A Little Bit More
Most of us have had plants in our homes and noticed that they grow toward the window or toward the light. We can easily understand how this growth helps a plant once we realize that plants need light for photosynthesis, but exactly HOW this takes place was not so easy to determine.
Scientists have known for centuries that plants grow toward the light; but how they do it remained a secret until the discovery of a plant hormone called auxin in 1926. An early step in unlocking the secret came when Charles Darwin, the world-famous biologist, experimented with oat seedlings in an attempt to understand phototropism (photo-light; trop-turning) or the directional growth of a plant in response to light. He eventually determined that the elongation of oat cells and their consequent bending toward the light were controlled by something – he didn’t know what - in the tip of the seedling.
Sure enough, in 1926, Netherlander Frits Went discovered the first known plant hormone, which he named “auxin”, and established its role in plant phototropism. Auxins are most evident in seedling tips, but are also found in growing stems, young leaves, and even root tips. Research has shown that auxin moves away from light to the darkest side of a stem, where its presence stimulates the cells to elongate. The amount of cellular elongation is directly proportional to the concentration of auxin at a given site. So, in reality, plant stems actually grow toward light by growing away from the dark!
Activities
Windowsill Magic
Objectives: See auxin in action
Materials: Potted plant or seeds
The sun moves across the sky from east to west so plants are usually not forced to grow in a single direction to maximize their sun exposure. However, there are many instances in nature when a given plant is not exposed to sun evenly from all sides. Plants growing in the shade of other plants, the shade of a building, or even on a steep cliff or hill may exhibit “lopsided” phototropism to a greater or lesser degree depending on their situation.
Here is a simple enough exercise, but one that’s worth doing. Now that your students know what causes a plant to grow in the direction of light, seeing it in action should help to reinforce this information.
Exercise #1A
1. Take one of your houseplants (this works best with fast-growing plants – not a cactus, for example) and make an arbitrary mark on the pot. You determine whether you want the mark to indicate the “dark side” or the “light side” of the plant.
2. Place the potted plant on a windowsill, or other area with a unidirectional lighting source. The greater the difference between the light-side and dark-side, the greater the growth difference too. Be sure your mark is properly lined up with the side you determined.
3. Once your potted plant has been situated, keep your eye on the mark to ensure that it remains stationary –you don’t want the plant to be rotated at all during this exercise.
4. Over time (this will depend on the species you’re growing) you should begin to notice unidirectional growth of the plant as it leans toward the source of light. Obviously, the longer you allow this setup to continue, the greater (up to a point) the plant’s bending.
Exercise #1B
1. If you don’t have any potted plants, or would rather not deal with them, you can use some seeds – plain old birdseed works well for this - for this exercise. Plant your seed in a shallow container with soil then water and wait for the seeds to sprout. Your setup should be kept in an area that gets adequate, unidirectional light. Keep in mind too that the seeds will sprout more quickly and grow more quickly if kept on the warm side.
2. Set up your seed container with a mark as in exercise #1A so you can determine at all times which is the dark or light side. Once the seeds sprout, their fast growth will make it especially easy for you to see phototropism at work.
Here are some things you can do with a second setup to further students’ understanding:
1. Slowly rotate a plant or tray of plants daily (maybe a sixth of a turn each hour) on the windowsill so it makes one full rotation dawn to dusk. How does the plant respond?
2. Place a plant in a totally dark room or cover with a container that does not allow light in, and see what happens over time.
3. Place a potted plant on its side as in the illustration and see what happens over time.
Key Concepts
Growth and Development, Structures and Functions
Questions
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Posted by Melissa W on September 04, 2006 at 06:01 PM
Q: Why do some trees grow sideways?
A: Good question Melissa. There are a couple of reasons why a tree might grow sideways that come to mind.
If a tree is growing on a hillside, there are times when the soil on which it is growing begins to slide down the hill and the tree begins to tip. One can often tell that this is happening if they notice that the branches come out from the tree in a normal fashion and then bend at a rather sharp upward angle. This would tell you that the tree started growing upright and then as the trunk tilted, the branches began growing differently to reach maximum sun.
Of course, trees need sunlight for growth so they will often grow at an angle if they're being shaded by a larger tree. This can cause a tree in the forest to grow a bit differently than other larger trees that surround it and make the tree end up at a strange angle. Once a tree gets started at a strange angle it may straighten out later, but the original part will remain crooked or tilted.
Hope this helps you out.
Posted by Daisy Guaman on March 21, 2006 at 04:29 PM
Q: How big can a plant be?
A: Each species (kind) of plant has an average size but there are always some individuals that are larger and some that are smaller than that average. The largest plant that I know about is the Giant Sequoia that grow in California - these trees grow to 300 or 330 feet in height. In the East, trees that grow to about 100 feet are considered tall but there are some that reach greater heights. The American Sycamore may achieve 150 feet, Tulip Trees may reach 175 feet, and the American Elm may reach 125 feet. Thanks for your question.
Posted by monica hinojosa on March 20, 2006 at 11:13 PM
Q: does plants need darkness to grow?
A: Plants don't need darkness to grow. In fact, plants growing in Alaska, for example, where summers have very long daylength, grow extremely well with the extra hours of sunlight.
Plants can grow in darkness and that may make you "think" they grow better in the dark. If a seed germinates (sprouts) under a rock, for example, it will use all of the seed's stored energy to grow as much as it can to reach the light. If you lift a rock that has sprouted seeds beneath, you may see that they're really long and seem to have grown well -they'll also be pale looking too. Actually, they are not doing well and if they don't soon get to the light, will use up all of their energy and die. Seeds that germinate and quickly reach sunlight may not be as long as those under the rock but they'll be healthy and able to use that sunlight to make food for further growth. Hope this helps.
Posted by Alvaro Ferrufino on January 23, 2006 at 04:25 PM
Q: How can plants grow int he dark?
A: Plants are able to extend their growth in the dark ONLY by using food reserves they already have. A plant in darkness can't make its own food at all but if it has enough stored, it can grow for a time. It's not unlike you if you were to stop eating - you'd continue to grow until all of your own fat reserves were used up. Hope this helps.
Posted by latonya on December 06, 2004 at 12:28 PM
Q: does plants grow better in light or dark
A: To answer this question, I need to know exactly what you mean by "grow better". Are we talking simply about growth (elongation) of a stressed plant or are we talking about overall growth of a healthy plant? A plant that is kept in the dark will put all of its energy into lengthening as it attempts to reach the sun. You may have lifted a board in your yard at some time and found a plant beneath that has grown quite a bit and is long and straggly. That plant isn't considered a healthy plant and although it may be larger than surrounding plants that are in the sun, certainly can't keep this up for long before it dies. So, a healthy plant grows best in the light, not the dark.
Posted by Michelle on May 20, 2004 at 10:00 PM
Q: Why do some seeds (like lima beans) grow quicker (and better)in the dark?
A: Good observation! Although it may appear that a given seed (plant) is growing better in the dark, it really isn't. Plants need light to make their own food and grow normally however when in a dark or very light-limited situation, the plant uses all of its energy into getting to the light. So, germinating (sprouting) seeds often grow quickly and extensively as they "search" for enough light in which to grow. If you've ever looked under a board in the spring you may have seen that seeds there have produced very pale, lengthy plant stems that were unable to reach the light and just sort of curled around. Once a light source is reached, a plant can then begin functioning in a normal fashion, making food and growing.