I have a new contribution to this topic that I want to share.

Figure 1: Rotating cans simulate reduced gravity for seedlings growing within them. Varying the tilt angle changes the effective pull of gravity from zero (top) to its full falue (bottom).

My recent advance builds directly on Grandpa Don's early insight. He germinated corn seeds in simulated weightlessness by taking advantage of the fact that plants respond only sluggishly to gravity: it typically requires about a minute for growth hormones called auxins to shift position, thereby allowing a tipped-over plant to start righting itself by adjusting its growth. So, Grandpa reasoned, if he continuously tumbled a plant such that it made a complete revolution in less than a minute, the specimen would be unable to tell up from down. He was right. And his experiments with corn seedlings proved that this plant would fare poorly in a spaceship.

My grandfather's apparatus averaged the earth's field to zero by slowly rotating the seedlings in the vertical plane. In February 1996 I described a more elaborate apparatus, one that spins the seedlings slowly in one plane (to cancel gravity) and quickly in another (to create a centrifugal force). That column sparked hundreds of science-fair projects, several of which earned honors for their creators at national competitions. The device was, however, rather difficult to construct.

But there's an easier approach. Slowly rotating the seedlings in a vertical plane cancels gravity completely; slowly rotating them in a horizontal plane does nothing special. So if the plants rotate at some intermediate angle, the seedlings experience on average an intermediate amount of gravity.

Figure 2: Gravitational acceleration (g, about 10 meters per second per second) has one component alnog the axis of rotation and one component perpendicular to it. The latter averages to zero as the can rotates.

Why? When something is tilted at an angle and spun around (as in the device shown above), part of the gravitational force pulls along the axis of rotation and part pulls across it. The cross-axis component averages to zero, yet the along-axis component remains unaffected. So seeds germinated in a canted, rotating chamber experience an effective gravity that is reduced by a factor equal to the sine of the tilt angle (q).

This fact makes it easy to experiment with geotropism. Just attach a surplus clock motor to a juice can and set up the contraption at an angle. Stuff the toe of a nylon stocking with moist potting soil and a few test seeds, then wedge it inside the can. That's all there is to it.

You can easily adapt the rotating-can technique to germinate many seeds simultaneously at several different effective gravities. My first step in this direction is shown in the illustration on the opposite page. The apparatus has 20 half-pint cans (McMaster-Carr, Los Angeles, 562-692-5911; www.mcmaster.com; part no. 4084T42; about $10 per dozen) positioned on four shelves inside an old bookcase. Each shelf holds five cans canted at an angle. I chose to mount them at 0, 30, 60 and 90 degrees (for my control plants). A drive belt rotates the two outer cans. Friction drives the three inner ones.

Figure 3: Reduction gearing allows a 1,600-rpm electric motor to turn the output shaft at a rate of just one revolution per minute.

To make the axles, use a wood screw to hold the center of each can to a half-inch-diameter wooden dowel. Next, use a hole saw and your electric drill to create openings in the base for thrust bearings (McMaster-Carr, part no. 5906K512; 33 cents each). Slip the bearings over each dowel and secure them through the holes. Create the rims from a small sheet of gum rubber (McMaster-Carr, part no. 8633K32; about $4 for a 12-by-24-inch sheet) and an appropriate adhesive (McMaster-Carr, part no. 7587A35; about $3 for a three-ounce tube).

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 plantÑsomething 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.

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

American Science & Surplus offers a unique mix of industrial, military and educational items, with an emphasis on science and education. We supply a wide range of unusual and hard to find items (some say bizarre stuff) to the hobbyist, tinkerer, artist, experimenter, home educator, do-it-yourselfer, and bargain hunter.

American Science & Surplus
P.O. Box 1030
Skokie, IL 60076
847-982-0870 Voice
800-934-0722 Fax

info@sciplus.com

http://www.sciplus.com