Wikipedia:Reference desk/Archives/Science/2014 October 24

= October 24 =

Duckweed on ponds


I was surprised to see the rather sharp boundary between duckweed and no duckweed: I would have expected the raggedy zone to be several feet at the minimum, or the fountains to cause enough movement to prevent duckweed growth over a far wider area. Any ideas why the raggedy zone is so small, why the fountain water is so suddenly unable to prevent weed growth? Nyttend (talk) 04:32, 24 October 2014 (UTC)
 * The majority of the duckweed plants are single organisms with two or four leaves which reproduce largely by cloning. They are not rooted to the bottom or connected by branches.  The separate plants are not connected, so they float as individual particle on the surface, and are subject to the dynamics any other unstructured surface covering would be.  Unfortunately our main article just mentions the diversity of form in a chart, but there's no description section. μηδείς (talk) 05:03, 24 October 2014 (UTC)
 * might have been that those fountains were recently switched on for some reason after a long period of duckweed growth? only explanation i can think of atm for the sharp boundary  ~Helicopter  Llama~  12:14, 24 October 2014 (UTC)
 * In my experience, the little roots of individuals often get intertwined, leading to clumps of many individuals that are attached to each other. Carefully separating one individual is the hard part. I often see it with fairly sharp boundaries. It's not that the fountain is preventing growth, it's just a steady push, roughly radially uniform, so you get a roughly uniform boundary where the surface flow outward drops below the a certain level necessary to counteract the other sources of random flow. Also, I can't tell for sure from the photo, but it looks like the duckweed forms a pretty continuous mat to the edge of the pond. So the fountain is basically just pushing out until all the duckweed is compacted against the edge. Each little clump pushes on each each neighbor, and most plants will be effectively connected to the edge in this way. From that perspective, little random jostles from the fountain would tend to increase packing density and smooth out the boundary. SemanticMantis (talk) 13:54, 24 October 2014 (UTC)


 * There are five genera of duckweed, and I don't know how many species, they do have quite different morphologies. I don't doubt some "entangle" but the ones I remember from swimming in the lake as a kid didn't do so.  I think its safe to assume in most cases they will act enough like ideal particles confined to a plane (the water surface) and repulsed by the fountain.  What I want to know is, why haven't the maintainers of this pond gotten a pair of ducks with their flight feathers clipped to remove the eutrophication? μηδείς (talk) 23:28, 25 October 2014 (UTC)

Wind turbine on a car
If we setup a wind turbine or several wind turbines on a car, will the turbines harvest more energy than the energy spent to run the car? roscoe_x (talk) 08:34, 24 October 2014 (UTC)


 * No in general, but yes if the engine is turned off.   D b f i r s   08:44, 24 October 2014 (UTC)
 * Also see Conservation of energy, First law of thermodynamics (you can't win), and Second law of thermodynamics (you can't even break even). But, as Dbfirs has pointed out, one could recover some energy when using wind resistance for braking. Unless I'm very much mistaken, it's much more efficient to do that via the wheels, however, and that is indeed done. --Stephan Schulz (talk) 09:45, 24 October 2014 (UTC)


 * Assuming the air relative the moving car is at rest, the wind turbines will feed off the car's fuel. This is because the expenditure of fuel is causing the car to move, and the moving car is causing a relative air current, which is moving the turbines. Thus you're actually consuming more energy than without the turbines. Even in an ideal universe where there is perfect energy conversion, you'll still only break even, creating a perpetual motion machine. Only in the case where you are trying to actively decelerate the car by converting its linear momentum, can the turbines generate energy. Plasmic Physics (talk) 10:37, 24 October 2014 (UTC)


 * Also see automotive aerodynamics; efficiency is improved by reducing air resistance, whereas a wind turbine will massively increase it.--Shantavira|feed me 12:06, 24 October 2014 (UTC)
 * Plasmic Physics' first sentence is key — it's possible for the car to generate more energy with the turbines, but it requires wind, which is independent of the car's engine. The point is that you must have outside input, whether wind or something else, in order to generate more energy than is produced by the engine alone.  After all, a sailing ship (potentially) has no engine at all, but it's still able to generate energy by using the wind.  Nyttend (talk) 12:17, 24 October 2014 (UTC)
 * Indeed. And there are Wind-powered vehicles that show unintuitively good performance - the Blackbird can go downwind faster than the wind! --Stephan Schulz (talk) 12:34, 24 October 2014 (UTC)
 * Fairly sure this has been discussed at least once and I think more than once before. Nil Einne (talk) 13:19, 24 October 2014 (UTC)
 * Yes, probably within the last month. As Dbfirs and others indicate, the only way this might work is to not use an engine, but rather to put sails on the car and possibly capture some of the wind for use with other things. However, as with a sailboat, you would have to tack if the wind is against you, and if there's no wind at all, you're stuck. With actual wind turbines, I think it would only work if the wind is behind you. ←Baseball Bugs What's up, Doc? carrots→ 14:28, 24 October 2014 (UTC)
 * No and no. There is no need to tack, and you can go any direction you like (in theory). See the articles on Wind-powered vehicle and Blackbird I linked below. A simple way to understand at least the plausibility is to mentally separate power generation and movement. While the vehicle stands, its wind turbine is charging a battery (or flywheel, or whatever). Then it uses that stored energy to go wherever it wants. Doing it on the fly and with purely mechanical means is the same thing, only without the dog. --Stephan Schulz (talk) 14:51, 24 October 2014 (UTC)
 * How does a sailboat move toward the wind without tacking? ←Baseball Bugs What's up, Doc? carrots→ 19:46, 24 October 2014 (UTC)
 * A narrowly defined sailboat probably can't (although I can easily do something with a wind turbine and two anchors). But a wind-driven vehicle can drive directly into the wind - and do so at much more than the speed of the wind, too. Check the articles I linked. In particular, a wind turbine can extract energy from the wind and use it to drive wheels even directly against the wind. --Stephan Schulz (talk) 20:47, 24 October 2014 (UTC)
 * Some previous discussions Reference desk/Archives/Science/2014 July 24 (second part), Reference desk/Archives/Science/2010 July 21, Reference desk/Archives/Science/2010 June 11, Reference desk/Archives/Science/2007 October 15, and also a small amount of discussion at Reference desk/Archives/Science/2009 March 31 & Reference desk/Archives/Science/2010 January 25. I think the key points were already covered here by PP, StS and Nyttend and others. Everyone agrees that you it will be dumb to try and recover anything from the movement of air solely arising due to the movement of the car. There is some disagreement over whether you're likely to ever generally get a net gain (compared to any added fuel cost from carrying the turbine and possibly increase drag) when the air is already moving relative to the car, i.e. if there is already wind. Of course this will also depend on how windy the place is. Nil Einne (talk) 16:37, 24 October 2014 (UTC)


 * The point is that the EXTRA air speed that comes about from the motion of the car doesn't help you at all...the extra drag would at best cancel out the energy from the windmill - and in truth, because nothing is every 100% efficient, it'll be much worse.  So the only energy could possibly be harvested from the wind itself.   Now you have a situation where a hypothetical zero-weight, zero-friction, lossless windmill would generate exactly as much net energy if it were bolted to the ground as it would on the car.   In any practical sense, the windmill that's bolted to the ground will generate much MORE net energy than one bolted to the car because the car doesn't have to haul the extra weight around, or suffer the net drag.


 * If this is an electric car (and I guess that's a part of the assumption here) - then it's VASTLY more efficient to buy a windmill, bolt it beside your house and use it to generate electricity that you use to power your house or run your electric meter backwards. The energy you save can then be used to charge your car battery whenever it's parked at home.   You don't have to haul the windmill around with your car - and you can situate it to better face the prevailing winds - and it doesn't have to be lightweight or low drag.  Better still, it's saving on your energy bill even when your car is parked in the garage. SteveBaker (talk) 19:32, 24 October 2014 (UTC)


 * Really? If the air is moving relative to the ground, you can potentially harvest substantially more net energy from a wind turbine of a given area by moving it, by virtue of the larger volume of air with which it interacts (and hence kinetic energy of the wind that it can extract). —Quondum 22:44, 24 October 2014 (UTC)


 * But the drag on a moving body is proportional to the square of the speed - and that erases that benefit. The laws of thermodynamics definitely apply here. SteveBaker (talk) 03:51, 25 October 2014 (UTC)


 * Until you've quantified the drag relative to the power extracted, such a statement makes no sense at all. This is a reference desk; let's keep what we say here on firm ground (no pun intended). —Quondum 21:34, 25 October 2014 (UTC)

Cotyledon function
Quoting from our Cotyledon article: "Cotyledons may be either epigeal, expanding on the germination of the seed, throwing off the seed shell, rising above the ground, and perhaps becoming photosynthetic...."

1) I take this to mean that some epigeal cotyledons do not become photosynthetic. Is this true?

2) If they become green, does that mean they are photosynthesizing?

3) Are dark green leaves better at photosynthesis than lighter green ones?

4) a)Are cotyledons that DO photosynthesize as efficient at doing so as the "regular leaves" of the same plant? b) Can they  be photosynthesizing but only weakly such that removing the cotyledons actually improves the growth of the plant?

Thanks, C7nel (talk) 18:33, 24 October 2014 (UTC)


 * I took the liberty of numbering your questions for ease of reference.
 * 1) This page says that this plant  in the Stemonaceae has non-photosynthetic cotyledons.
 * 2) Yes, in general. Chlorophyll is green, and I'm not aware of any other green pigment in plants.
 * 3) Not in general, but it depends on what you mean. "better at photosynthesis" depends on several factors. One is if the plant does C3 photosynthesis or C4 photosynthesis or even CAM photosynthesis. Other factors are leaf area index, specific leaf area, quantum efficiency, temperature, concentration of atmospheric CO2, and many other things. The biotic factors can vary between species and between plant functional types. The abiotic factors can vary based on time and location. One loose trend for forest species is that understory growth tends to be a bit darker than the canopy, and this is basically because most of the light is diffuse and scattered, instead of direct from the sun. But all bets are off if you compare plants from radically different taxa and having very different morphologies.
 * 4) a)Roughly - if anything the are probably better. That's the plant's first chance to get started in life, so they better make it good. b)Probably not. There is a notion of competition for light between ramets of a plant, but in general the plant makes its own best decisions. Removal of the cotyldons removes total photosynthetic area, and also causes damage that leads to water loss, and also removes non-structural carbohydrates, nitrogen, and other resources that the plant can use. Most cotyledons don't hand around that long, and when they wither and fall, the nutrients have already been retranslocated to use in other parts of the plant. We don't have a WP article on retranslocation, but see e.g. these lecture notes.
 * Hope that helps, SemanticMantis (talk) 19:30, 24 October 2014 (UTC)
 * SM has pretty much covered this all. I would point out the two halves of the "nut" of an acorn as an example of epigeal cotyledons which really don't photosynthesize, although they may turn a little greenish, while cannabis cotyledons do serve as the first pair of quasi-leaves.  It's never a good idea to remove cotyledons, they contain fat and protein that the leaves don't have in abundance and which is the plant's limiting growth factor at that point.  Even if brown and ugly they'll drop off at the right time, and removing them won't encourage growth. μηδείς (talk) 20:57, 24 October 2014 (UTC)
 * The acorn example is better than the weird plant I linked above. Come to think of it, the nut meat in e.g. pecans, walnuts, etc. are also cotyledons. But I can't easily figure out if they are only used as storage organs or if in some species they do photosynthesize... SemanticMantis (talk) 13:05, 27 October 2014 (UTC)

Bird chirp sounds like water sloshing?
Today I heard a very strange bird chirp that sounded like water sloshing around followed by a click. I was unable to see the bird clearly as it was up in the trees. I live in southern Arizona of the United States. Would you happen to know what bird it might have been? -- Tohler (talk) 22:17, 24 October 2014 (UTC)
 * I'd point out that there are a lot of birds like crows and mockingbirds that are mimics, and they sometimes mimic sounds made by inanimate objects/machines, assuming this isn't a "normal" call. If no one here offers a likely answer (I'm an Easterner) you can also ask at WikiProject Birds. μηδείς (talk) 00:19, 25 October 2014 (UTC)