Abderhalden, E.—"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze, Kautschuk," 822 pages, Berlin, 1912.Allen'sCommercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).Cook, M. T. andTaubenhaus, J. J.—"The Toxicity of Tannin," Delaware College Agricultural Experiment StationBulletinNo. 91, 77 pages, 43 figs., Newark, Del., 1911.Dekker, J.—"Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.Gore, H. C.—"Experiments on the Processing of Persimmons to Render them Nonastringent," U. S. Department of Agriculture, Bureau of ChemistryBulletinNo. 141, 31 pages, 3 plates, 1911; and No. 155, 20 pages, 1912.Lloyd, F. E.—"The Tannin-Colloid Complexes in the Fruit of the Persimmon,Diospyros," inBiochemical Bulletin, Vol. 1, No. 1, pages 7 to 41, 34 figs., New York, 1911.
Abderhalden, E.—"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze, Kautschuk," 822 pages, Berlin, 1912.
Allen'sCommercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).
Cook, M. T. andTaubenhaus, J. J.—"The Toxicity of Tannin," Delaware College Agricultural Experiment StationBulletinNo. 91, 77 pages, 43 figs., Newark, Del., 1911.
Dekker, J.—"Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.
Gore, H. C.—"Experiments on the Processing of Persimmons to Render them Nonastringent," U. S. Department of Agriculture, Bureau of ChemistryBulletinNo. 141, 31 pages, 3 plates, 1911; and No. 155, 20 pages, 1912.
Lloyd, F. E.—"The Tannin-Colloid Complexes in the Fruit of the Persimmon,Diospyros," inBiochemical Bulletin, Vol. 1, No. 1, pages 7 to 41, 34 figs., New York, 1911.
Practically all plant structures contain pigments. These may be considered as of two types: (a) the vegetative pigments, which have a definite energy-absorbing rôle in the metabolic processes of the tissues which contain them, and (b) the ornamental pigments. It is probable that the same chemical compound may serve in either one of these capacities under different conditions, but, in general, it is possible to assign either a definite vegetative, or physiological, use, or else a simple ornamental, or biological, significance to each of the common pigments. The first type is found widely distributed through the protoplasm, or cell-sap, of the plant structures; while the ornamental pigments are located chiefly in the epidermal cells, especially of flowers.
With respect to their colors, the plant pigments may be grouped as follows:
Green—the chlorophylls.Yellow—the carotinoids, flavones, and xanthones.Red—phycoerythrin, lycopersicin, anthocyanin.Blue—anthocyan derivatives.Brown—phycophæin, fucoxanthin.
Green—the chlorophylls.Yellow—the carotinoids, flavones, and xanthones.Red—phycoerythrin, lycopersicin, anthocyanin.Blue—anthocyan derivatives.Brown—phycophæin, fucoxanthin.
Of these, the chlorophylls, the carotinoids, phycoerythrin (in red sea-weeds) and phycophæin (in brown sea-weeds) are generally vegetative pigments; while the others form the basis for most of the ornamental pigments, although they may have a definite energy-absorbing effect, in some cases.
The importance of the green coloring matter in plants has been understood for more than a century, its connection withphotosynthesis having been known as far back as 1819. But definite knowledge as to its chemical constitution is of very recent origin. As recently as 1908, it was asserted that chlorophyll is a lecithin-like body, yielding choline and glycero-phosphoric acid on hydrolysis. It is now known, however, that chlorophyll contains neither choline nor phosphorus, the earlier observations being due to mixtures of various other materials with the true chlorophyll in the extracts which were examined. Beginning with 1912, Willstätter and his collaborators, in a series of classic papers which were finally collected in book form, clearly demonstrated the chemical constitution of the green pigments of plants, which had been previously designated under the single name "chlorophyll." In 1912, Willstätter and Isler first showed that the green coloring matter which is extracted from plants by alcohol, ether, etc., is made up of two definite chemical compounds, to which they assigned the names "chlorophylla" and "chlorophyllb," associated with two yellow pigments, carotin and xanthophyll, and, in some cases, with the reddish-brown fucoxanthin. The percentages of total pigment materials, and the relative proportions of the five different pigments, in several types of plants, are as follows:
The two chlorophylls have the following formulas: chlorophylla, C55H72O5N4Mg, and chlorophyllb, C55H70O6N4Mg. Hence, they differ only in having two hydrogen atoms in the one replaced by one oxygen atom in the other. Both are amorphous powders, from which crystalline chlorophyll (see below) can be obtained by hydrolysis. Chlorophyllais blue-black, is easily soluble in most organic solvents, and when saponified by alcoholic potash gives atransient pure yellow color. Chlorophyllbis dark green, is somewhat less soluble than the other form, and when saponified by potash gives a transient brilliant red.
Amorphous and Crystalline Chlorophyll.—When the chlorophyll of plants is extracted by alcohol and the alcoholic extract evaporated nearly to dryness, beautiful dark green crystals are obtained. Willstätter has shown, however, that in these crystallized forms the ethyl group (from the ethyl alcohol used) has replaced the phytyl group (see below) which is present in the pigments as they exist in the plant tissues; and that, when extracted by other solvents than alcohol, the pigments may be obtained in the amorphous forms in which they exist in the plant.
This change from amorphous to crystalline compounds may be understood from the preliminary statement that the chlorophylls are esters of tri-basic acids, in which one acid hydrogen is replaced by the methyl (CH3) group and a second by the phytyl (C20H39, from phytol, or phytyl alcohol, C20H39OH) group. When treated with ethyl alcohol (C2H5OH) for the purpose of extracting the pigments, the ethyl (C2H5) group replaces the phytyl group, thus yielding a methyl-ethyl ester, and these esters are the crystalline forms of the chlorophylls. This replacement is made possible through the action on the original pigment in the tissues of an enzyme,chlorophyllase, which is also present in the tissues, which splits off the phytyl group, forming phytyl alcohol, and leaving a free COOH group in the pigment, with which the alcohol used in the extraction forms the ethyl ester (seeChapter IXfor a discussion of the formation and hydrolysis of esters).
While the chlorophylls are tri-basic acids, only two of the acid COOH groups actually function in ester-formation. The third acid group seems not to exist as a free acid group; but in chlorophylla, it is in what is known as the "lactam" arrangement, represented by the —CONH— group, and in chlorophyllb, it is probably in the "lactone" arrangement, represented by the —COO— group; the two bonds in each case being attached to different structural units in the molecule (seepage 106).
The change from amorphous to crystalline forms may be represented by the following formulas, in which the R represents the whole of the complex group to which the acid ester groups are united:
"Chlorophyllin," the compound in which the ester groups have been converted into free acid groups, as indicated above, may be obtained from either amorphous or crystalline chlorophyll by treatment with caustic potash dissolved in methyl alcohol.
Phytol.—This alcohol, which furnishes the characteristic ester group in the chlorophyll of plants, is a compound of very unusual composition, which has never been found in any other form or in any other type of compound which is present in either plant or animal tissues. Careful studies of its addition and oxidation products prove that it has the following structural arrangement:
As this formula indicates, the compound contains one unsaturated, double-bond linkage, one primary alcohol group, and eleven methyl groups. As has been said, this alcohol occurs nowhere else in nature, and its presence and function in the chlorophyll molecule are, as yet, wholly unexplainable. Phytol itself is a colorless, oily liquid, with a high boiling point (145° in vacuo, 204° at 10 mm. pressure).
As has been mentioned, chlorophylladiffers from chlorophyllbby having one more oxygen and two less hydrogen atoms in the molecule, and in having one of its nitrogen atoms in the "lactam" arrangement. These differences in structure are represented by the following formulas which are commonly used to represent the two compounds, but which do not show the arrangements of the major groups of the complex molecules:
The chlorophylls are unstable compounds, readily acted upon by acids or alkalies, and by the enzyme chlorophyllase, which splits off the phytyl alcohol group. The progressive action of acids and of alkalies in breaking down the molecule, and the products of its oxidation and reduction, have served to establish the chemical composition of the compound in each case. Because of the importance of these pigments in the whole metabolic processes of the plant, it seems to be desirable to consider the nature of these reactions in some detail, as follows:
Decomposition of the Chlorophylls by Alkalies.—The first action of dilute alkalies on the chlorophylls is to split off, by hydrolysis, the alcoholic groups of the esters, producing the crystalline tri-basic acids, orchlorophyllins aandb. Each of these chlorophyllins exists in two forms, the normal and the iso, in which the attachment of the COOH groups to the other groups in the molecule is in different positions. Hence, chlorophyllayields chlorophyllinaand isochlorophyllina, and chlorophyllbyields chlorophyllinband isochlorophyllinb, all four of which are tri-basic acids.
These compounds, when heated with alkalies, split off carbon dioxide in successive stages, losing one COOH group at each step, thus yielding a series of simpler compounds of the following types: First, di-basic acids; second, monobasic acids; and finally,ætiophyllin, a compound in which no COOH group is present. In all of these compounds, derived from chlorophylls by the action of alkalies, the Mg remains in the molecule, and all the Mg-containing derivatives from the chlorophylls are known as "phyllins." At the stage at which only one COOH group remains in the molecule, only one group arrangement is possible, and the derivatives from chlorophyllinaand isochlorophyllinb, and those from chlorophyllinband isochlorophyllina, are identical. At the final stage, the derivatives from all four forms are identical. This may be graphically illustrated by the following diagram indicating the progressive decomposition of the two chlorophylls under the action of alkalies:
Decomposition of Chlorophylls by Acids.—The first action of dilute acids upon chlorophylls is to remove the magnesium, without otherwise changing the molecule. Two hydrogens go in in the place of the magnesium. Dilute acids act in precisely the same way upon each of the "phyllins" shown in the above scheme. In this way, a whole series of compounds, corresponding to each of the chlorophylls and their alkali-decomposition products, but with the magnesium lacking in each case, has been prepared. Thus,
Similarly,
Isochlorophyllina, becomes Phytochlorine, Chlorophyllina, becomes Phytochlorinf, andg,
Isochlorophyllinb, becomes PhytorhodingChlorophyllinb, becomes Phytorhodiniandk,
And bodies known as "porphyrins" are similarly derived from all the other known phyllins.
For example: cyanophyllin, MgC31H32N4(COOH)2, becomes cyanoporphyrin, C31H34N4(COOH)2; ætiophyllin, MgC31H34N4, becomes ætioporphyrin, C31H36N4, etc.
Phytochlorineand phytorhodingare the chief products of the decomposition by acids of the chlorophylls. Indeed, it was the production of these compounds which led to the discovery of the existence of the two chlorophylls. When treated with alkalies, they lose their carboxyl groups and become ætioporphyrin.
Decomposition of the Chlorophylls by Oxidation and Reduction.—When acted upon by oxidizing agents, such as chromic acid, the porphyrins yield two chief oxidation products, which are pyrrole derivatives having the following formulas,
By reduction, there have been obtained from the chlorophylls and the various porphyrins, three isomeric pyrrole derivatives having the following formulas,
As a result of the study of these decomposition units, Willstätter has suggested the following formulas for the structural arrangement of ætiophyllin and ætioporphyrin, the compounds which result from the removal of all of the acid groups and finally of the magnesium from the chlorophylls,
The COOH groups which are attached to these compounds to form the various phyllins and porphyrins, as well as the original chlorophylls, are supposed to be attached to the C2H5groups in the above formulas, the different modifications, or compounds, depending upon the position in which one or more of these attachments are made.
It seems to be desirable, at this point, to call attention to the remarkable similarity in the chemical composition of chlorophyll, the most important pigment of plants, and hæmoglobin, the all-important respiration-regulating pigment in the blood of animals.Hæmoglobin is a complex compound, consisting of about 96 per cent of albumin (a protein, seeChapter XIII) united with about 4 per cent ofhæmatin, a brilliant red pigment which has the formula FeClC32H32O4N4. When treated with acids, the iron (and its accompanying Cl) is removed, and hæmatoporphyrin, C32H36O4N4, is obtained. When either hæmatin, or hæmatoporphyrin is oxidized, hæmatinic acid imide identical with that obtained fromætioporphyrinis obtained. Also, when hæmatoporphyrin is reduced, hæmopyrrole identical with that fromætioporphyrinis obtained. Thus, it would appear that the unit structural groups in hæmatin and in chlorophyll are identical; although chlorophyll may exhibit more variations in isomeric arrangement of these structural units than have been found in hæmatin. Hence, it is apparent that the only essential difference in composition between chlorophyll and hæmatin is that in the former the structural units are linked together by iron, while in the latter, the same units are united through magnesium as the linking element. Further, it is known that while iron is not a constituent element in the chlorophyll molecule, it is, in some unknown way, absolutely essential to the production of chlorophyll in plants; plants furnished with an iron-free nutrient solution rapidly become etiolated and photosynthesis stops.
The following skeleton formulas have been suggested to indicate the way in which these elements are linked between the structural units in their respective compounds.
It is understood, of course, that the mineral element does not furnish the definite means of holding the structural units together as otherwise it would not be possible to remove the iron, or magnesium, without breaking down the molecule, as is done in the case of the porphyrins. The actual binding linkage is undoubtedly between carbon atoms, as indicated in Willstätter's formulas for ætiophyllin and ætioporphyrin (seepage 109). The attachment of the magnesium to each one of the four nitrogen atoms in the skeleton formula assumes the existence of subsidiary valences of 2-4 for magnesium (and of 3-5 for iron), or of possibleoscillatingvalences similar to those supposed to be exhibited by carbon in its closed-ring arrangements.
The phytyl esters, or natural chlorophylls, are amorphous solids; while the methylethyl esters (chlorophyllins) and the free acids (phyllins) are crystalline compounds. All of these compounds are easily soluble in ether and alcohol, but insoluble in water. The chlorophylls and chlorophyllins are practically insoluble in petroleum ether and chloroform; but the monobasic acids (pyrrophyllin and phyllophyllin) and the neutral ætiophyllin dissolve easily in chloroform.
Solutions of the chlorophylls arefluorescent,being green by transmitted, and red byreflectedlight.
Chlorophyllais a blue-black solid, which gives dark green solutions in all of its solvents. Chlorophyllbis a dark-green solid, which yields brilliant green solutions. Solutions in ether of glaucophyllin and of cyanophyllin are blue; of rhodophyllin, deep violet; of rubiphyllin, light violet; of erythrophyllin, red; and of pyrrophyllin and phyllophyllin, bluish-red. Solutions of the porphyrins are all red, the di-basic ones being usually a bluish-red, and the simpler ones a brilliant red to deep brownish-red in color.
The several chlorophyll derivatives are further distinguished by characteristic differences in their absorption spectra. These differences have been pictured by Willstätter in his book dealing with the results of his investigations concerning the chlorophylls, and reproduced in one or two other texts which treat in detail with the physical-chemical properties of thesepigments,but need not be presented in such detail here.
The characteristic brilliant green of healthy plant tissues is due to the fact that there are always associated with the dark bluish-green chlorophylls two (or more) yellow pigments. Theseare known as the "carotinoids." This group includes the two brilliant yellow pigments, carotin and xanthophyll, and the reddish brown fucoxanthin and the brilliant red lycopersicin, which are similar in their chemical composition. The first two are found universally distributed in plants, associated with the chlorophylls, and may be regarded as vegetative pigments, although the characteristic ornamental yellow and orange colors of many flowers and fruits, as well as that of the roots of carrots,etc.,due to thesepigments.
Carotin.—This pigment occurs in various forms in plants, both amorphous and crystalline. It crystallizes out of solution in flat plates, which are orange-red by transmitted light, and greenish-blue by reflected light, and have a melting point of 168°. Carotin is insoluble in water, only very slightly soluble in acetone or cold alcohol, readily soluble in petroleum ether, ether, chloroform, and carbon disulfide. Its solutions are strongly fluorescent.
Its molecular formula is C40H56. It is, therefore, a hydrocarbon of a very high degree of unsaturation. On exposure to dry air, it absorbs 34.3 per cent of its own weight of oxygen, which corresponds to 11-1/2 atoms of oxygen, computed on the basis of the molecular formula C40H56, and would indicate a formula of (C40H56)2O23for the oxygenated compound; this being three oxygen atoms less than would be required to bring the compound to the theoretical stage of saturation represented by the unimolecular formula CnH2n+2. In moist air, two more oxygen atoms are absorbed, probably forming two OH groups in the molecule. Moreover, carotin absorbs iodine. When the calculated amount of iodine is used, a definite compound having the formula C40H56I2is produced; but in the presence of an excess of iodine another compound having the apparent formula C40H56I3(or 2C40H56I2+I2) is obtained. (Note that 2 atoms of iodine plus 12 atoms of oxygen, or 3 of iodine plus 11-1/2 of oxygen, produce the degree of saturation required by the formula CnH2n+2.) It is evident from these experimental data, that a part of the unsaturated linkage in the carotin molecule is of a type which can easily be saturated by direct addition of oxygen, while the remainder may be saturated by iodine.
The reaction of carotin toward bromine is peculiar. With this element, it forms a compound having the formulaC40H36Br22, indicating the direct addition of two atoms of bromine and the substitution of twenty atoms of this element for the same number of hydrogen atoms.
The oxygenated carotins are colorless substances, while the iodide crystallizes in beautiful dark-violet prisms, having a coppery red fluorescence.
Xanthophyllis closely related to carotin. It has the molecular formula C40H56O2. It absorbs 36.55 per cent of oxygen (corresponding to 13 atoms, which would indicate the formation of two OH groupsinaddition to the saturation required by the CnH2n+2formula); and an iodine addition product having the formula C40H56O2I2, which crystallizes in dark-violet needles.
Xanthophyll differs markedly from carotin in its solubilities, being insoluble in petroleum ether and only sparingly soluble in carbon disulfide. It may be fairly easily reduced to carotin. This transformation is reversible, and suggests a similarity to the change from hæmoglobin to oxyhæmoglobin, and the reverse, in the blood of animals, as a part of their respiration process.
Separation of the Chlorophylls, Carotin, and Xanthophyll.—These pigments, which exist together in most plant tissues, may easily be separated from each other by taking advantage of the differences in their solubilities, according to the following procedure. Grind up a small quantity of the fresh tissue (leaves of the stinging nettle furnish a conveniently large supply of each of these pigments) with fine sand in a mortar. Cover with acetone, let stand a few moments and then filter on aBüchnerfunnel. Pour the filtrate into a separatory funnel, add an equal volume of ether and two volumes of water. Shake up once and then allow the ether layer to separate; the pigments will be in this layer. Drain off the water-acetone layer. Now to the etherial solution, add about half its volume of a concentrated solution of potassium hydroxide in methyl alcohol. Shake well and allow to stand until the mixture becomes permanently green. Now add an equal volume of water and a little more ether, until the mixture separates sharply into two layers. The chlorophylls will now be in the lower dilute alcohol layer, and the carotinoids in the upper ether, and may be separated by draining of each layer separately. To separate the carotin from xanthophyll place the ether solution in a small open dish and evaporate to a small volume. Now add about ten volumes of petroleum spirit and an equal volume of methyl alcohol, stir up well, transfer to a separatory funnel andallow the two layers to separate. The carotin will now be in the upper layer of petroleum ether, and the xanthophyll in the lower alcohol layer; these layers may be drained off separately and the solvents evaporated in order to recover the pigments in dry form.
Lycopersicin(or lycopin) is a hydrocarbon pigment having the same formula as carotin. It is, however, brilliantly red in color, and crystallizes in a different form and has a different adsorption spectrum from carotin. It is the characteristic pigment of red tomatoes, and is found also in red peppers. Yellow tomatoes have only carotin as their skin-pigment, while lycopersicin is usually present in the flesh of the ripe fruits of all varieties and in the skin of red ones. It has been shown, however, that if varieties of tomatoes which are normally red when ripe, are ripened at high temperatures, 90° F. or above, their skins will be yellow instead of red when fully ripe. Hence, the occurrence of carotin, or of lycopersicin, as the skin pigment is determined in part by the varietal character (being different in different varieties when ripened at normal temperatures) and in part by the temperature at which the fruit ripens. The two pigments are, of course, isomers; but the difference in their structural arrangement is not known.
Fucoxanthin, C40H54O6, is a brownish-red pigment, found in fresh brown algæ, and in some brown sea-weeds. Its formula indicates that it is an oxidized carotin. With iodine, it forms a compound having the formula C40H54O6I4. It is unlike carotin and xanthophyll in that it has basic properties, forming salts with acids, which are blue in color.
These are the principal pigments of red and brown seaweeds, respectively. Their most characteristic difference from the pigments of non-aquatic plants is that they are easily soluble in water, and insoluble in most organic solvents, such as alcohol, ether, etc. At first thought, this would appear to be impossible, since the plants grow in water and it would seem that their water-soluble pigments would be continuously dissolved out of the tissues. The reason why this does not occur lies in the fact that these pigments exist in the cells of the seaweeds in colloidalform (seeChapter XV), and, hence, cannot diffuse out through the cell-wails. The only way in which they can be extracted from the tissues is by rupturing the cells, by grinding with sharp sand, etc., after which the pigments can readily be dissolved out by water.
Phycoerythrinis the red pigment. It is a colloidal, nitrogenous substance, allied to the proteins (seeChapter XIII) but not a true protein compound. Hydrolysis by acids indicates that it containsleucineandtyrosine, two amino-acids which are constituents of proteins, along with other bodies of unknown composition.
The colloidal solution of phycoerythrin in water has a brilliant rose-red color, with an orange fluorescence. It readily sets to a gel (seeChapter XV), so that the solution is almost impossible to filter. On this account, purified solutions of this pigment are very difficult to secure, and no satisfactory analysis to indicate its composition has yet been obtained.
Actinically, it is a complementary pigment to chlorophyll, that is, it absorbs the blue and green rays and permits the passage of light which is of the wave length that is absorbed by chlorophyll.
Phycophæin.—Still less is known of the composition of this pigment than of that of phycoerythrin. It is the characteristic pigment of brown seaweeds. It is supposed to exist in the cells of algæ, chiefly as a colorless chromogen, which becomes first yellow and then brown on exposure to air. Associated with it are other pigments, which have been variously reported as carotin, phycoxanthin, etc.
These are a group of pigments of red, blue, or violet color, which occur in the flowers, fruits, or leaves of many species of plants. They are essentially ornamental pigments, and constitute a large proportion of the brilliant colors of flowers, etc. They occur not only dissolved in the cell-sap, but also as deposits of definite crystals or amorphous compounds in the cell protoplasm.
They are all glucosides. When the anthocyans are hydrolyzed, the sugar molecules are split off and the characteristic hydroxy-derivatives of the three-ring anthocyan nucleus (figured onpage 83),known as "anthocyanidins," remain. These anthocyanidins are themselves pigments. They have been shown to be all derivatives of the anthocyan nucleus. The oxygen atom in this nucleus is very strongly basic and exhibits its quadrivalent property by forming stable salts by direct addition of acid radicles. The variation of color of the anthocyanins has been explained by Willstätter, as follows; the red is the acid salt, the blue is a neutral metallic salt, and the violet is the anhydride of the anthocyanidin in question, thus
All of the natural anthocyanin pigments appear to contain a chlorine atom attached directly to the ring oxygen, as shown in the above partial formulas. In addition, they have four, five, or six hydroxyl (OH), or methoxy (OCH3), groups attached at various points around the three rings. The following formula forœnidin, one of the most complex of these anthocyanidins, will illustrate their structural arrangement.
Delphinidinis the corresponding compound without the two CH3groups; whilecyanidincontains only five OH groups; andpelargonidin, only four OH groups.
The anthocyanin pigments are soluble in water, alcohol, and ether, the solutions being red or blue in color according to the acidity or alkalinity of the medium. Their presence in many species of plants is hereditable, as these plants come true to color from seed, as in the case of red beets, red cabbage, several species of blue berries, etc. In other cases, the anthocyanin development depends largely upon the conditions of growth, particularly those which prevail during the later stages of development: as in the case of apples, where the amount of red color in the skin depends to a large extent upon the conditions under which the fruit ripens.
Anthocyanin pigments often make their appearance late in the season; in fruits, etc., as the result of the normal ripening process but in leaves as the result of shorter daylight illumination accentuated also by sharp frosts.
The yellow plant pigments, other than the carotinoids, are almost without exception glucosides having a xanthone or flavone nucleus. These typical nuclei are illustrated onpage 83. In these nuclei, as in the anthocyan one, the oxygen atom is strongly basic and combines with mineral acids to form salts (the oxygen becoming quadrivalent) and the color of the pigment depending upon the nature of the combination formed in this way.
The anthoxanthin pigments are yellow, crystalline solids, which are only slightly soluble in water. They dissolve readily in dilute acids and alkalies, giving yellow or red solutions which are of the same color in either acid or alkaline media. They are extensively used as yellow dyes.
Many of the common members of this group have been mentioned in the chapter dealing with the glucosides. The characteristic pigment nucleus of several of these is as follows:
Chrysin, found in various species of poplar and mallows,
Chrysin
Apigenin, found in parsley and celery, as the glucoside apiin,
Apigenin
Campferol, found in Java indigo, as the glucoside campferitrin,
Campferol
Fisetin, found in quebracho wood and fiset wood,
Fisetin
Quercitrin, found in oak bark, horse-chestnut flowers, and in the skin of onions,
Quercitin
Morin, found in yellow wood (Morus tinctoria).
Morin
Gentisin, found in yellow gentian (Gentiana lutea),
Gentisin
As a rule, the most brilliant of these yellow pigments are found in the largest quantities in the bark and wood of various species of tropical plants; although they are also present, in smaller amounts, in the blossoms of species growing in temperate zones.
The anthoxanthins are easily converted into anthocyanins, andvice versa, by the action of oxidizing and reducing enzymes which are commonly present in the tissues of the plants which develop the pigments.
The breeding of flowering plants having blossoms of almost any desired color has become a commercial enterprise of large importance. The results which have been obtained, in many cases, have been made the object of scientific study of the genetics of color inheritance. These studies have developed certain interesting facts with reference to the chemistry of the development of these ornamental pigments, which may be briefly mentioned here.
In many of the plants which have been studied, the color of the flowers depends upon several different factors, as follows:
C, a chromogen (or color-producing substance) which is generally a flavone or xanthone glucoside, and which may be either yellow or colorless.
E, an enzyme which acts uponC, to produce a red pigment.
e, another enzyme which acts upon the red pigment, changing it to some other anthocyanin color.
A, an antioxidase, or antienzyme, which prevents the action ofE.
R, an enzyme which changes reds to yellows.
Thus, if a plant whose flower contains only the factorCbe crossed with one which contains the factorE, a red blossom will result, or if it contains the factoremore intense pigments are developed. But if eitherAorRare present, no change in the color of the original parents will result from the crossing.
The vegetative pigments undoubtedly serve as agencies for regulating the rate of metabolic processes. At the same time, it is extremely difficult to determine whether the presence of a pigment in any given case is the cause or the effect of the changes in the plant's activities which result from changes in its external environment.
The chlorophylls are, of course, the regulator of photosynthesis, absorbing solar energy with which the photosynthetic process may be brought about. The simultaneous presence of carotinoids in varying amounts undoubtedly serves to modify the amount and character of the radiant energy absorbed, as these pigments absorb a different part of the spectrum of light and hence undoubtedly produce a different chemical activity or "actinic effect" of the absorbed energy. The variations in depth of color of foliage during different growing conditions, from a pale yellow when conditions are unfavorable and growth is slow to the rich dark green of more favorable conditions, is a familiar phenomenon. Whether this change in pigmentation is the result of an adjustment of the plant protoplasm, so that it can absorb a more highly actinic portion of the light, or is a direct effect of the lack of conditions favorable to chlorophyll-production and active photosynthesis, has not yet been determined.
But there must be some influence other than response to environmental conditions which controls the vegetative color in plants, since shrubs, or trees, which have green, yellow, red, and purple leaves, respectively, will grow normally, side by side, underidentical external conditions of sunlight, moisture supply, etc. The hereditary influence must completely overshadow the apparent normal self-adjustment of pigment to energy-absorbing needs, in all such cases.
Again, it appears that there is some definite connection between pigment content and respiration. It is known, of course, that the gaseous exchanges involved in animal respiration are accomplished through the reversible change of hæmoglobin to oxyhæmoglobin, these being the characteristic blood pigments. The easy change of carotin, C40H56, to xanthophyll, C40H56O2, andvice versa, and the reversible changes of the yellow anthoxanthins to the red anthocyanins, under the influence of the oxidizing and reducing enzymes which are universally present in plants, would indicate the possibility of the service of these pigments as carriers of oxygen for respiratory activities in plants in a way similar to that in which the blood pigments serve this purpose in the animal body. The fact, which has been observed in connection with the experimental studies of the development of the lycopersicin, that tomatoes which normally would become red remain yellow in the absence of oxygen, indicates that this pigmentation, at least, is definitely connected with oxygen supply; and the further fact that the development of lycopersicin in red tomatoes, red peppers, etc., is dependent upon the temperature at which the fruit ripens, may indicate a definite connection of this pigment with the need for more oxygen (or for more heat, as suggested in the following paragraph) at these lower temperatures.
Again, many investigators have concluded that at least one function of the anthocyanin pigments is to absorb heat rays and so to increase transpiration and other chemical changes. In support of this view, there may be cited the general presence of such pigments in arctic plants, their appearance in the leaves of many deciduous trees after a frost in the fall, etc. Indeed, there is much to support the view that the autumnal changes in foliage pigments have the physiological function of absorbing heat in order to hasten the metabolic processes of ripening and preparation for winter defoliation. The rapid and brilliant changes in foliage coloring after a sharp frost which kills the tissues and makes rapid translocation of the food material of the leaves to the storage organs immediately necessary, have been explained as theresponse of the pigmentation of the leaves to the need for increased heat-absorption. On the other hand, the red pigments of the beet-root, etc., which seem to be identical in composition with the other anthocyanin pigments, can have no such function as those which have just been described. Furthermore, the fact that the pigment often varies in color from red to yellow or brown, depending upon the temperature under which the tissue is ripening, makes it an open question whether the pigment is the regulating agency or whether its nature is the result of the environmental conditions. Or, in other words, it is a question whether these changes in color are a mechanism by which the plant cell adjusts its absorptive powers, or whether they are only the inevitable result of the changes in temperature upon a pigment material which is present in the cell for an entirely different use.
A very interesting side-light upon the color changes which many species of plants undergo when the external temperature falls has been shown by the investigations of the relation of the sugar content of the plant tissues to their pigmentation. It is a well-known fact that not only do many species of deciduous plants show the characteristic reddening of their leaves after frost in the autumn but also many evergreens (Ligustrum,Hedera,Mahonia, etc.) exhibit a marked reddening, or purpling, of their foliage during the winter months, with a return to the normal green color in the spring. Earlier investigations, which have been confirmed by several repetitions, showed that the red or purple leaves always contain higher percentages of sugar than do green ones of similar types. More recent studies have shown that artificial feeding of some species of plants with abnormally large portions of soluble sugars produces a reddening of the foliage tissues which is apparently identical with that which these tissues undergo as the result of low temperatures. Thus, the connection between the natural winter reddening of foliage and the development of sugar in the tissues during periods of low temperatures (seepage 64) seems to be clearly demonstrated. It appears that at least a part of the seasonal changes in color of plants is either the cause of, or the effect of, variations in sugar content of the tissues of the plants, accompanying the changes in external temperatures.
Oftentimes, the anthocyanin pigments seem to be associated with sugar production, as contrasted with the chlorophylls, whichseem to be more favorable to the production of starch. But in this case also, it is impossible to say whether the pigment is the direct causative agent in the type of carbohydrate production or whether it is the effect of the same external factors which determine, or modify, the character of the carbohydrate condensation.
The ornamental pigments undoubtedly have definite biological significance. When present in the storage roots, such as beet-roots, carrots, etc., or in the above-ground parts of plants, they may have served to protect these organs against herbivorous animals which were accustomed to consume green foods.
In flowers, the brilliant ornamental pigments undoubtedly serve to attract the insects which visit these blossoms in search of nectar, and in so doing promote cross-fertilization. Recent experiments have demonstrated that colors are much more efficient than odors in attracting insects.
Taken altogether, it is apparent that the pigments may have a variety of important rôles in plants. At the same time, some of them may be waste products, with no definite use in the plant economy.