SOON AFTER a potted plant has been laid on its side the stem turns up and the roots turn down. Experiments indicate that such changes in the direction of growth are induced by gravity: plants tend to align themselves in the direction of the earth's gravitational field. Botanists refer to this tendency as geotropism and have discovered by experiment that it arises from the influence of gravity on certain substances in plants, namely the organic compounds known as auxins, which stimulate the growth of the upper parts of plants but appear under some conditions to suppress the growth of roots.

When a potted plant is inclined from the vertical, auxin concentrates in the lower sides of the stem and roots. The concentration causes the lower side of the stem to grow faster than the upper side. The stem bends upward. Conversely, auxin in the lower side of the roots retards growth, but normal growth continues in the upper side. The root turns downward.

These effects can be observed by making a simple experiment. With India ink draw a set of evenly spaced marks along the lower side of both the root and the stem of a seedling. Place the seedling horizontally in a moist container for 24 hours and then examine the marks. The spacing between the marks will have increased on the lower side of the stem where it bent upward but will not have increased on the lower side of the roots.

Of course this experiment does not prove that gravity is solely responsible for reshaping the plant. The tops of plants grow toward sources of light, and the leaves of many plants follow the sun. The unequal distribution of auxin is also responsible for this effect, which is known as phototropism, but botanists have not yet learned how light influences the substance. Three mechanisms have been suggested. Light may inhibit the production of auxin on the exposed side. Alternatively, it may denature a portion of the normal production. On the other hand, as in the case of gravity, light may cause auxin to move to the shady side of the stem. Whatever the mechanism, you can identify the side of the stem that grows faster by drawing evenly spaced rings of India ink around the stem of a potted plant, placing the pot upright near a window and measuring the spacing of the rings as the stem bends toward the light. This experiment casts doubt on the assumption that the stem of an inclined plant turns upward in response to gravity. Perhaps the stem is merely seeking light, which usually comes from above.

All doubt concerning the role of geotropism in plant growth can be resolved by another experiment that was first performed about 150 years ago. In this experiment upright pots that contain seeds or seedlings are uniformly illuminated on all sides, but the gravitational field is tilted from the vertical by mounting the pots upright on the rim of a wheel that turns in the horizontal plane. (Each pot is enclosed in a transparent container for protection against currents of air.) When the wheel turns, the pots are acted on by two components of inertial force: a horizontal component arising from the circular motion of the wheel and a vertical component resulting from the acceleration of gravity. The resultant force acts at an intermediate angle that is determined by the speed of the wheel.

The roots of plants that are grown on the continuously rotating wheel extend outward and downward at precisely the angle of the resultant force. The stems grow inward and upward in exact alignment with the roots. The lines of resultant force along which the plants grow trace a cone in space as the wheel rotates. The altitude of the cone varies inversely with the speed of the wheel, an experimental result that can be explained only on the assumption that inertial force strongly influences the direction in which plants grow. Having established this fact, experimenters appear to have closed the books on geotropism and shifted their attention to other matters.

Interest in geotropism has now been dramatically rekindled, at least for one amateur, by the development of space vehicles. About a year ago Don Graham, who is a commercial artist in Petrolia, Calif., began to wonder how a plant might react if it were grown in a weightless state. Graham decided to undertake the experiment but could think of no way of eliminating gravity without putting plants in a space vehicle. Instead he devised an apparatus that interferes with the natural response of auxin to the gravitational field. Plants that are grown in the apparatus apparently lose their sense of direction. Graham built a pot that rotates slowly but continuously in all coordinates of three-dimensional space and undertook to grow corn in it. He describes the experiment as follows:

"My apparatus consists essentially of a cylindrical pot that rotates simultaneously on its axis and in the horizontal plane [see illustration above right]. The hollow cylinder, made of the wire mesh known as hardware cloth, is a foot long and about four inches in diameter. The ends of the cylinder are closed by two wood disks. The cylinder is supported by a shaft that passes through snugly fitting holes in the center of the disks and is rotated on its axis by a pulley on one end. The shaft is supported at its ends by a pair of vertical brackets that are fastened to a horizontal wood base.

"The base is rotated in the horizontal plane by a vertical shaft that is coupled to a slow-speed motor by a belt. The motor turns at eight revolutions per minute. A 1 : 8 pulley ratio reduces the speed of the vertical shaft to one revolution per minute. The pulley that drives the cylinder is coupled by a belt to a fixed pulley attached to the frame on which the motor is mounted. None of the dimensions are critical, but the diameter of the fixed pulley should not be a multiple of the diameter of the driven pulley, because this ratio would generate a cyclical pattern of cylinder positions. A 7:11 ratio works well.

"The cylinder is filled with a mixture of four parts of sphagnum moss to six parts of rich loam. I moistened the soil and packed the cylinder as though it were an ordinary pot. Sweet corn was selected for the experiment because the seedlings of corn develop in the form of a series of concentric whorls that appear to be stronger and sturdier than most plants are during the first few days of germination.

"Seven uniformly spaced openings, each 1/2 inch square, were cut in the a wire mesh to form a helical path of one b, full turn that extends to within an inch of the ends of the cylinder. With tweezers I pushed a seed through each opening and into the soil to a depth of two inches, which is to say to the middle of the cylinder. The cylinder was wrapped with a single sheet of clear polyethylene to conserve moisture.

"The apparatus was placed on the ground in the backyard, where it would receive full sunlight. The motor was turned on and operated continuously for 14 days, except during brief intervals when it was stopped for a check on the temperature and moisture of the potted soil. During this entire period seven additional seeds of the same stock were growing in an adjacent garden area that contained identical soil. These plants served as controls.

"On the 14th day all seedlings (both the experimental ones and the controls) were removed from the soil, washed gently, measured and replanted in the garden. All seven control seedlings had grown to an average height of 2-1/2 inches. They appeared to be normal in every respect. The most vigorous measured seven inches from the tip of the root to the tip of the longest leaf.

"The experimental seedlings had grown as vigorously as the controls. The largest measured nine inches from root tip to leaf tip. There the similarity ended. Whereas the controls grew straight up and down, most of the seedlings were sadly misshapen. Only one plant had found daylight; it grew about 2-1/2 inches beyond the wire. The root, which was about 3-1/2 inches long, bent randomly through the soil. One seedling grew in reverse: the root penetrated the wire and the stem remained in the soil. The root and stem of another seedling grew parallel in the same direction! One seed failed to germinate. Another produced a short root and an even shorter parallel stem. No experimental seedling had grown in the normal up-down direction. As the plants were removed I made a record of the direction in which each had grown with respect to its position in he cylinder. The record indicated that the direction of growth had been random.

"All seedlings matured in the garden, where they were cultivated and weeded regularly. The controls grew to heights ranging from five to nine feet and yielded an average of four ears of corn per stalk. In contrast, the confused seedlings matured at a height of less than three feet. Only one experimental plant produced an ear, and it was a distorted, underdeveloped runt [see illustration at right].

"Other experiments involving geotropism come to mind. For example, how long can a germinating plant survive without damage in the absence of a normal gravitational field? My plants were rotated for 14 days. How much damage might have been evident if I had transplanted the seedlings after the fourth or the eighth day? How would a plant react to an increase or a decrease in the intensity of the gravitational field?

"I can think of no practical apparatus for lowering the strength of gravity on the earth to, say, that of the planet Mars. On the other hand, it is easy to investigate the influence on germinating seeds of an inertial force greater than the earth's gravity by growing plants on the rim of a wheel that is spinning. It might be interesting to find out how sweet corn would grow on Jupiter, where gravity at the surface is 2.6 times stronger than it is on the earth.

"One should not place too much confidence in the outcome of a single experiment. Nonetheless, having observed the reaction of my confused corn, I suspect that no plant in an advanced stage of evolution can grow normally in a weightless environment. Nor can such a plant reproduce itself for more than a few generations, notwithstanding the fact that one of mine developed seeds. Perhaps lower marine organisms such as algae, corals or fungi could multiply in the absence of a gravitational field. So far as higher plants are concerned, however, gravity appears to be as essential to growth as sunlight. In my opinion, an orbiting spacecraft would make a poor garden."

WHEN ONE turns on a television set, makes a photograph, looks through a telescope or sends a party of astronauts to the moon, one calls into service thin films of metal or metallic compounds. The films are essential elements in fluorescent screens, photocells, thermistors, transistors, antireflection lenses and s scores of other devices with which amateurs can experiment. Not many amateurs bother to make their own films or to experiment with them, perhaps because most thin-film devices are inexpensive. Why make a poor transistor when you can buy a good one for less than $1? Roger Baker of Austin, Tex., explains that he makes his own transistors (and other thin-film devices) largely because he enjoys tinkering with small devices that occasionally behave in unexpected ways.

"Recently," Baker writes, "I learned of a governing principle known as Murphy's first law of biology. It states: 'Under any given set of environmental conditions an experimental animal behaves as it damn well pleases.' The same law appears to govern the behavior of thin metallic films, at least those I make. Some of my 'transistors' make dandy thermistors, and an occasional photocell works better as a fluorescent screen. There might be fewer surprises if I used better tools and had more experience, but some of the fun might also be lost. The techniques used by industry for making thin films are not beyond the reach of amateurs, but they require vacuum pumps, electronic heating device and controlled sources of high voltage that are costly and inconvenient to use. Thin films can also be deposited chemically. I use this method.

"Most of my films are deposited on substrates of glass. Usually I heat the glass and spray the surface with a solution of selected chemicals. The sprays react immediately to form the film.

"Films can be annealed in various atmospheres and at various temperatures that alter their composition, structure and properties. The properties of a deposited film can also be modified by recrystallization, by solid-state diffusion or by a vapor-phase displacement reaction. These procedures are much simpler to perform than their imposing names suggest. The properties of films can be radically altered by the addition of minute amounts of impurities, either when they are formed or by subsequent diffusion.

The microstructure of the substrate can also influence the properties of a film. For example, calcium sulfide forms an amorphous film when sprayed on a metal surface, but on glass it becomes a crystalline film.

"The required tools include an electric hot plate, a diamond point for cutting thin glass, an atomizer and a microammeter. Desirable accessories are a fume hood, which can be improvised if you have an exhaust fan, an oven thermometer for measuring the temperature of the hot plate, a triple-beam chemical balance, a pair of tweezers and chemical glassware for preparing solutions. For substrates I use mostly cover glasses of 35-millimeter Kodak slides, which I cut into small rectangles with the diamond point. These glasses can be heated (up to 600 degrees Celsius) and sprayed without breaking. Thin disks of alumina can be used at higher temperatures. I salvage them from discarded vacuum tubes. I immerse the glass slides for three days in a solution of one part by volume of nitric acid to 12 parts of distilled water. The acid leaches sodium and calcium ions from the glass, exposing a surface layer of relatively pure silica.

"A great variety of semiconducting oxide films can be made by thermally decomposing the resinate salts of metals. Resinate salts are prepared by stirring an excess of pure granulated resin into a one-normal (1 N) solution of sodium hydroxide. The solution turns milky as it cools. Pour off and retain the milky solution. To make a metal resinate, reheat the milky sodium-resinate solution and combine it with a weak solution of the metal salt, stirring the mixture vigorously.

"A relatively large volume of sodium resinate reacts with a small volume of metal salt. An excess of sodium is indicated by a pH of 8 or more. Add metal salt to lower the pH. The desired metal resinate appears as a thick precipitate.

"Filter the solution to recover the precipitate and wash it thoroughly with hot distilled water. Spread the moist filter cake and dry it at a temperature of about 50 degrees C. Dissolve the dried material in an organic solvent such as turpentine. Allow the sediment to settle. Use the clear upper layer for experiments.

"With a disposable capillary tube apply a few drops of the clear fluid to the center of a prepared cover glass and rock the glass to spread the fluid into a uniform film that extends to the edges. Heat the coated glass on the hot plate. The film will smoke and turn dark. In time, at a temperature that depends on the nature of the resinate, the dark color will clear, leaving a thin film of metallic oxide. The cover glass can then be scribed with the diamond point and broken into rectangles of convenient size for further processing and experimentation.

"Sulfide films can be formed directly from a number of oxide films. Sprinkle a few milligrams of sulfur on the back side of the coated substrate, wrap it with several layers of aluminum foil, fold the ends of the foil over the package and heat the package. The hot vapor of the sulfur will react with many oxides to form adherent films with interesting electrical properties. Two drops of different resinates can be allowed to diffuse partially together so that the properties of various ratios of the two can be explored.

"So far I have experimented with gold, nickel, cobalt, copper, iron, manganese, silver, indium, chromium, zinc and cadmium resinates. The salts of noble metals decompose into metallic films instead of oxides. They can be used for making electrical connections between various films of oxide previously applied to a substrate.

"The preparation of a field-effect transistor illustrates a typical experimental procedure. The substrate, which has been treated with nitric acid solution, is first coated with a film of cadmium sulfide. With distilled water prepare 500 milliliters of a stock solution containing .01 molar (.01 M) thiourea and .01 M cadmium chloride.

"Place the substrate inside a 250-milliliter beaker so that it rests diagonally against the side of the beaker. Cover the substrate with stock solution and slowly add concentrated ammonium hydroxide until the mixture turns faintly cloudy and then clears. Cover the beaker and put it in a double boiler. Heat the vessel slowly and boil for about 15 minutes. The contents of the beaker will turn yellow-orange, indicating the precipitation of cadmium sulfide.

'Pour off the contents and replace them with distilled water. Swab the substrate lightly with a tuft of absorbent cotton to remove adhering particles of cadmium sulfide. Rinse the substrate with distilled water. Repeat the entire procedure to double the thickness of the film, after which you can clean the beaker with hydrochloric acid.

"Bake the substrate in air at 500 degrees C. for 30 minutes. The color of the hot substrate will gradually change from yellow to red and, as it cools, to a deeper shade of orange. With the diamond point scribe and break the cooled glass into rectangular chips 1/4 inch wide and 1/2 inch long.

"The transistor requires two contacts that function as electrodes, one a source and the other a drain. The electrodes are conveniently made of indium, a soft metal that can be pressed into firm contact with the film. Indium is available from dealers in chemicals. Place a small pellet of indium on clean plate glass and roll it into thin foil with a short length of clean glass tubing. Transfer the foil to a yielding surface such as glossy white cardboard and, by pressing straight down with a sharp razor blade, cut the metal into strips about 1/32 inch wide and 1/4 inch long.

"With a sewing needle maneuver two of the strips to a clear portion of the paper so that they are parallel and spaced about 1/16 inch apart. Lay one of the coated chips over the strips so that the ends of the strips are even with one end of the chip. Press the chip firmly and evenly against the metal. The strips will adhere lightly to the film. Burnish them firmly into place by turning the chip strip side up on the plate glass, covering it with a glossy magazine cover and rubbing it with a fingernail. Place a small dab of conductive silver paste on the outer end of each indium strip [see illustration at right]. The dabs serve as terminals for connecting the electrode device to a power source.

"A layer of insulation is applied to the film and the indium strips in preparation for adding the third electrode, which is known as the gate. With a sewing needle apply a thin, uniform layer of vinyl cement by stroking the cement across the upper surface of the device. Do not coat the silver terminals. When the cement dries, apply a coat of silver paste over the insulation. Do not let the silver make contact with the cadmium sulfide film, the indium foils or the source and drain terminals. This completes the gate electrode.

"Finally, to protect the active region of the device coat the upper surface with a layer of silicone rubber that cures in air. This material is available from dealers in hardware. Leave one small region of the gate electrode exposed. This small area will be used for making electrical contact with the gate. Do not coat the source and drain terminals with rubber.

"To operate the device improvise a test fixture such as the one shown in the accompanying illustration [left] for holding the transistor and connecting it to a battery. Power is applied to the source and drain electrodes by a ninevolt transistor battery that is connected in series with a 10,000-ohm resistor and a 0-50 microammeter. If the transistor is reasonably good, the meter will indicate a current of about 10 microamperes. This is called the leakage current.

"Connect a one-megohm resistor between the gate electrode and the positive terminal of the battery. The positively charged gate will attract free carrier electrons into the cadmium sulfide film. Current through the film should rise to about 50 microamperes, indicating that the transistor is a so-called N-channel device and that it is operating in the enhancement mode. The gate electrode draws little current.

"If negative charge is now applied to the gate by transferring the one-megohm resistor to the negative terminal of the battery, current in the source-drain circuit should fall below 10 microamperes. The transistor is now operating in the depletion mode. I do not know why some homemade transistors work better than others. I suspect that their performance may be related to the crystalline structure of the films.

"Capacitors can be made by sandwiching insulation between films, resistors by etching away portions of film to form narrow conducting paths, photocells by doping cadmium sulfide with trace amounts of silver, copper or manganese. Films of zinc sulfide fluoresce strongly. Of course, devices are available on the market that work better than those one can make at home, but mine are better playthings.

"Certain hazards must be mentioned. Metallic salts and acids are toxic. Work either in a fume hood or outdoors when you spray chemicals onto a hot substrate. Wear gloves and an apron of neoprene when you handle acids. Remember that chemicals are hazardous and handle them accordingly."

Bibliography

BOTANY: AN INTRODUCTION TO PLANT SCIENCE. Wilfred W. Robbins, T. Elliot Weier and C. Ralph Stocking. John Wiley & Sons Inc., 1964.

THIN FILM MICROELECTRONICS: THE PREPARATION AND PROPERTIES OF COMPONENTS AND CIRCUIT ARRAYS. Edited by L. Holland. John Wiley & Sons Inc., 1965.

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