Lesson CXXXI

The Milkweed

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.

Observations—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 saddle-bags 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?