TEACH!: Unit 8, Lesson 2.

Supplies

• Varies by activity

Background

Dave Crawford, a professional naturalist for 30 years with Minnesota State Parks, developed these activities based on his years of working with visitors. They are gimmicky demonstrations that, by themselves, may entertain a visitor. However, with just the right, succinct interpretation, they can teach a useful and memorable tidbit of natural history.

Activities

1. Str-r-r-etch. Take a dogwood leaf between thumb and forefinger of left hand. Place thumb and forefinger of right hand tightly against left, and carefully pull (not tear) leaf in half. Separate halves by a quarter inch or so and let go of one half. It will hang suspended (magically?) from the other half.

Why does it do this? Dogwood leaves have elastic veins that can be stretched up to a half inch before breaking. This is actually a useful trick; since it gives positive verification that you do indeed have a dogwood leaf. Viburnums (highbush cranberry, arrowwood, and nannyberry) can also be stretched in this way, but not nearly as far. What’s not clear is what advantage having elastic veins provides for a dogwood leaf, if any.

2. Motorized twig. Pop a resin blister on the trunk of a balsam fir tree and dip the tip of a small (1-inch) twig in the resin. Place the twig in water. It will move across the surface under its own power.

Why does it do this? Resin, being less dense than water, floats, but also diffuses to form a thin layer, making it spread away from the twig, and the equal and opposite reaction propels the twig away. Another behavior you might see is that a resin-dipped twig placed in slow-moving water may turn and move against the current. The wide patch of expanding resin acts like the feathers on an arrow, resisting the current more than the slender twig, causing the current to push against the resin patch harder than it pushes the twig.

3. Squeak! Cup hands together, fingers interlaced. Place a fairly wide, rigid grass leaf parallel to thumbs and between them and blow over it to produce an obnoxious squealing sound remotely resembling the call of a green heron.

Why does it do this? The grass blade, once properly adjusted, acts like the vibrating reed in a clarinet or saxophone, producing a sound that resonates in the space between your cupped palms.

4. Biological nail file. Find a scouring rush, preferably a dark green one (it will be more rigid). Rub a fingernail edge across (not along) the striations on the stem. You’ll leave nail filings behind.

Why does it do this? As a form of physical protection, scouring rushes, also called horsetails, have an outer surface coated with tiny beads of silica in long lines. This plant was used by cabinetmakers for finish-sanding fine furniture because of its extremely fine-grained abrasiveness.

5. Tiny javelins. Throw a handful of porcupine grass seeds at a towel. Observe their stable, direct flight and the way they stick, looking much like porcupine quills.

is sharp and covered with reflexed hairs which allow the seed to penetrate soil and even leaves of other plants, but keep it from coming loose. The long tail twists and untwists with changes in humidity, drilling the seed into the ground. You might try making a hygrometer with one, or have races using a hair dryer and plant mister to see who can get a seed to plant itself the quickest.

6. Natural Velcro. Try sticking tick clover or tick trefoil seeds to clothing, or use the leaves and stems of sticky or rough bed straw. Note that there are no visible spines (unlike sandbur grass and burdock), but it sticks anyway.

Why does it do this? Humans didn’t invent the concept of Velcro. Plants have been using microscopic, hooked hairs to adhere to surfaces for thousands of years. Tick clover and tick trefoil seeds can hitch rides to new locations, while bedstraw may be uprooted wholesale, tagging along on passing animals, and dropping seeds along the way. Use a microscope or magnifying glass to inspect the hairs that do the trick.

7. Ballistic seeds. Ripe jewelweed pods will explode at a light touch. Nifty!

Why does it do this? The pod is composed of elements that are under stress that are only lightly held in check. The smallest deviation from this delicate balance results in eruption. The swollen, ripe pods can be triggered by the touch of a leaf, splash of a raindrop or bump by the wind or human hand. The pod seems to explode as the outside splits open and quickly curls back, shooting the seeds outward. This ejects seeds some distance, thus spreading the plant to new areas. The seeds are more or less round in shape, so they’ll potentially roll a ways once they hit the ground, increasing the dispersal distance even more. The distance jewelweed can spread in a native plant garden between one year and the next is no thing short of amazing.

8. Always look up. Place a wood tick on your hand. Hold your hand with fingers pointing up and observe the tick trudging wearily upward. When it gets close to the summit, invert your hand, and the tick will reverse direction. Repeat. Odds are you’ll get bored before the tick does.

Why does it do this? Ticks are programmed to climb upward on contact with a warm-blooded host, which is why so many end up attached where waistbands or hairlines block their upward path. Some ticks apparently have faulty programs (or limited patience) so don’t expect this trick to work every time.

Continue to:
Unit 8, Lesson 3.: Dressing the part: Leave it to beavers.

Go Back to:
Unit 8, Lesson 1: What’s it all about? Developing a theme.