reflect What kinds of things can do work? You probably answered that people do work. Perhaps you also thought of machines that help us do work, like scissors or bicycles. Perhaps you thought of animals in general. Is a lion doing work as it chases an antelope across a field? Is the antelope doing work as it runs away from the lion? What about plants: Can plants do work? Think about this question for a moment, then read on. Plants can exert forces. According to scientists, work happens when a force moves an force: a push or pull object. You may not think plants can do work in this way, because they are literally rooted in place. In fact, plants can move, exert force, and do work. One example of how plants move and exert force is turgor pressure. Unlike animal cells, plant cells have strong cell walls. These walls allow pressure to build up inside the cells when the cells absorb water. This pressure is called turgor pressure. When turgor pressure is high enough in a cell, the cell walls become firm and straight. As a result, the cell becomes stiff, or rigid. A rigid cell or plant is said to be turgid. As water flows into a plant cell, the cell becomes rigid, or turgid (left). As water flows out of a plant cell, the cell becomes flaccid (right). When a plant doesn t get enough water, turgor pressure inside of its cells decreases. Without the force of water pressing against their cell walls, plant cells lose their shape and shrink. As a result, the plant begins to droop, or wilt. A limp cell or wilted plant is said to be flaccid. If water becomes available to a flaccid plant, the plant s cells will become rigid again and the plant will gradually stiffen its leaves and stem will become firm and straight again. This stiffening is motion caused by a force: in other words, work! The force that causes this work, turgor force, is caused by turgor pressure. Turgor force is the reason that plants can remain tall and rigid even when blown by strong winds. 59
what do you think? Take a look at these plants. Which plant is turgid? Which plant is flaccid? In your own words, explain how forces inside the plants caused the plants to move like this. Another way that plants work is geotropism Tropism comes from a Greek word that means a turning. In plants, a tropism is a turning toward or away from a given stimulus. Geotropism (also called gravitropism) occurs when a plant turns toward (positive geotropism) or away from (negative geotropism) the pull of gravity. Gravity is the force that attracts all objects with mass to each other. When a seed germinates and sprouts, the roots always grow down and the stem always grows up. This is true no matter how the seed was oriented when it was placed underground. Scientists still do not completely understand the process of geotropism. At first scientists thought it was a response to sunlight. However, scientists have discovered that geotropism is not a response to sunlight because it happens to seeds buried underground in the dark. Just think you could be the botanist to discover the underlying cause of this phenomenon! No matter which way a seed is turned, the seed s roots grow down and its sprout grows up. Forces resulting from geotropism allow plants to perform a surprising amount of work. Have you ever tried to break pavement with a hammer? You can probably imagine how much work is needed to achieve this. Plants don t seem to mind it, though. A plant s roots and stem can exert a lot of force as they grow in opposite directions. Stems can break through pavement, and roots can create small cracks in boulders and split them open. The geotropism mechanism provides the direction, and turgor force provides the muscle of this work. 60
Sunlight also affects how plants move. Heliotropism (also called phototropism) is the tendency of plants to turn toward sunlight. (The prefix helio- comes from the Greek word for Sun. ) As with geotropism, turning toward sunlight is positive heliotropism and turning away is negative heliotropism. The best-known heliotropes may be sunflowers, but many plants that depend on the Sun for energy exhibit this tropism. Plants exhibit several kinds of heliotropism. The large buds of sunflowers track the path of the Sun across the sky from sunrise in the east to sunset in the west. In the morning, the flower buds swivel back toward the east. The leaves of some plants also orient themselves How can you tell the Sun was behind the photographer who took this picture? Look at the direction the sunflowers are facing. toward the Sun. If grass sprouts on a windowsill, the sprouts will also bend toward the Sun. Other plants bend away from the Sun, however. As the seasons change, some plants bend at different angles to protect sensitive parts of their flowers from the direct sunlight. look out! Plants respond to many stimuli other than light and gravity. Plants can change their growth patterns because of high winds, heat, and cold. Just touching some plants can stunt their growth. Touching one leaf of a mimosa tree with a hot needle causes all the leaves on the tree to droop within a few seconds. Vine plants with grasping tendrils are also stimulated by touch. When a tendril touches something it can cling to, it begins to coil around the object. Venus flytraps are some of the quickest moving plants. Their hinge-like leaves snap shut the moment an insect lands on them. (Left) The tendril of this vine wraps tightly around another plant s stem. (Right) A Venus flytrap snaps shut on an insect in less than a second. 61
Scientists in the Spotlight: Charles and Francis Darwin Charles Darwin is best known for his study of evolution by natural selection, but he also studied heliotropism. With his son Francis, Charles carried out some of the earliest experiments on this phenomenon. The Darwins discovered that grass seedlings would bend toward the Sun only if the tip of the grass blade were intact. If the tip were removed or covered, the plant would grow straight. Darwin hypothesized, correctly, that the tip of the grass is what senses the direction of sunlight and sends a message to cells farther down the stem. In 1926, several decades after Darwin s studies, the Dutch botanist Friedrich Went isolated the chemical messenger that triggers this effect and named it auxin. What do you know? Examples of plants moving and doing work are listed in the left column below. Mechanisms that make these actions possible Charles Darwin studied heliotropism in plants. are listed in the right column. Draw a line from each type of work to the mechanism that makes this work possible. Example of Work A wilted plant rises and becomes erect. The root from a sprouted seed turns downward. A houseplant bends toward sunlight through a window. A shoot from a sprouted seed turns toward the surface. A plant bends toward the ground to protect its flowers from sunlight. Mechanism Negative geotropism Positive geotropism Turgor pressure Negative heliotropism Positive heliotropism 62
connecting with your child Experimenting with Geotropism and Heliotropism In this experiment, you and your child will observe geotropism and heliotropism in plants. Purchase a package of bean seeds at a garden store. Following the planting instructions on the package, plant the seeds in two pots and place the pots on a windowsill. The plants in one pot will be for the study of geotropism and those in the other pot will be for the study of heliotropism. Plant at least six seeds in each pot. In the heliotropism pot, plant the seeds several inches apart with no particular orientation. (In other words, the seeds may point in all different directions.) In the geotropism pot, plant each seed with a different orientation: one should point straight up, another should point straight down, a third should point to one side of the pot, etc. Water the seeds as directed until they sprout. Make sure to expose all the seeds in both pots to the same conditions in terms of how much water and sunlight they receive, as well as the soil type the seeds are planted in. Carefully dig up the sprouted seeds in the geotropism pot and observe how the roots turned down and the sprouts turned up, no matter how the seed was oriented. Discuss the reasons for this with your child. As soon as the heliotropism sprouts break the ground, snip off the tips of two of the sprouts and cover the tips of two other sprouts with little hats made of aluminum foil. As the plants grow, observe which plants bend toward the light. After some sprouts show a definite bending, rotate the pot 180 degrees. As the sprouts continue to grow, observe whether the rotated plants bend back toward the light. Here are some questions to discuss with your child: 1. How do plants exert force? 2. How do plants do work? 3. Identify three stimuli to which plants are sensitive. 4. Give an example of a plant responding to each of the stimuli you named in the previous question. 63