Popular Science Monthly/Volume 5/June 1874/Have Plants a Pedigree?
By Professor W. D. GUNNING.
WE have lately been reminded, by a writer in the North American Review, that the builders are the metaphysicians, and that science is only a brick-yard. How many, then, would quit the service? No! wiser we think were the words of the late Cambridge professor: "We have reached the point where the results of science touch the very problem of existence, and all thoughtful men are listening for the verdict which solves the great mystery." There is a great truth here which people are beginning to recognize, and there can be little doubt that the strongest hold which Science now has upon the public mind, comes from the light she is supposed to be able to throw on "the problem of existence." The vegetable kingdom has many aspects of interest, but Science has now raised the profound inquiry how it came to be—that is, the problem of its existence. We propose to set forth in this paper part of the testimony which plants give on the question of their origin.
In the floral world that which first catches the attention is color, and, although in classification it passes for almost nothing, no other property or quality affects the mind so deeply. The great poets, who are always wiser than they know, hate literally painted the vices and virtues. Anger is scarlet—
"The ashy paleness of my cheek
Is scarleted in ruddy flakes of wrath."
Jealousy is yellow; modesty is crimson; melancholy is blue; cheerfulness, hopefulness, youth, are green; malice, vengefulness, falsehood, are black, and truth and purity have never been painted to the imagination more simply and beautifully than by Shakespeare in this:
and by Milton in this:
"The saintliest veil
If two colors are placed in juxtaposition, to produce the most pleasing effect they should be complementary, as red and green, orange and blue, yellow and purple. What, now, is Nature's method among her flowers?
He who has seen the calypso will remember it as the rarest of color-gems. The petals are of brilliant purple, the lip, deep within of gorgeous yellow, shades off into purple inoculated with darker purple. This floral gem is painted in two colors complementary to each other.
From calypso, which is rare, you pass to a flower of the same family which is not uncommon, the showy lady's-slipper (Cypripedium spectabile). In this we have three colors. The petals are snow-white, the lip is lustrous white melting into magenta, which in turn deepens into purple, and the sterile stamen, which mimics a petal and dips into the sac formed by the inflated lip, is pale yellow. The white of our lady's-slipper is the purple and yellow blended together; and we shall find, in general, that when a flower has three colors, two of which are complementary, the third will be white, representing the blending of the other two.
The wild-asters, like the calypso, are of two colors, the color of the ray being, in general, complementary to that of the disk, and thus the most common of our autumnal flowers are pleasing to the eye. The rose and English hawthorn are of one color, which harmonizes with the foliage, as red is the complement of green. But Nature has another side.
The corolla of the closed gentian, which is set in a green calyx, is deep blue. Here is chromatic discord. The "lilac"-colored flowers of the lilac, in contrast with the green leaves, form another discord. No lady would think of dressing in lilac and green.
Our buttercups and golden-rods are yellow. In the golden-rod neither the yellow of the flower nor the green of the foliage is strongly marked, and the contrast is not displeasing. But the bright yellow of the buttercup against the fresh green of the leaves and the spring grass makes a chromatic discord. Green and yellow are not in accord, and Shakespeare, taking green for youth and yellow for jealousy, uses this color-discord with fine effect:
"She never told her love,
Traveling on a June day from Bangor to Boston, we observed all the meadows and waste fields through the shale region of Central Maine overrun with bulbous crowfoot—the Ranunculus bulbosus. The face of the earth over large patches was literally green and yellow, and the chromatic effect began to tell very unpleasantly on the eye. Passing from the shale into a region of granite we found the butter-cups giving place to the white-weed (Leucanthemum vulgare). Over the pastures of Massachusetts this was as common as the buttercup in Maine, and the face of the earth was now white and green. The relief was felt at once. It was now a pleasure to look on the meadows. And yet the pleasure would have been still greater if the leaves and grass had been light blue instead of green, as white and light blue are in still better accord than white and green. But we will not be over-critical. A lady in white can wear a scarf, or sash, or breast-knot, of any color she may wish; and Nature, if she decks herself in white flowers, may set them in green, or brown, or red, or any tint she will, and there will be no discord.
The discords we have considered are between the flower and the foliage. There are others in the flower itself.
In the sweet-pea we have red and violet, a juxtaposition as discordant as that of green and blue. The bird-foot violet has the two upper petals of deep violet, the others of lilac-purple or blue—another discord. And such a collocation produces not merely a chromatic discord. Rays of blue and violet, entering the eye together, cause fluorescence of the cornea and crystalline lens. These parts become faintly luminous by the absorption of such light, and vision is rendered imperfect.
All this may seem fanciful. A field of green and yellow may appear to most eyes as pleasing as one of green and white. A violet, colored in violet and blue, may be called as beautiful as a calypso, in yellow and purple. But eyes which cannot see chromatic discord cannot see chromatic harmony. If a bad picture does not offend the taste, a good one cannot gratify it. There lies on our table a magazine printed in colored inks, an effort, the publisher tells us, to supplant the old monotony of black and white, and to minister to our love of color. It has a yellow cover bordered with scarlet and labeled in green! Flowers may not be guilty of chromatic offenses so atrocious as this, but such offenses they certainly do commit.
What is the end of floral decoration? By the old way of interpreting Nature, the botanist would have said, "To gratify man's love of the beautiful." He would not give such an answer now. If he would explain why certain flowers are not colored at all, and how it has come about that other flowers are colored, some in chromatic harmony, and others not, he must look to the flowers themselves, and to their servants the insects. Every one knows that the pollen must find its way from the anther to the stigma, else the flower, lacking impregnation, can set no seed. For many trees, and for grasses generally, the office of transporting the pollen is performed by the wind. And, "as the wind bloweth where it listeth," it may waste a million pollengrains for every one it lodges on a stigma. Hence the prodigality of pollen. If you walk through a field of corn, you will note how generously the tassel yields its pollen to the wind. If, toward the last of May, you shake a branch of pine, you will see floating from it a cloud of pollen-grains.
Having seen the largest prodigality, we will search now for the closest economy.
In many of the violets we find, in addition to the showy flowers, another set borne on runners and concealed under leaves. In the fringed polygala we have another case of dimorphism. In one flower the petals are of richest pink, two of them spreading out like wings, and the other, keel-shaped, crested and fringed. Another flower, which never opens, is borne close to the ground, or even in the ground, on subterranean shoots. Now, these ground flowers of the violet and polygala are self-fertilizing. One grain of pollen is enough for one seed, and that is all which Nature, in these flowers, will furnish.
You will observe now that the pine and the corn, in general all conifers and grasses depending for fertilization on the wind, have colorless flowers and much pollen, and that concealed flowers are without color and have but little pollen. We begin to suspect that color stands in some relation to the needs of the flower. In the grasses and pines it would be of no use, as the wind will find a dull flower as easily as a gaudy one. In self-fertilizing flowers it is also of no use.
Eighty years ago Sprengel maintained that breeding in and in would be as injurious in the vegetal as in the animal world, and he argued that colors and odors attract insects and thus secure cross-fertilization. It was the largest thought which had ever entered the head of a botanist. But Sprengel was ridiculed in his own generation and forgotten in the next. Thirty years ago Charles Darwin discovered the German's idea in an old, castaway volume, and at once he began to employ his matchless powers of observation in questioning Nature for its truth. He has taught us how to use our eyes. Let us look.
One of our most beautiful shrubs is the mountain-laurel (Kalmia latifolia). It blossoms all over in corymbs of bell-shaped flowers, white, tinged with red, having short nectaries and recurved stamens, whose anthers are inserted in little pits of the corolla. In Fig. 1 the flower is shown with the anthers out of their sockets. In Fig. 2 we have a section of the flower showing the recurved stamens and the anthers resting in their pouches. It will be seen that the stamens are shorter than the style. How is the anther to be liberated from the pouch? And how is the pollen to be carried to the stigma?
The kalmia is one of the most showy shrubs along the wooded way-sides of New England, and is very attractive to bumble-bees. Now, your bumble-bee is the clumsiest of insect. His action is as ungraceful as his person. He can do nothing expertly and neatly. He cannot even sting you while on the wing, but must first alight and adjust himself. At nectar-getting he is as clumsy as at stinging. He sprawls over a flower, pushes here and there among the delicate organs, and gets himself thoroughly bedaubed with pollen. This clumsiness makes him a good marriage-priest for the flowers. The color of the laurel attracts him, and the nectary promises honey. He lights, gets his legs entangled among the stamens, and as he jostles them they spring from their little pits with a sharp snap and scatter their pollen over his back. In visiting another flower some of the pollen will find its way to the stigma and thus secure cross-fertilization.
The iris would seem, at first thought, to have been specially planned for self-fertilization. As will be seen in Fig. 3, the petaloid stigma covers the petaloid stamen. But a most curious fact is that, while the stigmatic surface is brought right up against the stamen, the anther is turned outward away from the stigma. This arrangement will be seen in Fig. 4, which presents a longitudinal section of the flower after the petals have been removed. It is as if you apply powder to the surface of one sheet of paper and mucus to the surface of another, and, intending that the powder should be brought into contact with the mucus, place the two sheets together, surface to surface,
but turn the powdered surface out. What a seeming design, and yet, seemingly, what a fatal blunder! But this is nature as manifested in the iris. Pollen and stigma, so close together, are separated by the blade of the stamen. But for insects which, attracted by the color, light, and search the flower for its nectar, and push, now against an anther and now against a stigma, the iris could never set a seed.
Flowers, as Dr. Gray has said, seem to be made on the principle, "how not to do it." By traps, and pits, and springs, insects are made to do by indirection what it would seem could be better done directly by the flower itself. For many flowers the service can be rendered as well by one nectar-loving insect as another. But in some species the parts are so modified that only a single species of insect, correspondingly modified, can reach the organs of fructification. In the northern part of the United States the yucca (Fig. 5) has never been known to set seeds.
An entomologist has lately discovered a small moth with white head and thorax and wings, and legs of dingy yellow. To this moth he has given the name Pronuba yuccasella—yucca's go between. The structure of this moth will be seen in Fig. 6. In Fig. 7 the larva and the moth are drawn in the natural size. The peculiarity of this moth is, that in the female the basal joint of one of the maxillary palpi (Fig. 6, 5) is modified in a most wonderful manner into a long prehensile tentacle. With this she collects the pollen and thrusts it into the stigmatic tube against the stigma. In this pollen-mass she lays her eggs. A single palpus of the female of a single species of moth, modified in a peculiar way, is the means of perpetuating the yucca. In its native home yucca and yuccasella are inseparable. In the north, yuccasella does not thrive and yucca can set no seeds.
Well, if plants depend so much on insects, insects must have some way of knowing where they are. As an optical instrument, the eye of an insect is rather imperfect, but it is not color-blind. A butterfly may not be sensitive to the harmonies of color, but it certainly knows a colored object from an object not colored. We need more observations before we can safely say that insects apprehend the different shades of color, and prefer one shade to another; but, if any one will watch bees on a bed of hyacinths, he will see that a particular bee is apt to confine itself to a particular color.
We conclude that color and nectary are in correlation with the organs of fructification and with the eyes and appetites of insects.
Odor must come under the same law as color. We must seek for its rationale in the flower itself and the forces or agents which act on it. The hidden flowers of violet and polygala are odorless as well as
colorless. The grasses, pines, and palms, whose pollen is borne on the wind, are inconspicuous in color and are generally without odor. But some species of palm require the visitation of insects, and while their flowers remain inconspicuous in color they are rich in perfume. The fan-palm of the Rio Negro perfumes the air far and near with the odor of mignonette. Myriads of insects, attracted by the fragrance, hum and buzz among the flowers, and carry pollen from the staminate to the pistillate ones. This odor is pleasant to man as well as insects. But there are floral odors as well as colors which are offensive. We do not understand the laws of odor so well as those of sound and color. We educate the eye and the ear, but in the nose we are still cave-men. If we can trust a savant who has spent years in the cultivation of his olfactories, odors are under the same law as sounds and colors. Certain odors blend into a sort of music of smells as certain notes into a music of sounds. Other odors refuse to blend, but jar on the nose like discordant sounds on the ear. Septimius Piesse must live in a world of harsh notes and grating discords. Even to us the account seems nearly balanced between the odorous and the mal-odorous. The wild-rose is sweet, but the datura is sickening. The ailantus may be scored against the pink, and against the jasmine, the queen of flowers, whose fragrance is the only secret the floral world withholds from our chemistries—against the jasmine may be scored the carrion-flower. The odor of the flower is what its name implies. Surely it was not given for man's pleasure. Then for what? Perhaps from this very flower we may learn the rationale of odors.
The flower of the carrion-plant is of a pale yellow-green and is altogether inconspicuous. If it had no attraction but its color, it would never win the attention of an insect. Now, it is a fact of great significance that this carrion-flower is fertilized by the blow-fly. But what does the blow-fly want of a flower? If this flower were sensitive and rational and skilled in chemistries, we might imagine the correlation between itself and the fly to be the result of a mental process something like this: "At any cost I must secure fertilization. The wind cannot serve me, and bees and butterflies cannot find me. I will invite the blow-fly—I will practise deception. I will smell like decaying flesh!" Of course, this is fanciful. Suppose we imagine an intelligence outside of the plant, and the insect arranging by special creation such correlations between color, odor, nectary, and bees, moths, flies, and butterflies. Does it bring us any nearer to a mental resting-place? Rather, do not questions without end start up in the mind? Why such indirection? Why such seeming design marred by seeming chance as in the iris? And if all these structures and colors and odors are the result of special creation, what shall we say of deadly nightshades? of poison-ivies? of the fungi which live on the human body? Was there a special provision for certain fungi to grow on the forehead? for others to thrive in the mouth? for others still to infest the stomach? Was the body of man designed to be the habitat of pain-giving parasites? We have found plants good and not good, beautiful and not beautiful, odorous and mal-odorous. If the world were a theophany, would it not be good only, good everywhere and equally? To interpret the vegetal world as a "special creation" no more satisfies the religious sentiment than the reason.
Our common loosestrife (Lysimachia quadrifolia) is one of the most variable of species. Its European representative (L. vulgaris) appears in two well-marked varieties: one, found in sunny localities, whose petals are dark yellow, with red at the base; and the other, growing by shaded ditches, with petals of light yellow, and no red at the base. The more conspicuous variety attracts insects, and, as the stigma overtops the stamens, the agency of insects is required to secure fertilization. The flowers of the less conspicuous variety are not visited by insects, and, as the stigma does not overtop the stamens, the agency of insects is not required. The flower is self-fertilizing. These two forms graduate into each other, by connecting links which are found on the sunny edges of ditches.
Here is a very instructive lesson. Greater amounts of sunlight will account for the richer color of one variety; and the agency of insects, attracted by the color, will account for the change in structure. We see Nature in the act of species-making. Insects, acting mechanically on the delicate organs of a plant, effect something more than fertilization. Let us consider this action more carefully.
Dr. Ogle has observed the manners of the bee in visiting beans and scarlet-runners. These flowers are arranged to secure cross-fertilization. The honey they offer must be taken by an insect which will enter by the open door of the corolla-tube. But Dr. Ogle observed that while certain bees visit the flower in the legitimate way, and thus carry pollen from anther to stigma, others have a trick of evading their duty by piercing a hole in the calyx-tube, thus securing the nectar by a short cut. An important fact noted in these observations is, that the same bee always visits a flower in the same way. The inference is, that this habit of nipping the calyx is the result of individual experience. As some bees have acquired the habit, and others have not, another inference is that these insects are intelligent, and that they differ from each other in degrees of intelligence. Our final inference is that, if all bees are ever schooled up to this new art, there must come an end to our beans and scarlet-runners—unless some modification should occur in the structure of their flowers.
The salvia is constructed as if with special reference to fertilization by bumble-bees. But Mr. Meehan, who first pointed out the correlation, never saw a bumble-bee enter the flower. Under his eye the bee always cut the tube of the corolla. But another observer has seen the bumble-bee enter the flower and effect fertilization. And he has seen it nipping the corolla-tube to secure the honey by a short cut. And he has noticed that the smaller bees entered the corolla-tube, and that those which were too large to get into the flower nipped the tube. This is an important observation, and brings us to the very heart of the matter. The salvia with small corolla-tube, not securing fertilization, stands but little chance of surviving. The flower with large tube invites the bee, is fertilized, and ripens seed. "Natural selection" is going on under our very eyes. Now, if, in "the struggle for existence," a larger race of bumble-bees should appear, the salvia must either vary with the varying insect, or die out. And we have no right to assume that the body of an insect is a fixed bulk or structure any more than that the "instinct" or intelligence is fixed and invariable. Prof. Riley has shown the extreme probability that the peculiar modification of the palpus of the female yuccasella, which makes her the marriage-priestess of the yucca, was brought about little by little, as the peculiar structure of the flower came little by little. Flowers must vary with insects, and insects with flowers—yucca with yuccasella, and yuccasella with yucca—or both must die.
We have now the rationale of colors and odors. As vitality seems to have some general relation to color, perhaps the first show of color in a floral envelope was due to a slight diminution of vital force. Color being advantageous to the plant by attracting insects, when once it appeared its shades would be multiplied and intensified age after age, by natural selection. And as color is developed little by little as the result of insect-vision, modifications of structure are developed, in equal pace, by insect-touch.
This is not all. An organism is modified by all which environs it. Mr. Spencer, in his great work on "Biology," has shown us that the form of the cell, the leaf, the branch, the trunk, the flower, is determined in great part by the environment. Varying amounts of sunshine or shade modify the form of a branch. A prevailing wind modifies the form of a tree. A change of position on the stem changes the form of a flower. The drooping gloxinia in your conservatory is bi-symmetrical, and has a rudimental fifth stamen. Culture brings the flower up, erect on the stem, makes it radically symmetrical, restores the rudiment to a perfect stamen, prunes the flower of its eccentricities, and makes it regular—just as it does with a man.
Nature is, in the plant, what her name implies, "natura," a something about to be, a continual becoming. We see what changes are going on in the garden. Changes the same in kind are going on in the fields and the woods. A stroll over Goat Island on any May day will show the observant eye how variable in size and color and even in structure is the Trillium grandiflorum. The white-weed which overruns our Eastern meadows has sported into a score of incipient varieties, and, although but a new-comer on our shores, already in Connecticut it appears in a variety fixed and well marked. Incipient doubling is not uncommon, and now and then may be found a thalictrum or a saxifrage full-double. From the wild-strawberry (Fragaria Virginiana) has diverged a well-marked variety called the F. Illioensis. From this has diverged still another variety found by Mr. Gillman in great abundance on the shores of Lake Superior, far away from the influence of the gardener—a variety of a variety.
And now, as the visitation of insects, as more or less light, more or less heat, more or less wind, more or less room, more or less soil, are known to reflect changes in the color, size, form, and even structure
of plants, if the vegetal world has come to be what it is through the action of these secondary causes, we should find reminiscences of a primitive, undifferentiated type.
Theoretically the plant is a leaf. The axis or stem is a fused series of midribs. The floral organs are leaves variously modified. Nay, we may look even beyond the leaf to the components of the leaf, for thought cannot rest till it finds the ultimate unit. This is a cell, and the lowest plant is simply a cell. Cells adherent to cells form a frond. A frond, by a little modification, passes into a leaf, and a leaf into a part of the flower. A sepal is a leaf changed but little; a petal is a leaf changed in color and texture; a stamen is a leaf changed in color, texture, and form, the blade being eliminated except at the tip where it forms the anther, and the midrib remaining as a supporting filament; the pistil is a leaf with the lower part of the blade rolled up, and the edges united to form a carpel, and the midrib prolonged into a style, bearing atop a little shred of altered leaf-blade called a stigma. And here—at the very tips of these inner leaves where the nutriment is least and the vital force weakest—the investing membrane disappears; Nature slips back toward her simplest types, and shows us the naked primordial cells as pollen-grains! In the thistle one may see that the stem is continuous with the midrib of the leaf. Every observer knows that in the pond-lily (Fig. 8) we see the intermediate stages between a simple leaf and a pistil. And every one whose garden furnishes a syringa or double-flowering cherry, has only to look at the flowers to see pistils and stamens reverting to leaves.
Section of an Apple.
a a, carpels; b, remnant of pistils; c c, remnants of corolla; d d, remnants of calyx; f, fibrous line.
Fortunately for science, there grows in Kittanning, Pennsylvania, an apple-tree, which, in its flower and fruit, exemplifies the theory of the plant. Theoretically, a fruit is a branch with its leaves transformed. If the reader will take an apple—a rotten one is best—and cut it through from flower-scar to stem, he will find a core of five carpels, and about midway between the core and the rind he will see a green, fibrous line (Fig. 9). If he will look now at the flower, he will find the cup or calyx of five sepals, the corolla of five petals, the stamens many (a number of whorls), and the pistils five (one whorl). We have shown that these floral organs are simply transformed leaves, and we shall now see that the apple itself is merely these leaf-whorls still further transformed.
Let us cut the apple through around the equator and compare our section (Fig. 10) with the longitudinal one (Fig. 9). Looking at the flower-scar, we see the remnants of the sepals, d, d, d. Within and alternating with these are remnants of the petals, c, c. Still farther within is a little shred of a pistil, b. This shred can be traced down into the core, a. The other parts of the flower lose themselves in the fleshy fruit.
If we look now at the other section (Fig. 10), we shall see, on that fibrous line, ten greenish points, five opposite to the carpels and five alternating with them. Five other little points appear near the tips of the carpels and in line with them. Now, this fibrous line, and the points on it and within it, must bear some relation to the plan of the flower. And, as the stamens are a multiple of five, the points must have some relation to the staminate whorls. The core, as we see (Fig. 9), is continuous with the pistil. It is simply the base of the
pistillate whorl. The part of the fruit between the core and the dotted line represents the thickened and coalesced whorls of stamens and petals. The fruit between this dotted line and the fibrous line represents the thickened petals. The fibrous line represents the union of the coroline with the calycine leaves, and the fruit between this line and the rind represents the calycine leaves, grown thick and succulent.
Now, the flower of this Kittanning tree appears as a mere rudiment. How, if this theory is true, should the fruit appear? The pistillate whorl is wanting, and in theory the apple should have no core. It has none, but in its place a large cavity, as shown in Fig. 11. The stamens are rudimental, and the petals are represented by five little bud-scales. Theoretically, then, we should find but little fruit between the core-cavity and the fibrous line. There is but little, as the cut will show. The calyx is better developed, and we should find more fruit between the fibrous line and the rind—as we do. The flower starts with all the essential organs of a flower, and, before the inner whorls are arrested, enough vitality is given to the outer whorl to start it on the way toward an apple. And with the lack of development in the inner whorls the outer one develops cork cells, and the rind takes on the character of bark!
This is Nature teaching by what is abnormal. Let us scrutinize her where she seems most regular and orderly.
The acorn, like the apple, is seen by everybody and known by scarcely anybody. We will take a full-grown acorn in its cup and cut it through about midway from top to base. We shall find five little roundish bodies pressed up close against the shell. What are they? and how came they here? We consult the flower, and find (in the fertile one) a style with a three-lobed stigma. The pistil, then, represents three transformed and infolded leaves. When the flower is a little more advanced, we will cut through the lower part of the pistil and examine a section. This part becomes the ovary, and we find in our section three partition-walls radiating from the circumference to the centre and dividing the ovary into three compartments. In this tripartite structure we find our three leaves, the infolded blades cohering along parts of their surface and forming the partition-walls. On each of these partitions we see two ovules. The ovules represent leaves budding out on the margin of the pistil-leaf, and thus every ovary, in theory, should have at the very least as many ovules as there are leaves composing it. In the flower we have now the plan of the acorn. The surface of the ovary will become a shell. The six ovules will grow and ripen into six seeds. Cutting through the shell of the full-grown acorn we shall find it to contain three chambers, and each chamber two naked acorns. We find nothing of the sort! Where was the slip? Early in the acorn's life one of the six ovules gets the start of its neighbors and takes to itself all the nutriment. It grows too large for its chamber, and breaks the partition-walls. It grows to the measure of all the chambers, fills them, and pushes its shriveled brethren up against the shell-wall where you see them, five little starved-out things which once were possible oaks! Strange, is it not? And how passing strange if the oak were made so by "special creation!" What perplexity does the thought, coupled with the facts, bring into the mind! But if these aborted ovules are reminiscences of an earlier age, and an acorn less differentiated from the general type of the ovary, the oak becomes intelligible. And in this light of evolution all aborted organs, all rudimental organs, all floral eccentricities, become intelligible. Botany itself ceases to be a toy, and commands the attention of such imperial minds as those of Spencer and Mill. Her boundaries are enlarged. The plant does not stand apart, the result of a single antecedent. It represents the action of countless forces through countless ages. It almost justifies Tennyson's apostrophe;
"Flower in the crannied wall,
I pluck you out of the crannies—