Flowers - Common Milkweed or Silkweed
( Originally Published 1916 )
(Asclepias Syriaca) Milkweed family
(A. cornuti of Gray)
Flowers—Dull, pale greenish purple pink, or brownish pink, borne on pedicels, in many flowered, broad umbels. Calyx inferior, 5-parted ; corolla deeply 5-cleft, the segments turned back-ward. Above them an erect, 5-parted crown, each part called a hood, containing a nectary, and with a tooth on either side, and an incurved horn projecting from within. Behind the crown the short, stout stamens, united by their filaments in a tube, are inserted on the corolla. Broad anthers united around a thick column of pistils terminating in a large, sticky, 5-angled disk. The anther sacs tipped with a winged membrane ; a waxy, pear-shaped pollen-mass in each sac connected with the stigma in pairs or fours by a dark gland, and suspended by a stalk like a pair of saddle-bags. Stem: Stout, leafy, usually unbranched, 3 to 5 ft. high, juice milky. Leaves: Opposite, oblong, entire-edged, smooth above, hairy below, 4 to 9 in. long. Fruit: 2 thick, warty pods, usually only one filled with compressed seeds attached to tufts of silky, white, fluffy hairs.
Preferred Habitat—Fields and waste places, roadsides.
Distribution—New Brunswick, far westward and southward to North Carolina and Kansas.
After the orchids, no flowers show greater executive ability, none have adopted more ingenious methods of compelling insects to work for them than the milkweeds. Wonderfully have they perfected their mechanism in every part until no member of the family even attempts to fertilize itself ; hence their triumphal, vigorous march around the earth, the tribe numbering over nine-teen hundred species located chiefly in those tropical and warm, temperate regions that teem with insect life.
Commonest of all with us is this rank weed, which possesses the dignity of a rubber plant. Much more attractive to human eyes, at least, than the dull, pale, brownish-pink umbels of flowers are its exquisite silky seed-tufts. But not so with insects. Knowing that the slightly fragrant blossoms are rich in nectar, bees, wasps, flies, beetles, and butterflies come to feast. Now, the visitor finding his alighting place slippery, his feet claw about in all directions to secure a hold, just as it was planned they should ; for in his struggles some of his feet must get caught in the fine little clefts at the base of the flower. His efforts to extricate his foot only draw it into a slot at the end of which lies a little dark-brown body. In a newly opened flower five of these little bodies may be seen between the horns of the crown, at equal distances around it. This tiny brown excrescence is hard and horny, with a notch in its face. It is continuous with and forms the end of the slot in which the visitor's foot is caught. Into this he must draw his foot or claw, and finding it rather tightly held, must give a vigorous jerk to get it free. Attached to either side of the little horny piece is a flattened yellow pollen-mass, and so away he flies with a pair of these pollinia, that look like tiny saddle-bags, dangling from his feet. One might think that such rough handling as many insects must submit to from flowers would discourage them from making any more visits ; but the desire for food is a mighty passion. While the insect is flying off to another blossom, the stalk to which the saddle-bags are attached twists until it brings them together, that, when his feet get caught in other slots, they may be in the position to get broken off in his struggles for freedom precisely where they will fertilize the stigmatic chambers. Now the visitor flies away with the stalks alone sticking to his claws. Bumblebees and hive-bees have been caught with a dozen pollen-masses dangling from a single foot. Outrageous imposition !
Does this wonderful mechanism always work to perfection ? Alas! no. It is a common thing to find dead hive-bees and flies hanging from the flowers. While still struggling to escape, the unhappy victims will be attacked by ants, beetles, and spiders, or killed by heavy showers. Larger and stronger insects than honey-bees are required to regularly effect pollination and free themselves, especially when they are so unfortunate as to catch several feet in the grooves. Doubtless it is the bumblebee that can transfer pollen with impunity ; but very many other insects, not perfectly adapted to the flowers, occasionally benefit them. Among the large butterflies the Papilios, which suck with their wings in motion, are the most useful, because in using their legs to offset the motion of their wings they rapidly repeat those movements which are necessary to draw the pollinia from the anther cells and insert them in the stigmatic chambers of other flowers. "Large butterflies like Danais," says Professor Robertson, " hold their wings still in sucking, spending more time on an umbel, but generally carrying pollinia. Small butterflies are worse than useless. They remain long on the umbels sucking, but resting their feet superficially on the flowers. . Since several moths were found entrapped, pollination must often be brought about by night-flying Lepidoptera. As a rule, Diptera (flies) either do not transfer pollinia at all, or become hopelessly entangled when they do." Occasionally pollen-masses are found on the tongues of insects, especially on those of bees and wasps, which move about with their unruly member sticking out. Probably no one has ever made the exhaustive and absorbingly interesting study of the milkweeds that Professor Robertson has.
Better than any written description of the milkweed blossom's mechanism is a simple experiment. If you have neither time nor patience to sit in the hot sun, magnifying-glass in hand, and watch for an unwary insect to get caught, take an ordinary house-fly, and hold it by the wings so that it may claw at one of the newly opened flowers from which no pollinia have been removed. It tries frantically to hold on, and with a little direction it may be led to catch its claws in the slots of the flower. Now pull it gently away, and you will find a pair of saddle-bags slung over his foot by a slender curved stalk. If you are rarely skilful, you may induce your fly to withdraw the pollinia from all five slots on as many of his feet. And they are not to be thrown or scraped off, let the fly try as hard as he pleases. You may now invite the fly to take a walk on another flower in which he will probably leave one or more pollinia in its stigmatic cavities.
Dr. Kerner thought the milky juice in milkweed plants, especially abundant in the uppermost leaves and stems, serves to protect the flowers from useless crawling pilferers. He once started a number of ants to climb up a milky stalk. When they neared the summit, he noticed that at each movement the terminal hooks of their feet cut through the tender epiderm, and from the little clefts the milky juice began to flow, bedraggling their feet and the hind part of their bodies. " The ants were much impeded in their movements," he writes, " and in order to rid themselves of the annoyance, drew their feet through their mouths. . . Their movements, however, which accompanied these efforts, simply resulted in making fresh fissures and fresh discharges of milky juice, so that the position of the ants became each moment worse and worse. Many escaped by getting to the edge of a leaf and dropping to the ground. Others tried this method of escape too late, for the air soon hardened the milky juice into a tough brown substance, and after this, all the strugglings of the ants to free themselves from the viscid matter were in vain." Nature's methods of preserving a flower's nectar for the insects that are especially adapted to fertilize it, and of punishing all useless intruders, often shock us ; yet justice is ever stern, ever kind in the largest sense.
If the asclepias really do kill some insects with their juice, others doubtless owe their lives to it. Among the " protected " insects are the milkweed butterflies and their caterpillars, which are provided with secretions that are distasteful to birds and predaceous insects. " These acrid secretions are probably due to the character of the plants upon which the caterpillars feed," says Dr. Holland, in his beautiful and invaluable " Butterfly Book." " Enjoying on this account immunity from attack, they have all, in the process of time, been mimicked by species in other genera which have not the same immunity." " One cannot stay long around a patch of milkweeds without seeing the monarch butterfly (Anosia plexippus), that splendid, bright, reddish-brown winged fellow, the borders and veins broadly black, with two rows of white spots on the outer borders and two rows of pale spots across the tip of the fore wings. There is a black scent-pouch on the hind wings. The caterpillar, which is bright yellow or greenish yellow, banded with shining black, is furnished with black fleshy ` horns ' fore and aft."
Like the dandelion, thistle, and other triumphant strugglers for survival, the milkweed sends its offspring adrift on the winds to found fresh colonies afar. Children delight in making pompons for their hats by removing the silky seed-tufts from pods before they burst, and winding them, one by one, on slender stems with fine thread. Hung in the sunshine, how charmingly fluffy and soft they dry !
Among the comparatively few butterfly flowers—although, of course, other insects not adapted to them are visitors—is the Purple Milkweed (A. purpurasceus), whose deep magenta umbels are so conspicuous through the summer months. Humming-birds occasionally seek it too. From Eastern Massachusetts to Virginia, and westward to the Mississippi, or beyond, it is to be found in dry fields, woods, and thickets.
The Swamp Milkweed (A. incarnata), on the other hand, rears its intense purplish-red or pinkish hoods in wet places. Its leaves are lance-shaped or oblong-lanceolate, whereas the purple milkweed's leaves are oblong or ovate-oblong. This is a smooth plant ; and a similar species once reckoned as a mere variety (A. pulchra) is the Hairy Milkweed. It differs chiefly in having some hairs on the under side of its leaves, and a great many hairs on its stem. Both plants bear erect, rather slender, tapering pods.
The Poke, or Tall Milkweed (A. exaltata)—A. phytolaecoides of Gray—may attain a height of six feet if the moist soil in which it grows be exactly to its liking. Drooping or spreading umbels of flowers whose corolla segments are pale purplish green, and whose crown is clear ivory white or pink, appear from June to August from Maine to Georgia and far westward. Sometimes the tapering oblong leaves may be nine inches long. The erect seed-pods are drawn out to an unusually long point.
One may always distinguish the low-growing Four-leaved Milkweed (A. quadrifolia) from its relatives of ranker growth by its general air of refinement, as well as by the two pairs of thin, tapering leaves that grow in an upright whorl near the middle of the slender stem. Usually there are no leaves on the lower part. Small terminal umbels of delicate pink and white fragrant flowers, which appear from May till July, give place to very narrow pointed pods in late summer. From Maine to Ontario southward to North Carolina and Arkansas is its range, in woods and thickets chiefly.