Popular Science Monthly/Volume 49/September 1896/Dogbane and Milkweed
|DOGBANE AND MILKWEED.|
THE story of the trap-setting and insect-eating plants is a more than twice-told tale. The pitcher-plant, which beguiles the hapless fly to his drowning in its vase-shaped leaves, baited on the outside with nectar-bearing glands, and filled with water; the Venus's flytrap, which shuts up on him and crushes him; the sundew (Drosera), which chokes him in a sticky secretion, are all known, at least by pictures and descriptions, to the tyro in botanic study. And we have learned that they all have good and sufficient reasons for thus dealing with the hapless flies. For "the plants grow," says Grant Allen, "in places where the marshy and water-logged soil is markedly wanting in nitrogen compounds. Insect-eating leaves are thus a device to supply the plant with nitrogen by means of the foliage, in circumstances where the roots prove powerless for the purpose."
The insect slaughter which they carry on has the same excuse as the animal slaughter of the abattoir. It is killing for food, and the insects which these plants catch are honestly eaten and digested. But in the infinite analogy of the vegetable world we find a curious parallel to killing for sport. There are a few native flowers which entrap insects simply and solely, it appears, for the deed's own sake. The prisoners serve no apparent use in the plant's economy, nor do their poor little corpses nourish the plant's life. A botanist who let his imagination run away with him might accuse the guileless-looking flowers of that savage joy in another creature's pain which drew our forefathers in crowds to the badger-drawings and bear-baitings of bygone times.
One of these flower tormentors is the spreading dogbane (Apocynum androsœmifolium), which is common all summer, along shady roadsides and around the borders of thickets, in the Northern and Eastern States. The plant is about three feet high, erect and branching. The flowers are nearly as large as single blossoms of the lily of the valley, "and are very beautiful," says Mrs. Dana, "if closely examined. The corolla is bell-shaped and cleft, at the edge, into five slender points. Its deep pink veining suggests nectar," and the insect visitor is not disappointed, for at its base are five nectar-bearing glands. These stand in a ring around the pistil, and in a larger circle, outside the ring of honey glands, are the five stamens. The anthers stand erect, and in shape are like arrow or spear heads. Corresponding to the two points at the base of a spear head, there are, at the base of each anther, two little hard horns, and the stamens ring so closely about the pistil that horn is pressed against horn all around the circle.
On the inside of the corolla, near its base, are five triangular callosities, with their points up. These are placed in such a way as to alternate with the stamens, and stand a little below them, so that the two hard points at the bases of two neighboring anthers, and the hard tip of the callosity—three little horns—come together like the teeth of a trap. There are no fewer than five places inside the flower's cup where these traps are set, and inside the circle of traps are the glands which contain nectar. The flower is visited by bees and flies.
The insect caller must run his proboscis in between the long anthers, and just above the horny excrescences on the corolla. When he attempts to withdraw, after drinking his fill, the three points lock together, like the jaws of a trap, holding the tip of his proboscis in durance vile. If the winged captive is big and strong, he gets free, with a long and a vigorous pull. But small flies are often held prisoners till they die, probably from starvation. Sometimes one may see three or four of these hapless victims on one full-blooming plant of spreading dogbane.
Among the prisoners one may often see a little summer fly of dudish aspect, with body ringed with alternate bands of bronze and gold, and wings of gauze shot with opaline colors. To what end is this bright little fellow sacrificed? Held as he is by the tip of his proboscis, his body does not come in contact with the plant, and hence it can not be digested by the vegetable juices, as are the corpses of the sundew's victims. The dogbane is apparently unable to furnish any adequate justification for his taking off.
There is another variety of dogbane, the Indian hemp, orApocynum, cannabinum, which bears smaller blossoms than the androsæmifolium, blooms somewhat later, and is more widely distributed over the country. This flower has no callosities in its corolla, sets no snares for insect victims, and is apparently quite innocent of the crimes which one is inclined to lay to the charge of its first cousin.
The common milkweed (Asclepias cornuti) also imprisons insects, which sometimes die in captivity, and do no apparent good to the plant by their deaths. They have, however, invited misfortune, for though the milkweed is rich in honey, and is visited by a large and miscellaneous company, it can be fertilized apparently only by bees and perhaps by a few large flies.
The milkweed is a peculiarly constructed and very highly organized flower. The petals, five in number, fold back as soon as the blossom opens and press their backs against the flower stalk. Inside them, standing upright in a ring, are five honey jars, or nectaries, of peculiar form. Each nectary is hooded, and inside each is an incurved horn. 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 inclosing the pistil. On top of the ring of stamens is a large five-sided disk, which keeps the pollen from being blown away, or 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 will be two green pods. The pollen of each anther is collected into an Indian-club-shaped mass, which is fastened to a similar mass formed by the pollen of the next anther. Thus the two connected pollen masses belong to two separate stamens. They are united by a tiny black disk. This disk is set just above an opening between the stamens which runs "clear through" to the pistil inside the firkin. The fly or bee stands on the outside of the firkin, and, tramping about there, gets her foot caught on the black disk—which is glutinous—and pulls out (if she is strong enough) the whole affair, disk and attached pollen masses. A bee will gather three or four 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 and her burden loose. Following the custom of bees, she carried the pollen masses at once to another milkweed plant, and perched upon one of its flowers in exactly the position in which she had stood when visiting the first. This brought the pollen masses on her feet exactly into the slits running through the stamen ring to the pistil.
The bee seems the favorite guest of the milkweed. The pollen masses come out at once at 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, dark-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. On 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 the 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. Can he, for example, explain the fate of those hapless flies which, like Haman of old, come to a feast, but get only imprisonment and death? 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 off 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?