Plants respond to various stimuli through tropisms such as phototropism, gravitropism, and thigmotropism. Phototropism allows plants to grow toward light for photosynthesis, while gravitropism orients shoots upward and roots downward using gravity. Thigmotropism helps roots navigate through soil. Plants also respond to desiccation by opening and closing stomata to regulate water loss. Five major plant hormones - auxin, abscisic acid, cytokinin, gibberellins, and ethylene - influence growth, development, and environmental responses. Like humans, plants have an internal circadian rhythm that uses environmental cues like light and temperature to "set" and maintain a

1. Adaptations of Plant Sensory Systems, Physiological Regulation and Environmental Response -How do plants respond to stimuli? -What are the major plant hormones? -How do plants tell time?

2. How do Plants Respond to Stimuli? Tropism: “An 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” -Merriam-Webster Plants respond to different stimuli such as light, gravity, touch and desiccation in ways that benefit their survival.

3. Plant Response to Light Phototropism: The growth of a plant in the direction of a light source. Plants grow towards the light for the maximum amount of photosynthesis to occur. Caused by the hormone Auxin http://www.doctortee.com/dsu/tiftickjian/bio100/cell-communication.html http://www.gettyimages.com/detail/686097-001/Stone

4. Plant Response to Gravity Gravitropism: The plants growth direction in response to gravity. Plants use gravity to guide their growth direction in the absence of phototropism. Plants sense of gravity allows shoots to grow straight up and roots straight down. http://www.darienps.org/teachers/otterspoor/botany/tropisms/

5. Plant Response to Touch Thigmotropism: A plants growth response to touch. Roots use thigmotropism to find the path of least resistance. Caused by the plant hormones Auxin and Ethylene. http://visualsunlimited.photoshelter.com/image/I0000cjwWdVtlI5M

6. Plants Response to Desiccation Desiccation: A period of extreme dryness. Like the human body, plants will begin to shut down various parts of their body to survive. Plants will open and close their stomata to regulate the amount of water loss that occurs. http://www.labtechindia.net/stomata-1657.html

7. Five Major Plant Hormones There are five major plant hormones that are recognized in modern plant biology The list includes: 1)Auxin 2)Abscisic Acid 3)Cytokinin 4)Gibberellins 5)Ethylene These hormones are important in the growth and well being of the plant system.

8. Five Major Plant Hormones 1) Auxin: One of the most important plant hormones, this chemical helps in the growth of plant parts and the cell elongation process. Synthetic auxin is commercially used as a pesticide. Most common auxin is indoleacetic acid (IAA), however there are other types of auxin. The hormone responsible for many tropisms. http://www.pherobase.com/database/compound/compounds-detail-3-indoleacetic%20acid.php

9. Five Major Plant Hormones 2) Abscisic Acid:brings dormancy to a plant in certain conditions, and causes a plant’s stomata to open or close. Was originally thought to be the cause of abscission. 3) Cytokinin: Regulates plant growth, cell division, and lateral bud growth in plants containing flowers. Found mostly in plant roots. Works with the hormone Auxin.

10. Five Major Plant Hormones 4) Gibberellins:Regulates growth and developmental stages in the plant. Found in meristems and roots of plants Commercially used to increase the size of fruits and plants. 5) Ethylene:Promotes abscission and the growth of fruits. Abscission occurs when a plant need to separate its self from an appendage. Too much ethylene causes fruits to rot.

11. How do Plants Tell Time? Like humans, plants have a type of internal biological clock that is part of the Circadian Rhythm. The Circadian Rhythm is defined as a physiological cycle of about 24 hours that is present in all eukaryotic organisms and that persist even in the absence of external cues. These external cues such as light, temperature, and humidity are not what tell the plant what to do, they are used only to “set” the internal clock to a certain time period.

12. Circadian Rhythm Thought to work in three phases. Phase 1: An input “sets” the internal clock. Phase 2: The plant modifies proteins in the central oscillator Phase 3: The oscillator outputs the proteins to various parts of the plant body. http://howplantswork.wordpress.com/2009/01/25/how-plants-tell-time/

13. Sources -2008. Science Clarified. http://www.scienceclarified.com/Oi-Ph/Phototropism.html. January 20, 2011 -2004. Space Garden. Planet LLC. http://www.spacegarden.net/downloads/Gravitropism.pdf. January 21, 2011 -Vartanian, Steffan. 1997. Thigmotropism in Tendrils. Kenyon College. http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm. January 21, 2011 -Oliver,Melvin, Velten, Jeff, Mishler, Brent. 2005. Integrative and Comparative Biology. Oxford Journals. http://icb.oxfordjournals.org/content/45/5/788.full. January 21, 2011. -Moneo, Marta. 2004. ESPERE-ENC. http://www.atmosphere.mpg.de/enid/b53941b75a5c58a972d7e308d37e88c3,0/Food___Climate/more_pi.html. January 22, 2011 -2005, Biology Online, http://www.biology-online.org/11/10_growth_and_plant_hormones.htm. January 24, 2011 -Urry, Cain, Wasserman, Minorsky, Jackson. 2008. Biology. 8th Edition. Person. See page 838-839. -Gardner, Michael, Hubbard, Kathy, Hotta, Carlos, Dodd, Tony, Webb, Alex. 2006. How Plants Tell Time. PubMed Central. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1479754/. January 24, 2011.