Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: george on 04/01/2007 04:15:12
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Plants draw water from the soil and release it through their leaves, but how much do they consume, per hour?
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Of course water use will depend on many conditions- humidity, temperature, stress, size of the tree, species of tree etc. Here are some examples- they are all estimates because this depends on the number of leaves/needles and the rate can vary from the shaded side of the tree to the sunny side.
These are all measures of transpiration, but most of the water use in a plant is through transpiration.
From a paper in 1957 by Dr. Johnson at Yale University he estimated that on a clear 21degree C August day a non-shaded pine tree with an estimate 151kg of needles it lost 181 L per 10 hour day (the loss at night is very small). An oak tree with 39kg of leaf lost 154 L per 10h day. However, that water loss was not consistent across the whole day- it peaked from about 2pm to 5pm and really dropped off by about 7pm.
From a review in 1977- I guess not a very current topic!-
A single maple tree 47 feet high, with 177,000 leaves (area 1/6 acre), was estimated to lose 58 gallons (219 L) of water per hour on a summer afternoon.
Unfortunately, they don't say where these trees were- a dry Australian summer or a wet Californian summer could make a big difference. So just some estimates to start.
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Nevertheless that is an awful lot of water being evaporated away from the leaves. Which has not been addressed adequately.
When I first saw this post, I thought great, now we shall see some interest in this fantastic subject. No pump involved yet water flows effortlessly up from the roots at a pretty impressive rate according to some of the figures. (We need more examples please)
To believe this rate of flow is caused by water molecules leaving the leaf into the air as creating sufficient negative tension would require huge negative pressures at the leaf, which have never been measured or identified! One would certainly expect pretty impressive negative pressure to account for this using current explanations in plant physiology.
Imagine, we have leaves with open pores which enable oxygen to enter, carbon dioxide to leave, yet are supposed to provide suction.
Pardon me but what a complete croc of crap! My words were much stronger back in 1994 when I read this utter garbage.
Imagine sitting on top of a table trying to suck water up a tube with a few holes in it.
The argument that the leaf has valves has been put forward. Ok so let’s imagine standing on a table sucking on a tube with the other end at ground level submerged in water and experience how much tension you need to generate in order to pull the water up. Now consider that the tubes collapse under such suction, this does not happen in the trees tubular structure. The argument is lignifications strengthens the tubes. But this does not address the fact that many of the tubes in the new growth are not lignified and these would be exposed to the greatest negative tension at the top of the tree! And just like a paper straw sucking water to a few metres the walls of the tubes would become closed!
Bur we know the pressure inside some of the tubes is positive. How else would we harvest rubber or maple syrup? Here the sap is harvested because like the human artery it is under a positive pressure.
The formation of the tubular structure inside all trees requires a positive pressure to make sure the tubes are kept open as they become lignified. The cambium is the factory production line for manufacturing tubes, it has all of the materials for forming them and the positive pressure.
According to my own work in this field, tiny pulses of solutes flowing down inside the cambium from the leaves where they are concentrated by the huge evaporating losses in transpiration would provide the formation of tubular structures with ample positive pressure to maintain the diameters of the tubes while they are forming.
I presented a simple experiment with open ends to show both the negative tension generated in the return flow and the positive pressures generated by the downward flowing solutes. The negative tension in this experiment far exceeds the tensions measured inside trees, and why wouldn’t it, because a tree does not have its ends open like the experiment, the roots are covered in a fibrous sleeve similar to the bark on the outside of the tree. Roots inside moist soil would relieve the necessity for the huge negative tension in the open ended model.
For example, picture the open ends suspended in water but joined together. The tube is not completely air / water tight so can pull water in and let water out under pressure changes as is the case with a root system. Now, we introduce some falling salt solution on one side of the tube and this causes a positive pressure in the form of a pulse flowing down but at the same time generates a negative tension in the other side of the inverted loop. Now we will see water being drawn into the circuit and a slight expansion of the tube in the downward flowing side. This easily accounts for the influx of water at the roots and at the same time demonstrates that trees do not need huge negative tension in order to circulate fluids within. Remember we introduced a tiny pulse of salts at the top, say 0.5 mil of salt solution, which mixes with water as it flows down. This mixing is important for the negative tension lift on the opposite side because when the salts have mixed the dragging on surrounding water molecules becomes more efficient causing a more robust flow. Water molecules then follow each other in the direction of the pull causing a chain reaction to occur around the entire tree and in our experiments as observed in the videos. In fact 1 grain of sugar can initiate a flow when dropped in clear water. Again this can be observed happening in the video.
Recap: To explain the huge volumes of water evaporated often at great heights by trees we cannot ignore how sap is concentrated by such huge water losses at the leaves. And We cannot ignore what happens to these concentrated solutions in relation to gravity as gravity will inevitably pull them down towards the ground when they are in a container that has less dense solutions at the bottom than at the top. This is not rocket science, but it is factual science!
http://www.metacafe.com/watch/804181/the_gravity_of_life_part_1_the_origin_of_circulation/
http://www.metacafe.com/watch/804248/the_gravity_of_life_part2/
http://www.metacafe.com/watch/786493/water_flowing_up_a_cliff_to_24_metres_with_no_pump_experimen/
Andrew K Fletcher
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Andrew, you really do need to go away and read up on osmosis (in some proper text books, please, and as a hint if it says "A-Level" or "GCSE" on the side it's not a proper text book, it's a tome of oversimplified aproximations intended to be supplanted by a more rigorous understanding in anyone who goes on to further study).
Physical pressure and osmotic (chemical) pressure can work together or in opposition to each other, but if you've got a biomembrane in the way all sorts of interesting things can happen.
Your theories about how trees get water from roots to leaves fail utterly to account for how the water then gets out of your supposed closed loop system and into the leaves, the "falling solutes" idea is just a perpetual motion machine by another name, and of course the sap at the bottom of the tree is at a positive pressure, pressure changes with depth.
Andrew's experiments are quite pretty and a nice demonstration of a siphon effect (however much he resists the word) but don't show what he thinks they do.
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So you presume after all of these years I have not read further than GSCSE or A Level? That would make me the slowest learner on the planet. In fact I read these in 1994!
The cohesion tension theory you are relating to does not address the lack of negative tension observed in trees. It does not address the cavitations which do not appear to interrupt the flow in trees, yet would disconnect the tension required in this theory! I have never observed a working model that shows fluid transport that relates to the bulk flow capacity reported at the start of this thread, nor is there a working model anywhere to be found. The cohesion tension theory as it stands is little more than conjecture!
A water molecule leaving a column of water 300 feet in the air somehow does not convince me that it will pull up an entire column of water from ground level.
And before you spout off about the vast quantity of leaves pulling together as lots of water molecules leave the tree, bare in mind there are many trees including larch that stand very high above ground level that do not have a dense canopy. In fact they have very few leaves and branches, yet can lift water to great heights. How does the current cohesion tension theory address this?
Perpetual motion machine? “all sorts of wonderful things can happen?
Some very intelligent plant physiologists have agreed that nothing much has changed in plant physiology literature worth consideration!
The Late professor H.T. Hammel had no problem with my theory. Odd since he was a world authority on fluid transport in trees and plants.
As for siphon, I have mentioned before that the tube can be completely cut off from atmospheric pressure by joining it together after a small amount of salt is added. The flow still occurs! No siphon to my knowledge can be attached to this demonstration.
Nevertheless I am grateful for your post and hope you can come back with a much better argument.
Kind regards
Andrew K Fletcher
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-shrug-
Last time we discussed this in any depth (2005) you were referencing a GCSE textbook.
Infact, I've found the post. It's on the first page of this topic: http://www.thenakedscientists.com/forum/index.php?topic=1982.0
You say:
For the currently accepted view of osmosis and all other views on water transport I will refer to one of the standard GCSE text books entitled GCSE BIOLOGY, D.G. Mackean. ISBN 0-7195-4281-2 first published in 1986.
The implication I took from that statement was that you took the current understanding of osmosis to be as described in the text book mentioned above. I'm glad you're so offended by my assumption, because to be honest it ought to be a pretty offensive assumption.
Look. I don't say I understand exactly how trees lift water to their leaves, they're physiological systems and thus inherently complicated.
I do however say that I understand exactly what's going on in your much simpler plastic tubing experiments and that it is not applicable to the case of trees lifting water to their leaves. I'd be really interested to see from you a quantitative consideration of what you think is going on in one of your small scale experiments, how much liquid you're moving from where and to where, what the height differences are between the initial and final states, how the effects you observe are altered by using liquids of different densities in your tubes and as the injected solution, in effect an analysis in terms of the gravitational potential energies.
I don't believe you'll find anything I can't account for using standard highschool physics. But I'd be very interested if you did.
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Ok
Let us deal with an even more simplified flow and return system that should satisfy your requirements that it is open to the atmosphere at the top to represent surface evaporation.
This is very simple to apply, as it is the precise mechanism that drives the world’s ocean currents in a system called the Atlantic conveyor there are no structures to interfere with this example of a flow and return system, so again this completely rules out a siphon effect!
The Atlantic conveyor is being discussed in another thread, but there is a video albeit a rather simple video showing how these system flows causing an underwater river many times bigger than all of the rivers in the world put together.
This relates the density flow in a way that shows evaporation from the surface and a sink, together with a return flow, exactly the same as I have been explaining happens inside a tree.
On my experiments you will see a simple demonstration where salts are dropped into a vase of clear water. Here you can see a table top version of the Atlantic conveyor as the sunlight flows through it you can see the circulation distorting the sunlight. If you look carefully you can see a single grain of salt and sugar generating the flow by causing molecules of water to be dragged behind the falling crystals.
I do not need to show water flowing out of the top of the closed loop of tubing, this was designed to show how robust the flow system is.
If I construct a tree like example, I would have my tubes inside a larger tube filled with water which would represent the bark of the tree, which also covers the roots but much thinner at the root enabling water to enter.
With this type of model, we could easily show water being lifted above the height of the tallest trees on Earth. For example if we used a closed loop of tubing joined after salt solution was added at the top to one side, we would see circulation initiated by just a few grains of sugar or salt as it dissolves dragging on water molecules causing the entire bead of water to rotate. But the interesting thing here, again shown in the sprit level experiment is that the falling salts on one side cause the water in the other /opposing side to be lifted to a greater height, showing the impetus required for plants to grow taller. Also showing haw sap exudes from a cut stem.
Also I am not trying to show how much liquid is moved from one point to another but showing how fluids circulate inside vessels or indeed in a body of water, be it a glass of water, a tree, human physiology, animal physiology, the ocean or plant physiology or even microbial physiology the principles are the same. This flow system has no respect for the vessel that contains it, it is a non-living physical force that drives circulation.
You also related to it being perpetual motion, if you think about this for a while, when transpiration stops the circulation stops when it begins again the circulation begins, but as Strasburger proved, the tree does not need to be alive for this system to flow.
I do not take offence at your remarks previously, as I can understand how difficult it is to question current literature. All I ask is that you consider that density changes are inevitable as a result of evaporation and the density of the fluids in a tree or any other plant or creature bares this out. And once you accept that density changes are an inevitable consequence of evaporation you must also apply gravity to the denser solutes and determine what is going to happen when they move from one point to another.
You also mentioned pressure differences relating to height.
A tree is often felled from just above ground level, no water is observed to spurt out, in fact it is accepted that the fluid within behaves like an elastic band and is quickly withdrawn up the trunk rather than leaking out. This also fits with my observations in the Brixham experiment as this was repeated a number of times, even with no salts added and the result was the same as in the tree, water on both sides of the tubes was observed to retreat rapidly up both sides and become suspended about 25 cms higher forming an equal level on both sides much the same as a U tube would do for a spirit level. Once cavitations occur in the water bead with both open ends submerged in water at ground level and the inverted U tube suspended 24 metres the water level rapidly drops to the 10 metre level at sea level and forms an equal level on both sides at the new height with vacuum above the water levels.