what causes phototropism, gravitropism, and thigmotropism?
821130
"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 (as opposed to nastic movements which are non-directional responses). 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. The word tropism comes from the Greek trope ("to turn" or "to change"). Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to light) and may be either positive (towards the stimulus) or negative (away from the stimulus).\n\nTropisms are typically associated with plants (although not necessarily restricted to them). Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response)."\nFrom Wikipedia, http://en.wikipedia.org/wiki/Tropism\n\nTropisms evolved over time in each species as a product of the evolutionary process of adaptation. The mechanism, i.e. "what causes tropisms", is encoded in the genes (DNA) of each species.\n\nHope this helps!
75824
humans are naturally light-loving creatures. I forget the scientific name, but it's true. you might notice the same attitude if it is cloudy or raining. I forget what the disorder is called, but my mom has the same thing. some of us are more sensetive to the seasons than others in the fact that we can react more dramatically when the sun goes down or is not present. I would recommend turning all the lights on and curling up w/ a good book so you can forget that it is dark outside. put a poster or picture in your room that is of a sunny beach filled with palm trees. Brighten up your surroundings. Today it is cloudy and I was feeling kinda low, so I went and tanned for a few minutes and I feel ten times better. Its all about changing your surroundings to compensate for your grumpies. good luck!
1013494
I think it happens from an excess of silicon and suns rays. Oh, and drugs, lots of drugs. I don't know, I am just hanging around here to see what kind of answers you get.
675260
t has been suggested that high levels of axial aberration or specific patterns of peripheral refraction could play a role in myopia development. Possible mechanisms involving high levels of retinal image blur caused by axial aberrations include form deprivation through poor retinal image quality in distance vision, enhanced accommodative lags favouring compensatory eye growth, and an absence of adequate directional cues to guide emmetropization. In addition, in initially emmetropic eyes, hyperopia in the retinal periphery may result in local compensatory eye growth, which induces axial myopia. Evidence in support of these ideas is reviewed and it is concluded that, for any fixed pupil diameter, evidence for higher levels of axial aberration in myopes in comparison with other refractive groups is weak, making involvement of axial aberrations in myopization through image degradation at the fovea unlikely. If, however, some potential myopes had unusually large pupil diameters, their effective aberration levels and associated retinal blur would be larger than those of the rest of the population. There is stronger evidence in favour of differences in patterns of peripheral refraction in both potential and existing myopes, with myopes tending to show relative hyperopia in the periphery. These differences appear to be related to a more prolate eyeball shape. Longitudinal studies are required to confirm whether the retinal defocus associated with the peripheral hyperopia can cause patterns of eyeball growth which lead to axial myopia.
844582
Genetics and environment and stuff... oh and gravity! Unless if the tree is on a steep hill.  :-)
1049181
It's probably stress or caffiene. I take multiple vitamins everyday, and I get eye twitches that drive me nuts! My problem was curbed by cutting down on the caffiene and stress a bit.
1096572
I used to get this a lot when I used to regularly stay awake for 70+ hours in a stretch (I had problems and didn't sleep much).  Basically the way that the eyes work involves two main pigments (often called visual yellow and visual purple) which are photosensative (like camera film) and they constantly renew themselves, if you have not slept enough sometimes the production of these visual pigments slows down a bit leading to shapes and specks in th peripheral vision.  \nHave a great day and get more sleep!
317957
light (or sunlight)\n\nlight makes polar bears clear hair shafts appear white\nlight makes your eyes water\nlight first thing in the morning...guy gets out of bed to pee and girl combs her hair\nlight makes celebrities look stupid since it shed light on their antics\nlight makes normal people look like celebrities by putting them in the spotlight\npancakes brown, they are no longer light\nexpose champagne to light and it bubbles\nif you squeeze a lightbulb, it pops\ncan't figure last line but know that staring at sunlight is very bad for your eyes
783533
Sun is always good. The reason of leaves curling could be genetic defect of lack of potassion
798451
I would expect light exposure variance has much to do with it. Look at how it effects circadian rhythm, for instance. Probably begins a cascade of varying hormonal changes.
190786
Phosphemes \n\nDefinition from Wikipedia:\n\nA phosphene is an entoptic phenomenon characterized by the sensation of light from mechanical, electrical, or magnetic stimulation of the eye's retina, or from random firing of cells in the visual system, rather than from light.\n\nThe most common phosphenes are pressure phosphenes, caused by rubbing the closed eyes. The pressure mechanically stimulates the cells of the retina. Experiences include a darkening of the visual field that moves against the rubbing, a diffuse coloured patch that also moves against the rubbing, a scintillating and ever-changing and deforming light grid with occasional dark spots (like a crumpling fly-spotted flyscreen), and a sparse field of intense blue points of light. Pressure phosphenes can persist briefly after the rubbing stops and the eyes are opened, allowing the phosphenes to be seen on the visual scene. Christopher Tyler (1978) has published some good drawings of pressure phosphenes.\n\nAnother common phosphene is "seeing stars", from a sneeze, from a blow on the head, or from low blood pressure (such as on standing up too quickly or prior to fainting). It's possible these involve some mechanical stimulation of the retina, but they may also involve mechanical and metabolic stimulation of neurons of the visual cortex or of other parts of the visual system.\n\nBTW, it was pure luck that I knew that- just finished a course on cognitive neuroscience!
61665
Trees are phototrophs, they produce their own food by using carbon dioxide, water, and sunlight.
557413
(m)\n\nThe coleoptiles of grasses (like that of oat, the Avena - coleoptile) are popular test objects of plant physiology. A growing coleoptile that is illuminated unilaterally does grow towards the light source. This behaviour is common among plants and is known as phototropism. C. DARWIN (assisted by his son Francis DARWIN) attributed in his work "The Power of Movement in Plants" (1880) a decisive function in the recognition of a light stimulus to the coleoptile’s tip, and he observed that the actual bending occurs in a zone below the tip. He concluded that a transmission of impulse had to take place in the tissue. Studies of plant anatomical nature revealed that the growth towards light is caused by an elongation of the cells at the side that is shielded from the light. The phototropic reaction does not happen if the coleoptile’s tip is removed, though it can be induced again by the replacement of the tip. This indicates the existence of a substance that is spread from tip to bottom (basipetal direction) and that causes the elongation. \n\nThe Danish botanist P. BOYSEN-JENSEN interrupted the assumed substance flow by inserting a mica sheet into the shielded side thus separating the coleoptile’s tip from the tissue below (1913). The water-impermeable sheet interrupted the phototropic reaction. Consequently occurs no transport of the effector around the small tile. The phototropic reaction remained intact when the mica sheet was inserted into the illuminated side or along the coleoptile’s vertical axis. \n\nIn the late twenties was the material nature of the effector finally proved by the Dutch plant physiologist F. WENT. He assumed that a substance that flows from tip to bottom should also flow through a small cube of agar. In order to test his assumption did he place cut coleoptile tips with the cutting side on top of small cubes of agar. Some time later did he remove the tips and placed the agar cubes that he believed to contain the effector onto the decapitated coleoptiles
43911
there could be many causes such as environment factors, stress, diet or even allergy to certain substances...
755185
Mitochondia, because they transform glucose (sugar) to energy.\nChloroplast, because they transform sun energy to glucose.
806969
Hmmm change in gravity?\n\nYou need to send a free sample of hair dye to NASA so they can sent it out in space and test it for you.
1104638
Good answer owlwings - the cause of nystagmus is unknown - 1 theory is the brain is constantly searching for the area of highest visual acuity on the retina and is constantly scanning it.  People with nystagmus generally have a reduced visual acuity (even if you take the nystagmus into account). Nystagmus is also split into beat-nystagmus and pendular nystagmus.  Beat nystagmus is where the speed of the movement is greater in one direction than the other so the eye looks as if it is beating in one direction.  Pendular nystagmus is where the eye moves back and forth (or up and down) equally in either direction.  The condition is also more highly prevalent in albinos.  Finally you can get torsional nystagmus where the eye rotates along the axis of the pupil - but this is very rare!