The milkweed is a peculiarly - constructed and very highly-organized flower. The sepals and the petals, each five in number, fold back as soon as the flower opens and press closely against the flower-stalk (Fig. 87, a). Inside them, standing upright in a ring, are five honey-jars or nectaries of peculiar form (Fig. 87). Each nectary is hooded, and inside each is an incurved horn (Fig. 87, b). Within the circle of honey-jars are the five stamens, which are fixed to the base of the corolla, and stand in contact with each other, surrounding and enclosing the pistil (Fig. 87, c). On top of the ring of stamens is a large five-sided disk, which keeps the pollen from being wet with rain or dew. The whole stamen system is like a little tub or firkin, standing in the midst of the flower, upside down. Inside this firkin are two green pistils, which may become two green pods. Half the pollen of each anther is collected into a nine-pin-shaped mass, which is fastened to a similar mass formed by half the pollen of the next anther. Thus two connected pollen-masses belong to two separate stamens.
Fig. 87. - Trap of the milkweed.
a, single blossom seen from the side, showing the corolla turned backward and the ring of upright nectaries; b, single blossom seen from above; c, the stamen ring, showing one of the openings between the stamens, and the disk at its upper end; d, a freshly removed disk, with its attached pollen masses; e and f, positions taken by the drying pollen masses as they are carried through the air by insects.
They are united by a tiny black disk, which is seen, on closer examination to be thin, hard, and horny (Fig. 87, d). "Its sides are bent forward for its whole length," says Muller, "so that their edges lie close together, and in the middle of its lower border is a wedge-shaped notch." The disk is set just above an opening between the stamens which runs "clear through" to the pistils inside the firkin. This opening is a mere slit at its widest part, but it is distinctly narrower at its upper end. The fly or bee stands on the outside of the firkin, and slips and slides on the smooth surface till one of her feet enters the lower and wider end of one of the slits.
The winged captive draws her leg upward in the effort to escape, and her foot catches in the notch on the lower side of the little black disk. Then, determined to be free, she pulls out, if she is strong enough, the whole affair, disk and attached pollen-masses. A bee will gather several of these at once, and I have seen one buzzing away from a head of milkweed loaded with no fewer than nine. Thus encumbered she was for a moment held prisoner by the flower, unable to pull herself loose. Following the ancient custom of the bees, she carried the pollen-masses at once to another milkweed plant, and perched upon one of its flowers, in the same position in which she had stood when visiting the first. This brought some of the pollen-masses on her feet exactly opposite the slits running through the stamen-ring to the pistil.
The pollen-masses, when they are first extracted, stand wide apart. But as the insect flies through the air with them they dry somewhat, and in drying they droop so close together that they can both be introduced into the lower and wider part of the stamen-slit of another flower (Figs. 87, e and f). When the insect literally tears itself away from this second flower it snaps the cords which binds the pollen-masses to the little black disk. The disk still clings to the insect's foot as a souvenir of its visit to the first milkweed blossom, but the pollen-masses are left behind pressed close to the little green pistils of milkweed blossom number two.
The bee seems the favorite guest of the milkweed. The pollen-masses come out at once to her tread, and are carried directly to the pistil of another flower.
Wasps visit the milkweed for its honey, but I have never seen them withdraw the pollen-masses. Flies seldom do, though the flower is visited by flies of many species. Indeed, it is a general favorite, standing in the midst of a winged throng till dark, for twilight brings to it a number of small, sad-colored moths with very long proboscides.
But not all these visitors are permitted to go in peace. A small fly with his legs. stuck to the black disks is frequently unable to pull himself loose after he has drunk his fill.
In a bunch of twenty-five blossoms I have counted five flies thus held in captivity - three dead and two dying - and the same bunch had captured a long-legged, lace-winged caperer, whose struggles to free himself were as desperate as futile. On any large bunch of these flowers one can see mementoes of past tribulations. Here and there a blossom still holds a little black leg, the price of the liberty of some insect who has gone off free, but a cripple.
A flower so highly organized as the milkweed seldom receives and nourishes all comers. In one peculiarity of structure the milkweeds are like the orchids, that royal family of plants, for many orchids also send their pollen abroad massed into two clusters, which are united by a disk. But each orchid has its own very select and small circle of guests, and some among them endeavor to please one butterfly or moth friend, him and him alone. They are, in evolutionary language, "highly specialized".
On the other hand, a flower which keeps open house to all comers is generally primitive in color and structure, Such blossoms are apt to be yellow or white, with flat, open corollas, and without spurs, honey-jars, or covering to protect the pollen. So the milkweed is something of a problem to the evolutionary botanist.
And there is another puzzle for him in the inadequacy of Nature's very elaborate contrivances to ensure the fertilization of the milkweed. Flowers far simpler in structure and far less attractive to insects bring a larger proportionate number of fruits to perfection.
The great blossom-clusters which crown the milkweed in July and August are made up of from twenty-five to fifty flowers. But in September the same stalk will support only from two to six seed-pods, and a pair of pods represents a single flower.
So most of the flowers die leaving no memorial behind them, and the flies which they have victimized are avenged.
In the language of some elegiac poetry we pause to breathe a sigh over the fate of the hapless flies which, like Haman of old, come to a feast and thereat are captured and slain.
However, these unfortunates are but a small proportion of the milkweed's fly visitors. The great majority make off, after taking their fill of nectar, without carrying away any portion of the pollen which the flower is endeavoring to send to its neighbors. This waste of nectar is bad for the milkweed, which would be better off with fewer fly visitors. So the flower would profit by any device which would discourage these many flies, without deterring those useful and desired visitors, the bees. Will flies learn after a while to shun the milkweed's dangerous sweets, so that they may all be left for worthier and more welcome guests? And how many generations will it take this proverbially foolish insect to lay the lesson to heart? (Fig. 88).
Fig. 88. - A dogbane flower and its captive.