IMAGE: DANIELS AND DANIELS

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.

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.

IMAGE: DANIELS AND DANIELS

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.

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).

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