Life Cycle - Plants (6A)
Post Lab 

  • Exploring the uses of auxins.
  • Comparing different methods of weed control. 
  • defoliant
  • herbicide
  • empty bottles of different weed killers

Students investigate weed killers.


Learning more about how plants grow can help scientists find ways of controlling unwanted weeds. Scientists have found that growth hormones, like the ones that were studied in lab, can be artificially synthesized. A plant hormone (growth regulator) is defined as an organic substance produced by a plant that when acting in very small amounts regulates plant physiological processes. Three hormones have been isolated, auxins, gibberellins, and kinetin. We will only deal with auxins, which are the hormones that were discussed in lab.

Scientists have also found that larger amounts of hormones are toxic to plants and can kill certain plants with no adverse reaction to other plants. Some of the synthetic growth regulators affect certain types of plants more than others. Spraying an area with these growth regulators to affect only certain plants is called selective spraying. This is different from chemical weed killers that usually kill all vegetation because they are toxic to the individual plant cells. Many of these weed killers are generally used because they are cheaper, including ferrous sulfate, sulfuric acid, arsenical preparations, and aromatic fractions of fuel oils. Some plants may have a "tougher" outer skin, thereby not allowing the material to penetrate and hence not killing them, but for those cells that it does penetrate, it kills them by upsetting their photosynthetic process. Hormone herbicides are selective on the other hand and tend to affect broad leaved plants (dicots) more than narrow leaved monocots. Grasses on our lawns tend to be monocots, and the "weeds" tend to be dicots. A product on the market like 2,4 D-dichlorophenoxyacetic acid is a hormone herbicide which can eradicate plants like dandelions and morning glory. A chemical weed killer will kill the tops, but not the roots, so the weed will come back. Yes it is cheaper to buy the weed killer, but the hormone herbicide will do a better job.

There are many other applications for these growth hormones. Some of the synthetic herbicides are effective defoliants which are used to take the leaves from the cotton plants before harvesting so the leaves don't plug up the mechanical cotton harvester.

This practical application of weed control was never thought about by the early scientists who were just interested in learning why a plant bends toward light.

  1. Go over the information provided above.
  2. Have a few cans of weed killer, and show students how to read the ingredients.
  3. Ask students to go home and list the ingredients in any weed killer they may have at home. When they come to school go over the ingredients and see if you can identify some of the ingredients. If you are not sure of whether the ingredients are hormones or not, you probably can go to the web sites of the companies and ask them a question. You may also use some of the search engines like and type in the name of the ingredient to find out what it is.

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