Pondweed

Teacher's Story

HE study of any plant which has obvious limitations as to where it may grow should be made a help in the study of geography. Pondweed is an excellent subject to illustrate this principle; it grows only in quiet beds of sluggish streams or in ponds, or in the shallow protected portions of lakes. It has tremendous powers of stretching up, which render it able to grow at greater depth than one would suppose possible, often flourishing where the water is from ten to twenty feet deep. Often, when the sun is shining, it may be seen like a bed of seaweed on the bottom. Its roots, like those of most water plants, have less to do with the matter of absorbing water and nourishment than do the roots of land plants, one of their chief functions being to anchor the plant fast; they have a firm grip on the bottom; and if pondweed is cut loose, it at once comes to the surface, floats helplessly on its side, and soon dies.

The stem is very soft and pliable and the plant relies entirely on the water to keep it upright. A cross-section of the stem shows that its substance is spongy, with the larger open cells near the outer edge, thus helping it to float. The leaves are two or three inches long, their broad bases encircling the stem, their tips tapering to slender points. They have parallel veins and ruffled edges. They are dull olive green in color, much darker than the stems; in texture they are very thin, papery, and so shining as to give the impression of being varnished. No land plants have such leaves; they remind us at once of kelp or other seaweeds. The leaves are scattered along the stems, by no means thickly, for water plants do not seem to need profuse foliage.

In blossom time the pondweed shows its real beauty. The stems grow and grow, like Jack's bean stalk, and what was a bed of leaves on the pond bottom suddenly changes into a forest of high plants, each one standing tall and straight and with every leaf extended, as if its stems were as strong and stiff as ironwood; but if a wave disturbs the water the graceful undulations of the plant tell the true story of the pliant stems. There is something that arouses our admiration when we see one of these pondweeds grown so straight and tall, often three or four yards high, in order to place its little, greenish-brown flower-head above the water's surface. We have spent hours looking down into such a submerged forest, dreaming and wondering about the real meaning of such adaptations.

1. Flower of a pond-weed enlarged, early stage. 2. Same at later stage.

Although the stem is flexible, the somewhat curved, enlarged portion of it just below the flower-head is rigid; it is also more spongy than the lower part of the stem and is thus fitted to float the flower. The flower itself is one of the prettiest sights that nature has to show us through a lens. It is a Maltese cross, the four reddish stigmas arranged in a solid square at the center; at each side of this central square is a double-barrelled anther, and outside of each anther is a queer, little, dipper-shaped, green sepal. When the anthers open, they push away from the stigmas and throw their pollen toward the outside. There may be thirty or more of these tiny, cross-shaped flowers in one flower-head. In the bud, the cup-shaped sepals shut down closely, exposing the stigmas first, which would indicate that they ripen before the pollen is shed. The pollen is white, and is floated from plant to plant on the surface of the water; often the water for yards will be covered with this living dust.

Method—As this is primarily a lesson that relates to geography, the pondweed should be studied where it is growing. It may be studied in the spring or fall, and the pupils asked to observe the blossoming which occurs in late July. After the pupils have seen where it grows, the plants themselves may be studied in an aquarium, or by placing them in a pail or basin of water. There are confusing numbers of pondweeds but any of them will do for this lesson. The one described in the Teacher's Story is probably P. perfoliatus. 

2. Has the pondweed a root? Does the pondweed need to have water carried to its leaves, as it would if it were living in the air? What is one of the chief uses of the roots to the pondweed? Break off a plant, does it float? Do you think it would float off and die, if it was not anchored by its root?

3. Compare the stem of pondweed with that of any land plant standing straight. What is the chief difference? Why does the pondweed not need a stiff stem to hold it up? Cut the stem across, and see if you can observe why it floats.

4. Examine the leaves. Are all of them below the surface of the water? If some float, how do they differ in texture and form from those submerged? How are they arranged on the stem? Are they set close together? What is the difference in texture between its leaves and those of the jewelweed, dock or any other land plant? If any leaves project out of the water are they different in form and texture from those submerged? Sketch the leaf, showing its shape, its edges, and the way it joins the stem.

5. How far below the surface of the water does the pondweed usually lie? Does it ever rise up to the water's surface? When? Have you ever noticed the pondweed in blossom? How does the blossom look on the water? Can you see the white pollen floating on the surface of the water? Look down into the water and see the way the pondweed stands in order to float its blossoms.

6. Study the blossom. Note the stem that bears it. Is the part that bears the flower enlarged and stiffer than the stem below? Do you think that this enlarged part of the stem acts like the bob on a fish-line? Examine a flower cluster with a lens. How many flowers upon it? Study one flower carefully. Describe the four stigmas at the center. Describe the anthers arranged around them. Describe the sepal which protects each anther. When the anthers open do they discharge the pollen toward or away from the stigmas?

7. What happens after the flowers are pollenated? Do they still float? What sort of seed-capsule has the pondweed? Do the seeds break away and float?