"Little weavers of the summer, with sunbeam shuttle bright,
And loom unseen by mortals, you are busy day and night,
Weaving fairy threads as filmy, and soft as cloud swans, seen
In broad blue sky-land rivers, above earth's fields of green."
S there any other young plant that shows off its baby-clothes as does the young milkweed! When it comes up through the soil, each leaf is folded lengthwise around the stem, flannel side out, and it is entirely soft and white and infantile. The most striking peculiarity of the milkweed plant is its white juice, which is a kind of rubber. Let a drop of it dry on the back of the hand, and when we try to remove it we find it quite elastic and possessed of all of the qualities of crude rubber. At the first trial it seems quite impossible to tell from which part of the stem this white juice comes, but by blotting the cut end once or twice, the hollow of the center of the stem is seen to have around it a dark green ring, and outside this is a light green ring. It is from the dark green ring encircling the stem cavity that the milk exudes. This milk is not the sap of the plant any more than resin is the sap of the pine; it is a special secretion, and is very acrid to the taste, rendering milkweed disgusting to grazing animals. If a milkweed stem be broken or gashed, this juice soon heals the wound and keeps out germs, and thus is of great use to the plant, since many insects feed upon it. If cut across, every vein in every leaf produces "milk," and so does every small flower pedicel. When the "milk" is by chance smeared on cloth and allowed to dry, soap and water will not remove it, but it yields readily to chloroform, which is a solvent of rubber.
Milkweed in blossom.
Photo by Verne Morton.
The milkweed leaves are in stately conventional pairs; if one pair points east and west, the pair above and the pair below point north and south. The leaf is beautiful in every particular; it has a dark green upper surface, diversified with veins that join in scallops near the border; it is soft to the touch on the upper surface, and is velvety below. The lens reveals that the white under surface, or the nap of the velvet, is a cover of fine white hairs.
The flower of the milkweed is too complicated for little folks even to try to understand; but for the pupils of the seventh and eighth grades it will prove an interesting subject for investigation, if they study it with the help of a lens. In examining the globular bud, we see the five hairy sepals, which are later hidden by the five long, pinkish green petals which bend back around the stem. When we look into the flower, we see five little cornucopias—which are really horns of plenty, since they are filled with nectar; from the center of each is a little, fleshy tongue, with its curved point resting on the disk at the center of the flower. Between each two of these nectar-horns can be seen the white bordered opening of a long pocket—like a dress-pocket—at the upper end of the opening of which is a black dot. Slip a needle into the pocket opening until it pushes against the black dot, and out pops a pair of yellow saddle-bags, each attached to the black dot which joins them. These are the pollen-bags, and each was borne in a sac, shaped like a vest-pocket, one lying either side of the upper end of the long pocket. These pollen-bags are sticky, and they contract so as to close over the feet of the visiting bee.
1. Milkweed flower, enlarged.
2. Same, more enlarged. a,a, nectar-horns; p, pocket; o,o, openings to the pocket; s, pollen-bags in place; s', pollen-bags removed.
Since the stem of the flower cluster droops and each flower pedicel droops, the bee is obliged to cling, hanging back down, while getting the nectar, and has to turn about as if on a pivot in order to thrust her tongue into the five cornucopias in succession; she is then certain to thrust her claws into a long pocket, and it proceeds to close upon them, its edges being like the jaws of a trap. The bee, in trying to extricate her feet, leaves whatever pollen-bags she had inadvertently gathered in this trap-pocket, which gives them passage to the stigma. But the milkweed flower, like some folks, is likely to overdo matters, and sometimes these pockets grasp too firmly the legs of the bee and hold her a prisoner. We often find insects thus caught and dead—a result as far from the plan of the flower as from that of the insect victim, had both been conscious. Sometimes bees become so covered with these pollen-bags, which they are unable to scrape off, that they die because of the clogging. But for one bee that suffers there are thousands that carry off the nectar triumphantly, just as thousands of people travel by water for one that is drowned.
The milkweed pod has been the admiration of nature students from the beginning, and surely there is no plant structure that so interests the child as this house in which the milkweed carries its seeds. When we look at a green pod, we first admire its beautiful shape; on either side of the seam, which will sometime open, are three or four rows of projecting points rising from the felty surface of the pod in a way that suggests embossed embroidery. We open the pod by pulling it apart along the seam; and this is not a seam with a raw edge but is finished with a most perfect selvage. When we were children we were wont to dispossess these large green pods of their natural contents, and because they snapped shut so easily, we imprisoned therein bumblebees "to hear them sing," but we always let them go again. We now know that there is nothing so interesting as to study the contents of the pod just as it is. Below the opening is a line of white velvet; at one end, and with their "heads all in one direction," are the beautiful, pale-rimmed, brown, overlapping seeds; and at the other end we see the exquisite milkweed silk with the skein so polished that no human reel could give us a skein of such luster. If we remove the contents of the pod as a whole, we see that the velvety portion is really the seed-support and that it joins the pod at either end. It is like a hammock full of babies, except that the milkweed babies are fastened on the outside of the hammock.
No sooner is our treasure open to the air than the shining silk begins to separate into floss of fairy texture. But before one seed comes off, let us look at the beautiful pattern formed by the seeds overlapping—such patterns we may see in the mosaics of mosques.
Pull off a seed, and with it comes its own skein of floss, shining like a pearl; but if we hold the seed in the hand a moment the skein unwinds itself into a fluff of shining threads as fine as spiders' silk, and each individual thread thrusts itself out and rests upon the air; and altogether there are enough of the threads to float the seed, a balloon of the safest sort. If we wreck the balloon by rubbing the floss through our fingers, we shall feel the very softest textile fiber spun by Mother Nature.
If we look closely at our seed we see a margin all around it. Well, what if the balloon should be driven over sea, and the seed dropped upon the water? It must then drown unless it has a life preserver; this margin that we have noted is of the safest cork, and is warranted to float; if you do not believe it, try it.
If we pull off all the seeds, we can see that the velvety support is flat and that all of the seeds are attached to it, but before we stop our admiring study we should look carefully again at the inside of the pod, for never was there a seed cradle with a lining more soft and satiny.
Milkweed seed-balloons just leaving the sheltering pod.
Leading thought—The milkweed when wounded secretes a milky juice which is of a rubberlike composition; it flows out of the wounded plant and soon hardens, thus protecting the wound from germs. Milkweed flowers depend entirely upon insects for pollenation; the pollen is not a free, yellow powder, but it is contained in paired sacs, which are joined in V-shape. The seeds are carried by balloons, and they can float on water as well.
Method—Begin the study of the plant when it first appears above ground in April or May. Give the pupils the questions about the blossom for a vacation study, and ask that their observations be kept in their notebooks. The study of the pods and seeds may be made in September or October. When studying the milky juice, add a geography lesson on rubber trees and the way that rubber is made.
1. The plant. How does the milkweed look as it appears above ground in the spring? How are its leaves folded when it first puts its head up? Cut off a fully expanded plant a few inches above the ground. What flows out of the stem? Blot off the "milk" and study the cross-section of the stem. What is at the center? How many layers do you see around this center? Can you see from which the milkweed juice comes? How does the juice feel as it dries on your fingers? How does it look when dry? Place a few drops on a piece of paper and when it is dry pull it off and see if it is elastic. Break the edge of the leaf. Does the milky juice flow from it? Does it come from the veins? Do you think that this is the sap of the milkweed? Cut a gash in the milkweed stem and see how the "milk" fills the wound. How does this help the plant? Do cattle feed upon the milkweed when it grows in pastures? If not, why?
2. How are the leaves arranged on the stem? How do the upper and under sides of the leaves differ? Examine with a lens, and see what makes the nap of the velvet. What gives the light color to the under side? Sketch a leaf showing its shape and venation, noting especially the direction of the veins as they approach the edge of the leaf.
3. The flower. Where do the flower clusters come off the stems in relation to the leaves? Does the stem of the flower cluster stand stiff or droop? Take a good sized flower cluster and count the flowers in it. What would happen if all these flowers should develop into pods? How many flower clusters do you find on one plant? Which of these clusters open first? Last?
4. Take off a single bud with its stem, or pedicel. Does the milky juice come at the break? Is the bud stem stiff or drooping? What is its color and how does it feel? What is the shape of the bud? How many sepals has it? Look at the stem, sepals and bud with a lens and describe their covering. Look for a flower just opening where the petals stand out around it like a five-pointed star. What is their color? What happens to the petals when the flower is fully expanded? Can you see the sepals then? Look straight into the flower. Do you see the five nectar-horns? Look at them with a lens and describe them. What do you suppose is the use of the little curved tongue coming out of each? Where does the tip of the tongue rest? With a lens, look between two of the nectar horns; can you see a little slit or pocket, with white protruding edges? Note just above the pocket a black dot; thrust a needle into this pocket near its base and lift it toward the crown of the flower, touching the black dot. What happens?
5. Describe the little branched object that came out when you touched it with a needle. These are the pollen saddle-bags and each bag comes from a pocket at one side of, and above the long pocket. Do these saddlebags cling to the needle? Look with a lens at some of the older flowers, and see if you can find the pollen-bags protruding from the long pocket. See if you can find how the long pocket is a passageway to the stigma. To see how the little saddle-bags were transported, watch a bee gathering nectar. Describe what happens.
6. Since the flowers bend over, how must the bee hold on to the flower while she gathers nectar from the horns? As she turns around, would she naturally pull out some of the saddle-bags? Catch a bee in a collecting tube and see if her feet have upon them these pollen-sacs. After gathering these pollen-sacs upon her feet, what happens to them when she visits the next flower? Is the opening of the long pocket like a trap to scrape the sacs off? Can you find on milkweed flowers any bees or other insects that have been entangled in these little traps and have thus perished? Try the experiment of drawing a thread into one of these traps and with your lens see if the opening closes over it.
7. How many kinds of insects do you find visiting the milkweed flowers? Can you detect the strong odor of the flowers? Why must the milkweed develop so many flowers and offer such an abundance of nectar?