Tropisms Essay

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A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus. Viruses and other pathogens also affect what is called "host tropism" or "cell tropism" in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, or specific cell types within those species. Tropisms are usually named for the stimulus involved. There are many types of tropisms. Five of them are: 1} Thigmotropism is a movement in which an organism moves or grows in response to touch or contact stimuli. The prefix ‘thigmo’ comes from the Greek for "touch". Usually thigmotropism occurs when plants grow around a surface, such as a wall, pot, or trellis This happens in plants such as climbing vines when the tendrils curl around objects as they climb. This is called positive thigmotropism. Negative thigmotropism occurs in the roots of plants, allowing the roots to follow the line of least resistance through the soil. Touched cells produce auxin and transport it to untouched cells. Some untouched cells will then elongate faster so cell growth bends around the object. Redvine tendrils coil upon contact. 2} Thermo tropism or thermo tropic movement is the movement of a plant or plant part in response to changes in temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures. Rhododendron leaves in winter 3) Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant
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