SOMETHING REMARKABLE HAPPENS WHEN you tip a plant. Special hormones, called auxins, begin to collect in the underside of its roots and stem. Auxins stimulate stem cells to grow and divide. The bottom of the stem then outgrows the top, causing the stem to bend skyward. Auxins in the root cells act differently: they retard growth. The auxin-poor cells near the 5ai top of the root then outgrow the auxin-rich cells near the bottom, and the root bends downward. In this way, a tipped plant makes internal adjustments to -realign itself with the pull of gravity.

Botanists call a plant's response to gravity geotropism. In the early 1800s experimenters explored geotropism by growing plants on a rotating wheel, thereby exposing them to both the earth's gravity and centrifugal forces. The plants grew against the vector of the resultant force-that is, against the direction of the combined forces.

But scientists soon learned that plants respond to gravity only sluggishly. Most plants must be tipped for at least a minute before the auxins start to redistribute. By the turn of the century, scientists had invented a device, called a clinostat, that tricks plants into thinking they are growing in near-zero-gravity environments. Clinostats are still used today. By slowly rotating a plant vertically and horizontally, the clinostat prevents the plant from fixing on gravity. It then grows almost as if there were no gravity at all. Clinostats have fascinated amateur scientists for generations. Don Graham, my grandfather and an amateur scientist extraordinaire, published his explorations into geotropism in these pages 26 years ago [see "The Amateur Scientist," SCIENTIFIC AMERICAN, June 1970].

Oddly, professional biologists have paid little attention to what is perhaps the most interesting region to investigate-between 0 and 1 g (the acceleration caused by the earth's gravity, equal to an increase in speed every second of 9.8 meters per second). This is good news for the amateur, for a little dedication could reward you with original discoveries.

The rotating platform of the device described here is a bicycle wheel. By properly choosing the wheel's rotation speed and placing the seeds at different distances from the pivot, you can germinate seeds at any gravity. Observe the thresholds at which plants first respond to gravity and see how seeds would grow on Mars (about 0.4 g). Tumbling the wheel is also necessary. The earth's gravity then averages to zero, so that the seeds consistently experience only the centrifugal acceleration from the spinning.

Besides the wheel, you will also need to scavenge parts of the bicycle frame- in particular, the front mounting forks and its hollow shaft that slides up the head tube (the part through which the handlebar stem goes). I bought them all for $15 from a bike rental shop, which kept them around as spares.

Pieces of wood support the frame and wheel. Cut a hole through the upper ends of two slats of one-inch-thick pine shelving; attach the lower ends to a pine baseboard so that the slats stand upright [see illustration in Figure 1]. Thread the mounting-fork shaft through the holes. For a drive gear, try the plumbing department of a hardware store. You can make an excellent one by cutting two inches off the end of a large-diameter plastic or rubber pipe. Shore up the inside of the circular ring with a wooden plug. Cut a hole in the center of the plug, then thread the assembly over the shaft and epoxy it in place.

Next, thread a dowel through the drive gear and into the shaft, then attach a weight, such as those scuba divers use on their belts, to the dowel. Bolt a three-by-seven-inch wood slat to the end of the dowel. Temporarily tie the weight to the slat. Position the wheel so that the bend of the mounting forks points upward. Slide the dowel into the shaft tube and adjust the weight's position, both horizontally and vertically, until the wheel is balanced against its closest support. Epoxy the dowel in place and secure the weight with bolts.

To negate the effects of gravity, the bicycle wheel must complete a tumble about once each minute. I used a slow motor (0.28 revolution per second, or rps) and connected the motor shaft to the drive gear with a flat rubber belt (check the power-tools section of your local hardware store). So attached, the drive gear turned at the right speed. If your motor spins at a rate much different from 0.28 rps, then you will have to fiddle with the size of your drive gear or add another gear to the motor shaft to produce the correct ratio.

A second, faster motor spins the bicycle wheel to create the artificial gravity. I used a 12-volt DC motor (Radio Shack No. 273-255), which spins at 192 rps. Make sure the tire's tread is smooth- tires from road bikes work well. The radius of the motor's shaft is about one millimeter. To figure out at what frequency to spin your wheel, consult the box on this page.

In my clinostat the radius of the wheel was 30 centimeters; at 192 rps, my motor spun the wheel at 0.64 rps. You can increase the motor's effectiveness by wrapping a few layers of cloth tape around the spool; the extra material will easily boost the rotation frequency to 1.5 rps. Power comes from a DC adapter and feeds the DC motor through two slip rings. The circuit shown in the schematic on the opposite page regulates the power.

Commercial bicycle speedometers let you easily monitor the acceleration. Get the kind that uses a small magnet placed on one of the spokes. Mounted near the axis, the magnet can measure the speed to the nearest 0.1 mile per hour or (better yet) 0.1 kilometer per hour.

So little work has been done in this area that you can grow just about anything and find something new. I've been focusing on corn. I let the seeds germinate for several days and then measured the total length of the sprouts and their "angularity"-the sum of the bend angles along the stock. At 1 g, the plants grow very straight; near 0 g, they become quite crooked. Growing seedlings at a number of locations along the radius of the wheel enables you to see the effects of gravity "turn on" inside the plant.

Put five seeds into a small handful of potting soil and place them inside the leg of an old nylon stocking. Cut the ends around the soil, then tie them off with a bit of twine to create a small pouch. These lightweight packets hold the seeds in place and make them easy to water. To get good statistics, you will need to sprout roughly 30 seedlings for each acceleration, so you should place six of these bundles at the same distance from the center. Arrange them symmetrically between the spokes of the wheel to keep the wheel balanced. You can make more bundles and insert them at different distances, so that you can experiment at different accelerations simultaneously.

Remove the seeds after they have germinated for three to seven days (or until they begin to poke out of the pouches). Cut the seedlings at their bends, then lay all the pieces end to end so that all the bends are in the same direction. The angularity is the angle between the first piece and the last piece. To measure the length of a seedling, lay a string along the piece. For each acceleration, divide the angularities by the seedling lengths and average the results. Plot this average versus acceleration, and you'll see how the sensors in your plants respond to different gravitational fields. You might also consider replanting the seedlings in normal conditions to see how they subsequently do-or try to figure out the best food plant that would grow on Mars.

For more information about this project, consult the Society for Amateur Scientists' Web page at http://www.sas.org/or Scientific American's area on America Online, or send $5 and your address to the Society for Amateur Scientists, 4951 D Clairemont Square, Suite 179, San Diego, CA 92117. 1 am grateful to SAS member John Michaelson for perfecting the instrument's construction and for building the first prototype.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.