Popular Science Monthly/Volume 74/March 1909/The Influence of Radium Rays on a Few Life Processes of Plants
|THE INFLUENCE OF RADIUM RAYS ON A FEW LIFE PROCESSES OF PLANTS|
UNIVERSITY OF MISSOURI
THE purpose of the present paper is to present in non-technical form some of the more striking results embodied in the author's memoir on "Effects of the Rays of Radium on Plants."
It is now well known that radioactivity was discovered by Henri Becquerel in 1896. Earlier in this same year Niewenglowski had found that several substances, after being exposed to sunlight, gave off a new kind of rays that could penetrate matter opaque to ordinary light. Following up this work, Becquerel found that the salts of uranium gave off such rays, even when not exposed to sunlight. Uranium, he said, manifests a kind of "invisible phosphorescence." It was Madame Curie who, in 1898, proposed the term "radioactive" for substances possessing this property. In 1898, also, M. and Mme. Curie and Bemont announced the discovery of a new substance fortement radio-active, contained in pitchblende. In a moment of inspiration they named it radium.
The discovery of radioactivity and of radium introduced a new epoch into physical science. Not only was it necessary to revise old ideas and ways of expressing them, but new ideas and conceptions, and a new scientific jargon all developed in less than a decade. Atom, affinity, opaque, ray, electricity, matter and other more or less fundamental terms had to be redefined. To the layman, getting his science largely from the daily press, it was revolution; but to the patient worker in the laboratory it was evolution. He welcomed the new light as the sure reward of years of patient interrogation of nature; as the culmination of a long series of painstaking investigations.
Through the further classical researches of the two Curies, of Rutherford, Righi, Soddy, Becquerel and many others, the new science of radioactivity rapidly developed. The term atom became a figure of speech; matter and electricity became difficult to distinguish from each other, and what remained of scientific materialism received a blow from which it may never recover.
It need hardly be restated here, that radioactivity is an expression of the disintegration of atoms. The atom of radium is constantly breaking up and hurling into space minute particles at enormous velocities. The smallest of these particles, one one-thousandth the size of a hydrogen atom, bear negative charges (or, shall we say, are negative charges) of electricity, and are called electrons. They travel with about 95 per cent, of the velocity of light, penetrate "opaque" bodies, passing easily between and through their atoms, darken a photographic negative, and to a slight extent ionize a gas through which they pass. Streams of these particles constitute the ß rays of radium and other radioactive substances.
The larger particles projected from radioactive bodies are about twice the size of a hydrogen atom, or two thousand times as big as the ß particles. They move more slowly than the latter, and carry a charge of positive electricity. On account of their relatively greater size, they are much more effective ionizers, and correspondingly less penetrating to "opaque" matter than the ß particles. They were named α particles by Rutherford. Streams of them constitute α rays.
Whenever a ß particle, or electron, is started or stopped a penetrating electro-magnetic pulse in the ether (X ray) is developed. Such rays proceeding from radium are called γ rays. In addition to the emission of one or more of these three kinds of rays, radioactive materials give off a radioactive gas, called the emanation. In studying the physiological effects of radium, therefore, we have to consider these four factors—α rays, ß rays, γ rays and emanation.
Our interest in the effects of radium rays on living organisms is enhanced by the discovery that radioactivity is widely distributed in nature. It is probable that all plants and animals are adjusted to a normal degree of radioactivity in their environment, or, in other words, are in a state of radiotonus. Professor J. J. Thomson was the first to discover that air bubbled through Cambridge (England) tap-water became decidedly radioactive, and the subsequent researches of numerous other physicists have taught us that this property belongs to the waters of most deep wells, to mineral waters generally, to freshly fallen rain and snow, to the spray at the foot of waterfalls, to the water of the ocean in certain localities, and quite probably to all spring waters.
After Elster and Geitel found radioactivity a property of the "fango," or mud from the hot springs of Battaglia, in northern Italy, other investigators discovered the same property in mud from various widely separated sources, in lava from volcanoes, in the sediments of springs, the sand of the seashore, and in sedimentary rocks.
The discovery, also made by Elster and Geitel, of the presence of radioactivity in the earth's atmosphere has been abundantly confirmed. Soil-air is more strongly radioactive than air above the surface. Evidence leads to the conclusion that the radioactivity of water, air, mud, rocks, etc., is due to the presence of the emanation of radium and other radioactive substances.
Radioactivity, therefore, must be recognized as a factor of plant environment, and plant physiology and the newer physics join hands. Here, as elsewhere, the boundaries between the different "sciences" break down.
Fig. 1 is from a photograph of a few of the preparations employed in the experiments about to be described. The three marked R are sealed glass tubes containing radium bromide. The figures indicate the degree of activity of the preparations in terms of the activity of uranium taken as a unit. Radium bromide of 1,800,000 activity is the purest salt thus far obtained. The lower right-hand tube contains radio-tellurium, which gives off only a rays.
The rod below the tubes is of celluloid, coated on one end with radium bromide of 25,000 activity. The radium coating is overlaid with one of celloidin for purposes of protection.
By means of the rod, not only the three kinds of rays, but the emanation as well, are available. The walls of the sealed glass tubes permit the β and γ rays to pass, but the emanation and the α rays not at all.
Radium coatings, such as those on the rod, were devised by Mr. Hugo Lieber, of New York City, and are a valuable aid in studying the physiological rôle of radium. The experiments of the writer, carried on for over three years at the New York Botanical Garden, were made possible solely through the great liberality of Mr. Lieber, who freely supplied all the standard preparations, several thousand dollars worth in all.
In none of the experiments did the radium itself come in contact with the plant tissues. The results noted were due to the action of the rays alone. When the sealed glass tubes were used, the effect was produced by the β and γ rays, acting together; when the radium coatings were employed, by the combined action of the emanation and the rays, α, β and γ.
To review the results obtained by other investigators is beyond the scope and purpose of the present article. Koernicke, Dixon and Wighman, A. B. Greene, Guilleminot and Abbe, not to mention others, have experimented on the action of radium rays on germination and growth, and, to a slight extent, upon other plant processes. There seems to be general agreement among them that the rays exert a retarding or an inhibiting effect, depending upon the activity of the preparation employed and the duration of exposure to the rays.Germination is easily retarded or inhibited by exposing seeds while dry, or during imbibition of water. In one experiment ten seeds of "Lincoln" oats, after being soaked in water overnight, were exposed for eighteen hours to rays from the tube of 10,000 activity, by being placed with their embryo-sides in contact with the tube. Germination was retarded by this treatment. After being exposed for about 50 hours longer to the same preparation (67 hrs. 35 min. in all), the
seeds, together with ten unexposed, but otherwise similarly treated seeds, were planted in soil in pots. The relative amount of growth in the two cultures at the end of five days after planting is illustrated in Fig. 2, where R is the culture exposed to the rays, and C the control (unexposed) culture. The growth of the root system was also greatly retarded by this treatment, and the root hairs on seedlings from exposed seeds were much longer than normally.
The effect of duration of exposure on the germination and growth of lupines (Lupinus albus) is shown in Fig. 3. The activity of the radium was the same in each case, 1,800,000, the seeds were exposed dry, and the length of exposure, from left to right in the figure, was 72 hours, 50 hours, 26 hours, 0 hours (control). The size of the largest seedling in the pan at the left doubtless indicates that the seed
was poorly exposed. There is usually more or less difference in the resistance of individuals, but never as much as that apparently indicated, in the 72-hour culture. This and similar experiments confirm the results of Koernicke and others that the effect varies directly with the duration of exposure.
The relative effect of preparations of different activities is illustrated by the following typical experiment. Three sets, a, b and c, of six dry seeds of the white lupine were exposed to rays from sealed glass tubes of radium bromide by laying the tubes in contact with the hilum edges of the seeds. Care was taken to have the radium salt distributed evenly along the bottom of the horizontally placed tube. The activities of the preparations were: a, 1,800,000; b, 1,500,000; c, 10,0-00. A fourth set, d, served as a control. All exposures were for 91.5 hours. The seeds were then sown in soil in pots, and the comparative amounts of growth in the four cultures are shown in Fig. 4. The activity decreased from left to right. It is clearly demonstrated that the stronger the activity the greater the amount of retardation, under the conditions of the experiment.
An experiment to test the effect of a radioactive atmosphere on germination and growth was facilitated by the preparation by Mr. Lieber of a tube lined with the radium coating devised by him. This tube (T, Fig. 5) was connected with the upper tubulure of a glass bell-jar, resting air tight on a ground-glass plate. The lower tubulure was connected with an exhaust, so that air, entering the radium lined cylinder, carried with it into the bell-jar the radioactive emanation. This air was delivered over pots of growing plants or freshly planted seeds in various ways, one of which is shown in Fig. 5, where the radioactive air passed over the soil-surface from an ordinary dovetail
gas burner. The opening to the outlet pipe is under the flower pot. A control apparatus was similarly arranged, with the exception of the omission of the radium preparation. In one experiment, after a six days' exposure of timothy grass seed, sown unsoaked and covered with only an extremely thin layer of soil, germination and growth were shown to be retarded and the amount of retardation was greatest nearest the point of delivery of the radioactive air (Fig. 6), But where germinated seeds of the white lupine, with radicles marked 10
mm. back from the root-tip, were exposed for twelve hours in the radioactive atmosphere, growth was greater than that of a like number of roots similarly placed in the control jar. In one experiment, for example, the total growth in 24 hours was, for the exposed radicles, 28.66 mm., and for the control radicles only 16.08 mm.
Thus it is seen that exposure to radium rays, though followed in some cases by a retardation or inhibition of function may, under certain suitable conditions of exposure and with certain tissues, be followed by an acceleration.
Excitation of function is further illustrated by the following experiment: In a flower pot of soil unsoaked seeds of oat were sown in three concentric circles, distant, respectively, 7 mm., 22 mm. and 45 mm. from the center of the pot. Into the soil at the center was inserted the sealed glass tube of radium bromide of 1,500,000 activity. The end containing the radium was about 15 mm. below the soil surface. A second pot was arranged in a like manner except for the substitution of an empty glass tube for the radium tube. At the end of 106 hours the seedlings from the exposed seeds were much taller
than those in the control pot (Fig. 7), the amount of stimulation being greatest in the outer circle of plants and least in the inner circle. At the end of the 106-hour period the radium tube was placed in the control pot and the empty glass tube in the pot R. Following this change the seedlings in CR grew faster than those in R, now serving as a control. Thus it was possible to accelerate the growth of the seedlings in either pot at will by transferring the radium tube from one culture to the other.The fact that incandescent gas mantles contain a large percentage of thorium, a radioactive substance, suggested the following experiment. On the surface of soil in a pot was sown a row of timothy grass seed, and over this row and at right angles to it, was suspended a fresh, unburned mantle at a distance of three or four millimeters above the seeds (Fig. 8). Germination and subsequent growth were both retarded
by the rays from the mantle, and Fig. 9 shows the appearance of the culture seven days after the experiment was started.
The influence of radium rays on photosynthesis was tested in several ways. For example: A nasturtium (Tropœolum) plant was placed in sunlight after having been in darkness for 18 hours. Under one of the leaves, and lightly in contact with it, was placed a Lieber's coated rod of undetermined (probably 25,000) activity. After twenty-four hours the leaf was dechlorophyllized and stained with iodine to test the presence of starch. Starch was almost entirely wanting in the part of the leaf that was directly over the radium-coated rod, but was
present in other portions of the leaf. The result was recorded by exposing the leaf to sunlight in contact with the velox paper in a printing frame. The region lacking starch, being more translucent, gave the darkest image on the velox paper (Fig. 10).
It was found possible to increase the rate of respiration of germinating seeds by means of the rays, and alcoholic fermentation was also accelerated by suitable exposure, as follows: Five fermentation tubes were filled with equal quantities of a mixture of 2 gm. of a compressed Fig. 10. yeast cake in 250 c.c. of a 5 per cent, solution of cane-sugar. Into four of the fermentation tubes were placed sealed glass tubes as follows: RaBr2 1,500,000 X; 10,000 X; 7,000 X; radio-tellurium. The fifth served as a control. At the end of about three and one half hours the cultures were photographed (Fig. 11). It is clearly shown in the figure that the rate of alcoholic fermentation, as measured by the evolution of gas, was accelerated by the rays; most by the preparation of 1,500,000 activity, least by that of 7,000 activity, and to an intermediate degree by the other preparations.
Various attempts have been made to detect a tropistic response, or curvature of a growing organ toward or from a radioactive source. The phosphorescent light of radium has not been found intense enough to call forth phototropic curvatures, and the existence of a true radiotropism is yet to be demonstrated. Koernicke found that seedlings
grown from exposed seeds were still sensitive to gravity and unilateral illumination, and the experiments of the writer confirm this result. Under certain conditions of exposure of corn grains, however, the seedlings failed to respond to gravity, and grew horizontally, close to
the soil surface. Thus, in one experiment, the grains were exposed for twenty-seven hours to rays from radium bromide of 1,800,000 activity, and all of them showed this tendency to a greater or less degree (Fig. 12, pot 27). Whether geotropic sensibility was destroyed by the exposure is difficult to say, for histological examination showed the tissues to be so abnormal that it is possible the plants could not have stood erect even if they had been able to detect the stimulus of gravity.
All attempts to obtain a curvature of growing organs or plants toward or from a radium tube or radium-coated rod proved unsuccessful, but when a sealed glass tube of radium bromide is suspended horizontally
in tap-water, or in nutrient solution, in which radicles of white lupine seedlings are growing vertically, the tips of the roots may be made to curve toward the radium. Such a result is illustrated in Fig. 13. In this experiment the radium tube was originally about 5 mm. distant from the root-tips. Whether this result was due to the direct influence of the rays, or to some undetermined condition established by them in the liquid can not yet be decided.
The above experiments were all confirmed by repetitions, and clearly indicate that radium rays act as a stimulus to the various physiological processes of plants. If the strength of the radium, the duration of exposure, and other conditions are suitable, the response is an excitation of function, but if the method of treatment is otherwise, the radium too strong, the exposure too prolonged, the result is a retardation, or complete inhibition of function, or the death of the plant. There are not only differences in sensitiveness between individuals, but also between different species and different tissues. As in the case of animals, embryonic and younger tissues are more sensitive than those that are older and more mature.
- Contributions from the Botanical Department of the University of Missouri. No. 16.
- Mem. N. Y. Bot. Gard., 4: 1-286, November, 1908.