Popular Science Monthly/Volume 49/May 1896/The Physiology of Color in Plants
By D. T. MACDOUGAL,
ASSISTANT PROFESSOR OF BOTANY IN THE UNIVERSITY OF MINNESOTA.
THAT the color exhibited by the roots, stems, leaves, and especially flowers and fruits of plants received serious attention at a very early date is well attested by ancient record. It was only in comparatively recent time, however, that the daring conjecture was hazarded that even such an abundant, widely distributed, and characteristic color as chlorophyll (leaf-green) subserved a purpose in the life-process of plants. Doubtless certain masses of marked color, or combination of pronounced tints, must have afforded a gratification to man's sense of beauty quite, early in his development. At the same time and earlier these colors were also used as a distinguishing mark in the selection of plants for food, and later they were taken to be indicative of the absence or presence of magical curative properties. The first-named feature is still valid, and forms the basis of the art of the gardener and florist to-day. The last-named aspect of plant colors received its greatest attention during the prevalence of the practices of the Grecian Rhizotomoi and Pharmakopoli, and later in the "doctrine of signatures." The doctrine of signatures supposed that the color and form of plants indicated their relations, good or evil, to the human race, in reference to which they were especially created. This crude superstition attained greatest favor in the sixteenth century, and is still prevalent in obscure form among the lower classes in certain portions of Europe. The use of colors as a distinguishing mark between species, families, and groups began quite early in the history of attempts at classification, and still forms a minor character in modern systems. A wholly new point of view was that taken by Konrad Sprengel, in his history of the biological significance of color (Das entdeckte Geheimniss der Natur im Bau und Befruchtung der Blumen; Berlin, 1793). To Sprengel is due the idea that the colors of the secondary reproductive organs are a device for the attraction of insects, thus securing cross-fertilization. Investigations in many directions from this idea have revealed the fact that plants in a similar manner attract insects and other animals for many other purposes besides fertilization, and in some instances avoid such visitors, for various reasons connected with their development, in a similar manner. Such an amount of attention has been given to these ecologic color adaptations that the aggregate mass of the results recorded is nothing short of colossal. That these results are of immense value and importance goes without saying: yet, given such a thesis, it is impossible that the observations of both trained and amateur workers should not contain a large number of misinterpreted facts. The general principle has been drawn upon to furnish solutions to complicated or unusual arrangements of color, in a manner highly improbable and unscientific and in many instances verging upon the impossible and ridiculous. That it can not be assumed a priori that the colors exhibited by the flowers or any other organs of the plant are devices to attract and guide insect visitors is becoming more and more apparent. Timely attention has been called to the perversion of this principle by the writer of a recent article on floral biology (Willis, Science Progress, No. 21, 1895). That great care is necessary in the interpretation of areas of color in plants is emphasized by the fact that accumulating observations tend to show that a color sense is wholly lacking except among the higher insects, and that if the colors of flowers were fashioned to attract insect visitors the directive impulse must have been received at a very recent date—that is, since the acquisition of the color sense by insects. It is by no means the purpose of this article to discredit the great mass of well-confirmed facts concerning the uses of the colors as an adaptation to insect visitors, but chiefly to call attention to conclusions afforded by the last fifteen years of research upon the formation and physiological uses of color in plants. The functions subserved by many of the coloring substances besides chlorophyll are by no means secondary in distribution or importance to the individual plant to the exterior adaptations described above.
The principal coloring matters among the higher plants besides chlorophyll (leaf-green) are those which have been grouped under the terms erythrophyll, xanthophyll, and anthocyan. Of these substances the chemical and physical properties of chlorophyll are best known, although its exact composition is yet undetermined. Not only is our chemical knowledge of the non-green colors very vague, but it is thought that a great number of different substances are grouped under each of the above and other color terms. Thus, for instance, anthocyan is made to include the large number of substances to which are due the red, blue, violet,
Fig. 1.—I. Spectrum of Chlorophyll showing Seven Absorption Bands. The two in the red-yellow between B and D, and the three in the blue-violet, beyond F, are the most important and characteristic. The bands between D and E are most marked in the spectra of solutions which have been exposed to the air and light some time, and are believed to be due to disintegration products of chlorophyll.
II. Spectrum of Amaranth-red. All the rays except those falling between B and D have been absorbed.
III. Spectrum of Autumnal Color of Leaves of Ampelopsis. All the rays except a part of those falling between C and D have been absorbed.
and purple colors of such plants as the violet, beet, canna, rose and amaranth. Only so much is known of the formation of these color substances as to justify the assertion that many of them are produced as disintegration products of the glucosides and others from a mother substance—chromogen.
The coloring matters of plants may be in solution in the cell sap as in the beet and amaranth, in irregular solid masses in the sap or protoplasm, as in nasturtium (Tropœolum); or may be incorporated in the cell wall, as in logwood (Hæmatoxylon); or dissolved in minute oil drops suspended in special masses of protoplasm, as is the case with chlorophyll.
Although this article is particularly concerned with the non-green colors of plants, yet it will be necessary to outline the function and adaptations of chlorophyll, to which these substances
Fig. 2.—Curves showing Brightness and Synthetic, Thermal and Disintegrating Effects of the Regions of the Solar Spectrum.
bear a special relation. Chlorophyll is perhaps the most important coloring substance in the world, for upon this substance depends the characteristic activity of plants, the synthesis of complex compounds from carbon dioxide and water—a process upon which the existence of all living things is ultimately conditioned. Only in a very few unimportant forms devoid of chlorophyll can the synthesis of complex from simple compounds or from the elements be accomplished. The function of chlorophyll may only be comprehended when its chief physical properties are understood. These may be best illustrated if a solution of the substance is obtained by placing a gramme of chopped leaves of grass or geranium in a few cubic centimetres of strong alcohol for an hour. Such a solution will be of a bright, clear green color, and when the vessel containing it is held in such a manner that the sunlight is reflected from the surface of the liquid it will appear blood-red, due to its property of fluorescence, that of changing the wave length of the rays of light of the violet end of the spectrum in such manner as to make them coincide with those of the red end. It is by examination of light which has passed through a solution of chlorophyll, however, that the greatest insight into its physical properties may be gained. If such a ray of such light is passed through a prism and spread out on a screen, it may be seen that there are several large intervals or dark bands in the spectrum. The rays of light which would have occupied these spaces have been absorbed by the chlorophyll, and converted into heat and other forms of energy. This energy is directly available to the protoplasm containing the chlorophyll, and by means of it the synthesis of complex substance may be accomplished. Moreover, the amount of synthesis accomplished by plants exposed to separate portions of the spectrum will be directly proportional to the amount of that portion which can be absorbed and converted into useful forms of energy. This is graphically illustrated in Fig. 2, The amount of synthesis is shown to be greatest in the red light between B and C, where the greatest absorption takes place. (See Fig. 1, I.)
Chlorophyll is a very complex and highly unstable substance, and during the absorption of light it is slowly broken down, but Fig. 3.—Transverse Sections through the Frond of Lemna Trisulca (Duckweed), showing Different Positions of Chlorophyll Bodies. A, position in diffuse light; B, in strong light striking the surface perpendicularly; C, in darkness. ordinarily it is rebuilt by the protoplasm as fast as it is decomposed. If, however, the chlorophyll and the leaf containing it are exposed to a light of such intensity that the chlorophyll is decomposed faster than it can be rebuilt, then damage must ensue, which if sufficiently extensive will result in the death of the entire leaf. The intensity of the light which induces a maximum of activity in any plant, and which it may receive without damage, is determined by its specific constitution. The intensity of light falling on a plant in an open plain during twenty-four hours ranges from almost total darkness to the blaze of the noonday sun, and varies almost momentarily. As an adjustment to this condition many plants are able to regulate the intensity of the light impinging on the chlorophyll-bearing masses of protoplasm by altering the position of the surfaces of the leaves. In others in which this movement is not possible—such, for example, as the leaf-like duckweeds which float on the surface of the water—the intensity of the light received is regulated by alternations in the position and distance of the chlorophyll from the surface of the organ. (See Fig. 3.)
In many plants growing in the bright glare of the sun a thickened cuticle or a heavy coat of hairs serves to protect the chlorophyll against the more intense action of the rays. It is also in this purpose of protection of the chlorophyll that many of the colors grouped under anthocyan find their chief function in the plant. In such instances the color is generally in solution in the sap of the layers of cells exterior to the chlorophyll, and light must pass through the coloring matter in order to reach the interior of the leaf. This may be seen by reference to Fig. 4, in which is shown a cross-section of a portion of a leaf of coleus.
That such layers of coloring matter do materially alter the light which passes through them may be demonstrated if the spectrum of light which has passed through a solution of them is examined in the manner described above. Water, instead of alcohol, Fig. 4.—Cross-section of Leaf of Coleus. A, A, epidermal cells tilled with reddish cell sap; B, cells containing chlorophyll bodies. is used as a solvent, however. If the color of the leaf of the amaranth is used, it will be found that nearly all the light has been absorbed except a portion between B and D (Fig. 1, II).
It may be seen that a large proportion of the light is absorbed by the anthocyan and converted into heat, and furthermore it is inclusive of the portion of the spectrum which exercises the most violent disintegrating effect on chlorophyll, as may be seen by reference to Fig. 2. The portion which promotes synthesis of food materials, on the other hand, is transmitted almost unchanged to the chlorophyll beneath. That the anthocyan does partially retard the disintegration of chlorophyll by light may be seen if two vessels containing solutions of chlorophyll are so arranged that the light which strikes on one of them shall first pass through a parallel-walled vessel containing water, and that which strikes the other through a similar vessel containing a solution of anthocyan. The chlorophyll in the first will soon become much more discolored than in the second, which has received light transmitted through anthocyan. The number of plants in which coloring substance is present in the cell sap or walls of the outer layers of leaves is extremely large, and braces many well-known species, among which are the "foliage" plants of the gardener,
A very large category of plants have become adapted to living in the deep recesses of swamps and jungles, and in underbrush, where the direct rays of the sun never penetrate. These plants must carry on the synthesis of food material by the aid of the diffuse light which reaches them, and stand a little danger from its over-intensity. Still another group is found upon the higher slopes of mountains in regions of low air temperatures. In both instances these plants need all the energy they may be able to derive from the light which falls upon them. They are not able by means of their chlorophyll to absorb all this light, and some of it would ordinarily be transmitted through the leaf without advantage to the plant. As an adaptation to this condition, their leaves are provided with layers of coloring matter, which are placed near the lower surfaces in such manner that any light passing the chlorophyll will be absorbed and converted into heat. It is noticeable that plants growing in the swamps and on the mountain tops are not provided with layers of coloring matter, if the leaves are arranged in such manner that light passing the upper leaves will fall on those underneath. The heat-saving color screen is not needed in this instance, but is present most frequently in leaves which form a low, simple rosette, or which lie close to the ground. That the presence of anthocyan in flowers is also often for the purpose of converting light into heat seems well authenticated, from the number of plants which bear colorless petals or glumes at ordinary temperatures, yet develop color in these organs at lower temperatures at the beginning or end of the season, or at higher altitudes. That these bright colors in living plants do convert light rays into heat follows as a conclusion of the following experiment devised by Kny: Three similar glass vessels with parallel walls were filled with distilled water. In one vessel a number of green leaves of canna were placed, and in another such number as to offer the same amount of surface as those in the first, but which contain a large amount of anthocyan. The third vessel is left unchanged, and all are placed in sunlight of equal intensity. A certain rise in temperature naturally ensues in the water in the third vessel; a greater rise occurs in the first, showing that chlorophyll converts a portion of the light into heat, while the greatest increase takes place in the second, where, in addition to the action of the chlorophyll, the converting power of the anthocyan is exerted. The difference between the temperature of the vessels containing the green and red leaves often amounts to 4° C, which is due entirely to the action of the anthocyan.
It is often necessary for the plant to transport complex food substances from one portion of the vegetative tract to another along conduits which lie near the surface. On such compounds, as well as on chlorophyll, the blue-violet rays (see Fig. 2) exercise a disintegrating effect. In quite a large number of plants, the lines of vessels in stalks, midribs, and petioles of leaves are shielded from the direct action of such rays by means of external layers or bands of anthocyan, of some shade of red or purple. It is possible in some instances to trace the line of the conducting vessels by the lines of color appearing near the surface. The direct connection between the food substances and the presence of the coloring matter is strongly indicated by the example detailed by Kerner, in which the pearly-white rhizome of Dentaria when taken from the soil and exposed to the light will become a deep violet in a few hours. Whether the connection is a direct one or not, it is also true that many young and rapidly growing shoots exhibit marked reddish or violet colors at a time when reserve food is being conveyed to them in greatest quantity, and when the thin, tender tissues are otherwise so translucent as to allow the sun's rays to strike through them in a manner calculated to work great damage in the complex compounds in the young leaves. When the leaves mature and are not so pervious to light, the colors may disappear. This is well illustrated by the behavior of the young leaves of rhubarb, cherry, and grape. Many instances of this character are known, as well as the fact that storage organs are often provided with coloring layers or shields, when partially exposed to the light under normal conditions. In plants with deciduous leaves, or the shoots which die down to the root stock each year, it is highly important that the material in the protoplasmic structures of the portion dying away should not be entirely lost, as it represents a large outlay of energy. As a matter of fact, in plants of this character the protoplasm, chlorophyll, and other nitrogenous substances are usually broken down and begin to be gradually withdrawn into the surviving portion of the plant about the time of the formation of the first stages of the absciss layer which finally cuts off the leaf stalk, or about the time the activity of the herbaceous shoot begins to slow down. The disintegration of the chlorophyll would leave the leaf almost colorless and translucent, and the sun's rays would strike directly through it, resulting in the total decomposition of the proteids and a consequent waste to the plant, but during the decomposition of the chlorophyll there occurs, as a result or accompaniment of the process, the formation of much brilliant coloring matter of various shades, to which are due the brilliant autumnal tints of deciduous leaves. These coloring matters sustain the same general relation to sunlight as the other colors described above. They generally absorb the entire violet end of the spectrum, which, as has been pointed out, is the one which causes disintegration in the cells, as well as the lower red and infra-red rays. The spectrum of the autumnal red coloring of the leaves of the Virginia creeper (Ampelopsis) is shown in Fig. 1, III. It may be safely asserted that the above-described occurrences of coloring matter are undoubtedly marked factors in the physiology of a large number of plants, without reference to the manner in which such coloring screens and shields have arisen. It is also true, however, that a large number of plants contain coloring matters in the interior of organs or disposed in such other manner that they could sustain no possible relation either to light or to animals furnished with a color sense; still, many other occurrences of color are to be noted in which a physiological function is quite possible but is not proved. Among the latter is the color formation which ensues in evergreen leaves on the approach of a cold season, or in other leaves on the approach of a dry season. It must be admitted that in some instances physiologists have been led to conclusions concerning the use of colors quite as little justified as many of those reached by enthusiastic students of "adaptation to insect visitors." It is now somewhat generally admitted that color substances must very often be regarded as simply by-products in the chemical processes carried on by plants; a view which is undeniably valid of color masses in the interior of underground roots or tubers, or massive aerial organs, and also in a large number of instances in flowers. This latter application is further justified by the fact that some flower colors may change during the season without any relation to light conditions, insect visitors, or other ecologic factors. It is quite within the range of possibility that color masses in aerial organs bear an important modifying relation to the forms of irritability to radiant energy acquired by the plant.
As a summation of the foregoing, it may be stated that coloring matters stand in the following relations to the plants:
1. Chlorophyll converts light into energy by the aid of which protoplasm containing it is able to build up foods from carbon dioxide and water.
6. Colors serve as an attractive, guiding, or warning device for insects and other animals, more especially for the purpose of securing cross-fertilization and protection from injury.