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Alive & KickingThe Cleverness of Plants |
Plants are clever.
And if you have a penchant for horror movies (“Th e Invasion of the Body Snatchers” comes to mind) where plants accomplish strange and wonderful things while taking over the world, or if there is ivy, at least, growing up the side of your house, you already know this!
In real life, plants accomplish strange and wonderful things every day, if we only think to look at them.
Plants on a windowsill grow toward the light. In a similar way, forest trees grow toward an opening in the forest canopy or toward light along a dark forest’s margin. This is an example of Phototropism: “photo” meaning light, and “tropism,” a response to a stimulus.
The way it works is simple: hormones produced by growing cells cause young cells to increase their length. Strangely, direct sunlight destroys these hormones, slowing elongation on the sunny side, thus resulting in more growth on the plant’s less-lit side, turning the plant toward the sun.
This mechanism aids the light-competing plant to gain more energy for growth.
It also explains why larger tree leaves are more often found on branches in deeper shade, why daffodil stems are longer on the shady side of the house, and why grass grown in deep shade is usually taller and less sturdy.
This growth mechanism benefi ts the plant by providing more surface area for light absorption; more energy can thus be captured in a shady environment.
Plant leaves and green stems have millions of microscopic openings: thousands per square inch. These leaf openings (stomata) allow carbon dioxide from the air, water from the roots and other materials to pass both ways between the leaf interior and the atmosphere. These pore openings are adjustable like blinds on a window: surrounding an individual stoma (pore) are two guard cells, each shaped like a jellybean. The inside guard cell wall is thicker than the outside cell wall. As the guard cells gain moisture, the cells become more arched and the stoma opens wider.
If a plant wilts, water is lost from the guard cells; the cells become less arched and the stoma closes. Some leaves have stomata on both sides; some leaves, typically waxy surfaced leaves like holly, only have stomata on the underside.
Try your luck with a magnifying glass. If it is strong enough, you will be able to see the stomata. At 100x, the stomata will be obvious. Dry the leaf and look again. What do you think will have happened?
Try It Yourself
A plant’s responses to light and moisture are very important to a plant’s success. A fun but simple experiment for children that demonstrates Phototropism is to plant two or three bean seeds 1” deep in a paper cup nearly filled with garden soil. Keep the soil moist, but not sloppy wet.
The successful plant will sprout and grow two leaves. Soon after, a bean runner sprout will form. Find or make a light-tight box about the size of a shoe box (preferably with a removable lid), but tall enough to not crush your plant. Make a two-inch hole in one end of the box. Place the cup with its plant or plants in the box at the opposite end of the hole. Put the lid on the box, but be sure to keep the soil moist. Place the box where some light from a window can enter the hole and see what happens!
Guest columnist John Cooke taught high school biology for 30 years and is pleased to share his insights with our readers.