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Transcript of Tropism Project
March 26, 2011 Different Types of Tropisms and How They Work... Gravitropism Phototrophism Thigmotropism Hydrotropism [What's a Tropism?] According to our handy-dandy textbook, a tropism is a response of a plant to an environmental stimulus. Merriam-Websters was a tad more detailed with: involuntary orientation by an organism or one of its parts that involves turning or curving by movement or by differential growth and is a positive or negative response to a source of stimulation.
In other words, a tropism is a plant's reaction to a certain stimuli by movement, usually.
There are two main types: negative and positive. Negative tropisms consist of actions towards opposite directions. For example, plants grow upwards, despite the pulling downwards. Positive, on the other hand, is a tropism that causes the plant to move towards it. An example of this is a plant leaning towards a light source as its position changes. How do Tropisms Work? When the plant is exposed to the external stimuli, changes begin to occur almost immediately depending on the type, of course. Basically, a certain hormone in each from a group called "auxin" triggers a series of events that change the movement of the cell and direction of growth by altering the cells. With phototrophism, a hormone by the name of auxin, a type of phytohormone, is the chemical responsible for a plants movement. It releases protons in order to decrease pH in the dark side of the plant. This creates a very acidic environment that actives elastins. These expansins weaken the cell wall structure and because of this, cells swell and tilt the plant toward the light source. Keep clicking for a more precise description for the 4 types of tropisms I've used today. Stimuli: Gravity Stimuli: Light Stimuli: Water Stimuli: Touch or Contact Here's phototrophism in action: Here again, auxin released to allow plants (usually the vines) to climb onto walls, fences, etc. by the cells being touched. This lets certain cells elongate so a cell can grow around the object. Watch the plant spin around the stick using thigmotropism. Also known as geotropism, gravitropism focuses on roots' downwards growth due to gravity. (The root growth is positive tropism, however, the stem shooting upwards is negative gravitropism because of the growth opposite to the pull of gravity.) Here, auxin is relocated to the bottom of the root cells, dictating downward growth. Here's gravitropism in action! Through the magic of auxin, plants move towards both higher concentrations of humidity and large amounts of water. This is one of the least influencial of all tropisms and has a surprizingly lesser effect on roots than gravity, by a lot. Unfortunately, it has not been properly tested very well by scientists as a result of the fact that hydrotropism is found in environments with many other effectful characteristics. http://www.tutorvista.com/content/biology/biology-iv/plant-growth-movements/paratonic-movements.php http://virtualastronaut.tietronix.com/textonly/act25/text-plants.html Negative: Positive: http://www.sciencephoto.com/images/download_lo_res.html?id=670038195 http://botit.botany.wisc.edu/Resources/Botany/Root/Gravitropism.jpg.html http://marksteinmetz.photoshelter.com/gallery-image/Natural-History-Misc/G0000q4VyoZIxjBA/I0000mr5xheRP5gI MORE on
Phototropism http://scienceaid.co.uk/biology/plants/plantgrowth.html Here's a great diagram explaining the vital role auxin plays in tropism. http://www.tutorvista.com/content/biology/biology-iv/plant-growth-movements/paratonic-movements.php Real-life Experiment Go to the website to find an experiment perfect for comparing the power of phototropism to gravitropism: http://www.cals.ncsu.edu/nscort/outreach_exp_phototrop.html Can also occur in fungi! A plant bends within 30 minutes to an hour after receiving blue light. http://www.biology-blog.com/blogs/permalinks/7-2007/molecular-mechanics-of-phototropism.html