Gibson William Hamilton
My Studio Neighbors
THE MILKWEED
The singular hospitality of our milkweed blossom is nowhere matched among Flora's minions, and would seem occasionally in need of supervision.
Just outside the door here at my country studio, almost in touch of its threshold, year after year there blooms a large clump of milkweed (Asclepias cornuta), and, what with the fragrance of its purple pompons and the murmurous music of its bees, its fortnight of bloom is not permitted to be forgotten for a moment. Only a moment ago a whiff of more than usual redolence from the open window at which I am sitting reminded me that the flowers were even now in the heyday of their prime, and the loud droning music betokened that the bees were making the most of their opportunities.
Yielding to the temptation, I was soon standing in the midst of the plants. The purple fragrant umbels of bloom hung close about me on all sides, each flower, with its five generous horns of plenty, drained over and over again by the eager sipping swarm.
But the July sun is one thing to a bee and quite another thing to me. I have lingered long enough, however, to witness again the beautiful reciprocity, and to realize anew, with awe and reverence, how divinely well the milkweed and the bee understand each other. After a brief search among the blossom clusters I return to my seclusion with a few interesting specimens, which may serve as a text here at my desk by the open window.
Two months hence an occasional silky messenger will float away from the glistening clouds about the open milkweed pods, but who ever thanks the bees of June for them? The flower is but a bright anticipation—an expression of hope in the being of the parent plant. It has but one mission. All its fragrance, all its nectar, all its beauty of form and hue are but means towards the consummation of the eternal edict of creation—"Increase and multiply." To that end we owe all the infinite forms, designs, tints, decorations, perfumes, mechanisms, and other seemingly inexplicable attributes. Its threshold must bear its own peculiar welcome to its insect, or perhaps to its humming-bird friend, or counterpart; its nectaries must both tempt and reward his coming, and its petals assist his comfortable tarrying.
Next to the floral orchids, the mechanism of our milkweed blossom is perhaps the most complex and remarkable, and illustrates as perfectly as any of the orchid examples given in Darwin's noble work the absolute divine intention of the dependence of a plant species upon the visits of an insect.
Our milkweed flower is a deeply planned contrivance to insure such an end. It fills the air with enticing fragrance. Its nectaries are stored with sweets, and I fancy each opening bud keenly alert with conscious solicitude for its affinity. Though many other flowers manage imperfectly to perpetuate their kind in the default of insect intervention, the milkweed, like most of the orchids, is helpless and incapable of such resource. Inclose this budded umbel in tarlatan gauze and it will bloom days after its fellow-blooms have fallen, anticipating its consummation, but no pods will be seen upon this cluster.
What a singular decree has Nature declared with reference to the milkweed! She says, in plainest terms, "Your pollen must be removed on the leg of an insect, preferably a bee, or your kind shall perish from the face of the earth." And what is the deep-laid plan by which this end is assured? My specimens here on the desk will disclose it all.
Here are two bees, a fly, and a beetle, each hanging dead by its legs from a flower, an extreme sacrificial penalty, which is singularly frequent, but which was certainly not exacted nor contemplated in the design of the flower. A careful search among almost any good-sized cluster of milkweeds will show us many such prisoners. As in all flowers, the pollen of the milkweed blossom must come in contact with its stigma before fruition is possible. In this peculiar family of plants, however, the pollen is distinct in character, and closely suggests the orchids in its consistency and disposition. The yellow powdery substance with which we are all familiar in ordinary flowers is here absent, the pollen being collected in two club-shaped or, more properly, spatula-shaped masses, linked in pairs at their slender prolonged tips, each of which terminates in a sticky disc-shaped appendage united in V-shape below. These pollen masses are concealed in pockets (B) around the cylindrical centre of the flower, the discs only being exposed at the surface, at five equidistant points around its rim, where they lie in wait for the first unwary foot that shall touch them. A glance at the two views of this central portion of the flower, as it appears through my magnifying-glass—the honey-horns and sepals having been removed—will, I think, indicate its peculiar anatomy or mechanism. No stigma is to be seen in the flower, the stigmatic surface which is to receive the pollen being concealed within five compartments, each of which is protected by a raised tent-like covering, cleft along its entire apex by a fine fissure (A). Outside of each of these, and entirely separated from the stigma in the cavity, lie the pollen masses within their pockets, each pair uniting at the rim below in V-shape, the union at the lower limit of the fissure.
Milkweed Captives
The Pollen Masses and the Fissure
With this more intimate knowledge of the floral anatomy, let us now visit our milkweed-plant and observe closely.
A bee alights upon the flower—the object of its visit being, of course, the sweets located in the five horn-shaped nectaries. In order to reach this nectar the insect must hang to the bulky blossom. Instantly, and almost of necessity, it would seem, one or more of the feet are seen to enter the upper opening of the fissure, and during the insect's movements are drawn through to the base. The foot is thus conducted directly between the two viscid discs, which immediately cling closer than a brother, and as the foot is finally withdrawn, the pollen is pulled from its cell. The member now released seeks a fresh hold, and the same result follows, the leg almost inevitably entering the fissure, and this time drawing in the pollen directly against the sticky stigmatic surface within. The five honey-horns have now been drained, and as our bee leaves the flower he is plainly detained by this too hearty "shake" or "grip" of his host, and quite commonly must exert a slight struggle to free himself. As the foot is thus forcibly torn away, the pollen mass is commonly scraped entirely off and retained within the fissure, or perhaps parts at the stalk, leaving the terminal disc clinging on the insect's leg. Occasionally, when more than one leg is entangled, the dangling blossom is tossed and swayed for several seconds by the vigorous pulling and buzzing, and a number of these temporary captives upon a single milkweed-plant are always to be seen.
Not unfrequently the mechanism so well adapted exceeds its functions and proves a veritable trap, as indicated in my specimens. I have found three dead bees thus entrapped in a single umbel of blossoms, having been exhausted in their struggles for escape; and a search among the flowers at any time will show the frequency of this fatality, the victims including gnats, flies, crane-flies, bugs, wasps, beetles, and small butterflies. In every instance this prisoner is found dangling by one or more legs, with the feet firmly held in the grip of the fissure.
Almost any bee which we may catch at random upon a milkweed gives perfect evidence of his surroundings, its toes being decorated with the tiny yellow tags, each successive flower giving and taking, exchanging compliments, as it were, with his fellows. Ordinarily this fringe can hardly prove more than an embarrassment; but we may frequently discern an individual here and there which for some reason has received more than his share of the milkweed's compliments. His legs are conspicuously fringed with the yellow tags. He rests with a discouraged air upon a neighboring leaf, while honey, and even wings, are seemingly forgotten in his efforts to scrape off the cumbersome handicap.
The Tragedy of the Bees
An interesting incident, apropos of our embarrassed bee, was narrated to me by the late Alphonso Wood, the noted botanist. He had received by mail from California a small box containing a hundred or more dead bees, accompanied by a letter. The writer, an old bee-keeper, had experience, and desired enlightenment and advice. The letter stated that his bees were "dying by thousands from the attacks of a peculiar fungus." The ground around the hive was littered with the victims in all stages of helplessness, and the dead insects were found everywhere at greater distances scattered around his premises. It needed only a casual glance at the encumbered insects to see the nature of the malady. They were laden two or three pairs deep, as it were, with the pollen masses of a milkweed. The botanist wrote immediately to his anxious correspondent, informing him, and suggesting as a remedy the discovery and destruction of the mischievous plants, which must be thriving somewhere in his neighborhood. A subsequent letter conveyed the thanks of the bee-keeper, stating that the milkweeds—a whole field of them—had been found and destroyed, and the trouble had immediately ceased. I am not aware that Mr. Wood ever ascertained the particular species of milkweed in this case. It is not probable that our Eastern species need ever seriously threaten the apiary, though unquestionably large numbers of bees are annually destroyed by its excessive hospitality. I have repeatedly found honey-bees dead beneath the plants, and my cabinet shows a specimen of a large bumblebee which had succumbed to its pollen burden, its feet, and even the hairs upon its body, being fringed deep with the tiny clubs—one of the many specimens which I have discovered as the "grist in the mill" of that wise spider which usually spreads his catch-all beneath the milkweeds.
Allied to the milkweed is another plant, the dogbane (Apocynum), which has a similar trick of entrapping its insect friends. Its drooping, fragrant, bell-shaped white flowers and long slender pods will help to recall it. But its method of capture is somewhat similar to the milkweed. The anthers are divided by a V-shaped cavity, into which the insect's tongue is guided as it is withdrawn from the flower, and into which it often becomes so tightly wedged as to render escape impossible. I have found small moths dangling by the tongue, as seen in the illustration below.
A Moth Caught by the Tongue in Dogbane
