In the light of these reports interest in such experiments largely disappeared until the advent of electronic apparatus for generating ultrasonic vibrations. Last year Evalyn Horowitz, then a high school student in Bergenfield, N.J., won a prize at a science fair for a project based on the exposure of radish seedlings to vibrations of 50,000 cycles per second at an acoustic energy equivalent to about one watt. She demonstrated that the rate of growth is almost doubled during the three weeks following germination and that accelerated growth is observed even when the plants are kept in darkness.

The ultrasonic generator. used by Miss Horowitz consisted of an electronic oscillator connected to a small loudspeaker of the kind used for the "tweeter" in high-fidelity phonographs. "The idea of doing the experiment," Miss Horowitz writes, "came to me when one of my teachers explained that the growth of plants can be influenced by light of a selected color. Somehow the words 'light' and 'sound' became linked in my mind; I wondered if sound of a certain pitch might not have a comparable effect on plant growth. The literature was not encouraging. Most investigators who had subjected plants to vibrations in the audible range reported negative results. A bulletin issued by the U.S. Department of Agriculture, however, stated that 'the effects [of sound] on flowering, growth and yields have not yet been evaluated with respect to time of treatment and intensity of treatment.' This implied that the question was not settled. So I decided to set up an apparatus for exposing potted plants to ultrasonic vibrations. I chose radishes for the experiment primarily because they are hardy and the seeds were available.

"Two groups of plants were grown for each experiment. One was exposed to ultrasonic energy and the other was insulated as a control. The apparatus used in the first series of experiments consisted of two cabinets with a volume of one cubic foot each. They were lighted by 7.5-watt incandescent lamps. The experimental cabinet also housed a tweeter, which is a small loudspeaker powered by an audio generator of the kind used by electronic-service technicians. The audio generator developed a maximum frequency of 50 kilocycles at about one watt. I bought these components, together with timing switches, from a dealer in amateur radio supplies.

"The cabinets were constructed of acoustic ceiling tile two inches thick [see Figure 1]. I obtained the tile from a lumberyard. The material was cut to size and assembled with wood screws. A small hole in the top of each cabinet admitted a thermometer for monitoring the temperature of the air. The assemblies were closed with snugly fitting, removable covers of the same material.

"I later learned that the sound would have been more effectively confined if I had made the cabinets by nesting two plywood boxes with an inch of space between them and filling the space with sand. My construction worked, however, perhaps because during operation the cabinets were separated by about 10 feet. At this distance little ultrasonic energy from the experimental cabinet penetrated the control cabinet.

"Enough garden soil was procured for eight clay pots of two-inch diameter. The soil was thoroughly mixed in a cardboard box, tested for chemical composition, fertilized, moistened and packed lightly into the pots. Four radish seeds were planted in each pot at a depth of about an eighth of an inch. All the pots were watered equally and simultaneously. The time switches were set to turn on the lights and the audio generator at 8:30 A.M. daily and turn them off 12 hours later.

"Records were made of the dates of planting and sprouting. When the seeds germinated, each plant was identified by a numbered label affixed to the edge of the pot. Thereafter on Mondays, Wednesdays and Fridays notes were made of the general appearance of each plant, including height, leaf growth and coloring. The plants were measured in two ways: by straightening the stems and placing them against a ruler and by estimating the height against a background of metric graph paper. I am not satisfied with either method. The first entails handling the plants, with the risk of damage, and the second is subject to error. I did not succeed, however, in devising a better measuring scheme.

"The first experiment was stopped after 28 days, and I then plotted graphs of the relative growth of the experimental plants and the controls [see Figure 2]. Plants that were treated with ultrasonic vibrations grew an average of 87 percent taller than the controls. In general appearance the controls were small and sturdy, with thick stems and bright green leaves. In contrast, the experimental plants were comparatively spindly. They were tall and shaky, with thin stems and dark green leaves.

"This experiment demonstrates that exposure to ultrasonic vibration affects the growth of radishes, but it does not explain why. It has been suggested that the vibrations 'awaken' the plants, that is, that they first stimulate the metabolic processes as light becomes available for photosynthesis and that later they accelerate the biochemical reactions. If this were the case, the plants should produce more and larger leaves and thicker stems; in general they should become large counterparts of the controls. Instead they merely grow taller.

"The theory has also been advanced that the vibrations raise the temperature of the soil, thereby increasing the activity of soil microorganisms and providing the experimental plants with an abnormal amount of nutrients. A careful check of soil temperature by thermometers that could be read to within half a degree failed to show a significant difference in soil temperature between the two cabinets. Increased microbe activity should change the acidity of the soil. A careful check of soil acidity at the end of the experiment indicated a pH of between 3.5 and 4 in all pots.

"The plant hormones known as auxins can encourage elongated growth. It occurred to me that the most common auxin, 3-indoleacetic acid (IAA), might be involved. Perhaps the vibrations either increased its formation and activity or decreased the effectiveness of its inhibitors. As a rough check on this hypothesis I made up four solutions of the chemical: two test tubes containing five grams of a .1 percent solution of IAA (the approximate concentration found in plants) in ether, and another two test tubes containing five grams of a 1 percent solution of IAA in ether. One tube of each concentration was placed in the control and experimental cabinets respectively. The experimental solutions were exposed to 50-kilocycle vibrations 12 hours a day for 10 days. The .1 percent experimental solution lost .08 gram more IAA than its control and the 1 percent solution lost .17 gram more than its control. These losses do not necessarily imply increased chemical activity as a result of the ultrasonic irradiation, but they indicate that some phenomenon other than evaporation is at work.

"In another experiment I compared the growth of two groups of radish seedlings: one treated with IAA and exposed to ultrasonic vibrations, the other not treated with the hormone but exposed to the vibrations. Two other groups, one treated with the hormone and one untreated, were used as controls. These plants were potted in troughs that required more room than was available in the original cabinets. Larger cabinets were built [see illustration below]. These were similar to the ones made first but were lighted by 20-watt fluorescent lamps. The lamps overheated the cabinets. This problem was solved by placing the ballast coils of the lamps outside the cabinets and by keeping a tray of ice inside as required. A more elegant cooling system could have been set up by installing a heat exchanger in the cabinets, but my scheme worked nicely even though it required close attention.

"One group of control plants and one group of experimental plants were watered regularly as in the first experiment. The other group of controls and the second group of experimental plants were watered with a .1 percent solution of IAA. All other conditions were identical with those of the first experiment. The IAA-treated control plants showed 91.5 percent of the growth of the untreated controls. The IAA-treated experimental plants grew 140 percent higher than the untreated controls, and the untreated experimental plants grew 150 percent higher than the untreated controls.

"Although the hormone-treated plants in both cabinets grew somewhat less than their untreated counterparts, the treated experimental plants exhibited more growth with respect to the untreated experimental plants than the treated controls did in relation to their untreated counterparts. This result does not necessarily implicate IAA in the phenomenon of growth acceleration, although in my opinion some relation is implied. The lower comparative growth rate of the treated plants may be explained by the fact that auxin can cause a plant to spend more energy 'burning' food than using it.

"As a final experiment in the series, I compared the relative growth of a group of normally lighted controls with a comparable group of experimental plants exposed to ultrasonic vibrations but deprived of light. I was curious to learn if some or all of the energy necessary to support growth could be provided by ultrasonic vibrations. Plants normally grown in darkness are tall, thin and devoid of the green pigment chlorophyll; they die of starvation soon after sprouting. Unfortunately this experiment was undertaken just prior to the science fair, when my time was limited. Even so, it produced at least one interesting result.

"The control cabinet was lighted by a single 7.5-watt incandescent lamp. The experimental plants were irradiated with ultrasonic vibrations in darkness. Both cabinets were operated 12 hours a day for 20 days. Four plants were grown in each cabinet. The average growth of the two groups is shown in the accompanying illustration [Figure 4 ]. The control plants were small but sturdy; they had thick stems and the leaves were bright green. The experimental plants, which grew 134 percent taller than the controls, were spindly, thin-stemmed and yellowish, but they survived."

Nyle A. Steiner of Kaysville, Utah, submits what is perhaps the most inexpensive system that can be devised for demonstrating the many effects associated with electrical discharges in a gas at low pressure. The complete apparatus, including the gas-discharge tube and vacuum pumps, can be constructed for about $5 if the experimenter owns an induction coil and no more than $10 if he does not. Steiner writes: "My discharge tube was made from a glass straw bought at a drugstore. The tube has an inside diameter of about four millimeters and is about 25 centimeters long. The anode was prepared by wrapping enough black plastic tape snugly around a two-inch length of 14-gauge clean copper wire to make a snug fit with the inside diameter of the glass tube. The joint was made airtight with a coating of plastic cement. It could also have been sealed with paraffin or with a mixture of equal parts of rosin and beeswax applied smoking hot.

"The cathode, which is installed in the other end of the glass tube, consists of a 1 1/2-inch length of small brass tubing removed from a ball-point pen. Traces of ink inside the tubing can be removed with carbon tetrachloride or a comparable cleaning fluid. The cathode is sealed into the glass tubing by the same method used to install the anode. The brass tube serves both as an electrode and as an outlet for exhausting the glass tube.

"The air pump consists of two parts: a length of heavy-walled rubber tubing, mounted on a flat board, and a pair of rollers equipped with handles. I take a roller in each hand, press one against the flexible tubing near one end and roll it to the opposite end. This causes a closed constriction to move down the tube, pushing the air ahead of the constriction. The second roller is then pressed against the tubing at the same initial point before the first roller is lifted. This sequence of motions is continued. The action resembles that of milking a cow. The glass tube to be exhausted is connected to the end of the rubber tubing at which the strokes originate. Care must be taken to avoid lifting both rollers simultaneously or air will rush into the vessel.

"I used a 20-inch length of surgical tubing for the pump. The tubing had an outside diameter of 3/8 inch and an inside diameter of 3/16 inch. This tube was mounted by means of staples on a board 18 inches long, three inches wide and 3/4 inch thick. A guide rail 1/4 inch thick that extended 1/8 inch above the surface of the base at one side prevented the rollers from tipping off the tubing [see illustration at right].

"The connection between the pump and the discharge tube was made by a six-inch length of neoprene tubing that made a tight fit with the brass tubing serving as the outlet of the discharge tube. The neoprene tubing was coupled to the larger surgical tubing by means of a reducer made by soldering a one-inch length of brass tubing from a ball-point pen into a similar length of 1/4-inch copper tubing. The small neoprene tubing was obtained from a hobby shop. Such tubing is normally used for the fuel line of miniature gasoline engines. The rollers are ink brayers and cost $1 each at a shop selling art supplies. The operation of the pump can be improved by lightly coating the inner surface of the surgical tubing with vegetable cooking oil.

"The capacity of the pump is low. For this reason small leaks can be troublesome. In some versions of the apparatus I have installed oil seals at all joints. The joints are placed inside a small container, such as the cap of a catsup bottle, and flooded with oil [see Figure 6]. A coating of the rosin-beeswax mixture is about as easy to apply and more convenient because it does not spill.

"For the high-voltage power supply I used an induction coil with a self-contained vibrator for interrupting the primary circuit. The coil produced a half-inch spark between needle points in air. Such coils are available from dealers in scientific apparatus. The high-voltage output is unidirectional and equivalent to direct current. Comparable coils were formerly used in the ignition system of Model-T Ford automobiles and can be bought for a few dollars from mail-order houses that cater to antique-car enthusiasts. The ignition coil from a modern automobile can be used but it must be equipped with a switch for rapidly closing and opening the primary circuit. This could be done by coupling a small electric motor to the camshaft of a distributor from an automobile.

"Direct current for energizing the induction coil can be provided by rectifying the alternating-current output of a 12-volt step-down transformer. For the rectifier I used a single silicon diode. The coil performs much better, however, when it is powered by a 12-volt storage battery. The high-voltage output leads of the coil connect directly to the electrodes of the discharge tube. Avoid touching the leads or electrodes when the coil is in operation or you may get an unpleasant shock.

"To operate the system, switch on the high voltage and operate the rollers. After about 10 strokes reddish streamers that resemble miniature bolts of lightning will appear between the electrodes. As pumping continues the streamers will be replaced by a solid bluish glow that fills the entire tube. Shortly thereafter the glow will fade at one end; the so-called Faraday dark space will appear and the remaining glow will slowly break into striations. When the pump is operated at the rate of about one stroke per second, all these phenomena will be observed in less than a minute.

"If the system is working well, the Faraday dark space may grow to a length of about a quarter of an inch. Greater length requires lower pressure. To achieve it you can convert the surgical tubing into a two-stage pump by means of a pinch clamp. First pump the system to the lowest possible pressure. Then clamp the surgical tubing on the low-pressure side of the brayer at the end of the stroke. Pumping is resumed by stroking the brayers between the hose connection and the pinch clamp. Thereafter air that is removed from the discharge tube is compressed in the short length of surgical tubing between the pinch clamp and the end of the brayer stroke. The pressure in this region, even after prolonged pumping, will remain far below that of the atmosphere. Still lower pressures can be achieved by similarly installing a second pinch clamp, thus converting the device into a three-stage pump. Continued operation will then cause the striations to fade, and within minutes a greenish glow will appear on the inner walls of the glass. This glow is fluorescence excited by X rays.

"Faster pumping speeds can be achieved at a somewhat lower cost in terms of labor by backing up the pump with a water aspirator. Such an aspirator can be made easily by inserting the brass tubing from a ball-point pen, through the side of a seven-inch length of copper tubing (1/4 inch outside diameter) fitted with a hose connection [see illustration below]. The hose connection is screwed to a water tap. When water runs through the copper tubing, air is drawn into the stream through the brass tubing. I have also backed up the pump with the compressor unit from a discarded refrigerator. The inlet of the compressor is connected to the outlet of the surgical tubing. With this arrangement I have succeeded in pumping the tube to a pressure so low that all glow disappears, indicating a pressure of less than .001 millimeter of mercury."

Bibliography

THE USE OF ULTRASONIC ENERGY IN AGRICULTURE. Lowell E. Campbell and L. G. Schoenleber in Agricultural Engineering, Vol. 30, No. 3, pages 239-241; May, 1949.

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