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mistermack last won the day on August 16

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  1. capillary pump

    No, I think it's independent of the size of the tube. The atmospheric pressure at sea level is 760 mm of mercury, regardless of the measuring tube. Which equates to about 10m of water. I found one fact on the net, that water will boil at 23 deg C, at a pressure of one fortieth of an atmosphere. So the small diameter of the Xylem seems to come into play, by preventing cavitation at low pressures. That still doesn't overcome the 30m problem. If you have a simple tube, with one end in water, and the other a perfect vacuum, then the maximum that the water can rise is 30m, because that's the value of atmospheric pressure, in m of water. Because pressure is force/area, if your tube doubles in cross section area, the upwards force is doubled, and the weight of water in the column also doubles. So whatever the diameter, a given vacuum gives the same height of column supported. If you had a six inch pipe, with a near total vacuum at the top, I would expect the water to be pushed up 30m, and be boiling at the surface, if it was at room temperature. So I don't think you can make it work for any tree over 35 m in height, with maximum root pressure, and a 100% vacuum at the top. There must be something else in play for the very tall trees.
  2. capillary pump

    There seems to be something missing then. If the maximum that a vacuum can lift water is ten metres, what's lifting it above that? It doesn't come across from the pages that have been linked so far. Reading a bit more, it appears that the exact mechanism isn't agreed as yet. Wikipedia mentions root pressure, transpirational pull, and "pressure flow hypothesis". Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals. The phloem pressure can rise to several MPa,[11] far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure. This seems to be the answer to some of the problems I mentioned : Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading). It seems to be a lot more complicated than a simple tube.
  3. capillary pump

    I was thinking along those lines, but I couldn't reconcile the height of tall trees with the partial vacuum caused by evaporation. From memory, going back to my school days, atmospheric pressure will only lift water approximately ten metres, at sea level in response to a nearly full vacuum. So in trees that are 100m tall, you would need pressure from the roots to take it the rest of the way. If the evaporation was able to reduce the pressure in the tubes close to zero, you would think that the water would boil.
  4. capillary pump

    Trees can do it. Up to a maximum height of 115 metres. The way that they manage it, I believe, is by evaporating the water in the leaves, reducing the pressure in the tubes, so the liquid is forced up by root pressure. Obviously, the water evaporates at the top so it wouldn't be usable. I don't know how the roots create enough pressure to push water to the top of a Coastal Redwood, or Australian Mountain Ash at over 100m tall. Maybe energy manufactured in the leaves is used to create the root pressure, so it might not be possible to copy the process.
  5. Did Christianity start with a real human Jesus?

    There you go again with your "majority view". It's meaningless. And it's very often self perpetuating. After all, who is teaching the new crop of historians? People who hold the "majority view". It's a bit like the Christian religion itself. It's self replicating down the ages. And climate research IS a good example of a self perpetuating consensus. Nobody in their right mind would go into climate research, if they didn't already believe in MMGW. They would just be signing up to a lifetime of being shunned, and unemployable. Unless they were prepared to keep their mouth shut and live a lie. Climate research is now for believers only, and that's pretty close to bible history too. It's nice to see one or two dissenters, but they need to be pretty special people to make a go of it.
  6. Cool Facts

    Not due to the transparency, but to the very effective insulation. This includes control of the blood supply to the skin, as well as hollow fibre fur. I remember years ago installing passive infrared detectors in my burglar alarm system. You could defeat the detectors, by holding up a high tog duvet in front of you, and moving slowly. A polar bear would have hot spots, like the eyes and nose, and breath, so I would think that a PIR detector would pick them up in a cold space.
  7. Did Christianity start with a real human Jesus?

    It's there in response you your equally huge assumption, that the majority view among religious historians is somehow significant. Perhaps we should both drop the huge assumptions. There's an inbuilt problem with addressing the question of whether the Jesus was an actual man or not, if you're a professional historian. And that is the book-buying public. You are cutting yourself off from a lot of sales, if you argue for an out-and-out no. I'm not particularly keen on Richard Carrier, but I have to admire his stance. It would have been a lot easier to go for the easy money, or stay on the fence. Maybe there's a market there for his stance though, smaller but more exclusive.
  8. Did Christianity start with a real human Jesus?

    Religious historians, like religious writers in antiquity, need to be taken with a huge pinch of salt, for the same reasons. They favour what they WANT to be true. To lump them in with other historians is a bit like giving intelligent design equal billing with proper science. That doesn't mean that their arguments can be ignored. But it does mean that I don't think that their opinions should be given the sort of weight that you give them.
  9. I read the other day that song thrushes and black caps eat deadly nightshade berries. No more details than that. They would make a better seed dispersal candidate, as rabbits would be more likely to chew the seeds up, and they eat their own shit and digest it twice, so the seeds might not survive being eaten.
  10. A strange hypothetical planet

    I reckon that Venus would come pretty close. It's rotation has very little tilt, and the atmosphere is so thick and hot that the heat is probably spread around the planet very evenly. Not a pleasant place though, so conditions are predictable but pretty horrific.
  11. I'm still guessing that it's birds that spread it, as well as rabbits. I know what rabbit shit looks like, and I haven't seen any, nor any scrapes. But maybe we've just got some tidy rabbits. I'm not sure if the seeds would pass through a rabbit undamaged either. But I'll avoid eating any rabbit shit, just in case.
  12. And cattle too, I mentioned it above. It's surprising that those animals evolved an immunity, rather than an aversion to the plant. You would think that it would be less costly to just evolve a few circuits in the brain, than to evolve the physical ability to cope with the poison. But maybe by evolving the immunity, it gave those animals a food source that other animals are shunning, so they have a more exclusive access to it. It could be a life saver in hard famine times, when every other available scrap of food has been eaten.
  13. Did Christianity start with a real human Jesus?

    It's not impossible, if you're willing to look at what evidence you have, and take a guess at what happened. So long as you make it clear that you're guessing. Cross out distillate, and replace it with "guess at" and I'd be with you on that. But as I've said numerous times, I don't think historians are taking into account enough, the willingness of people to lie and invent and twist stories, to make them say what they would LIKE them to say, in the field of religion. I don't think you can do religious history in the same way as you do political history. The willingness to lie and invent is on a different scale.
  14. Did Christianity start with a real human Jesus?

    What is wrong, is the inference that these sources are the only sources. I'm sure that there were sources around those dates, but there's no way of knowing what happened in the interval between them being penned, and what we have now becoming finalised. The original gospels are no more. What we have are copies of later documents, that might or might not be similar to what was originally written. And when it only takes one word to completely change the meaning of a piece, it's totally misleading to claim that the gospels that we have now date from the late first century. They might, they might not. And they are somebody's preferred selection anyway, not an original collection.
  15. The interaction between plants and their seed dispersers can seemingly get really complicated. The deadly nightshade plant is one of the most poisonous you will find in northern Europe, but it's fruit is described as sweet. As an allotment keeper, I've noticed it growing where it definitely wasn't growing the year before, so something is distributing the seeds, and I'm guessing it's birds. The whole plant is poisonous, and even the honey carries the poison, if bees have been visiting a lot of it. Some animals have no immunity to it, but obviously others can eat it no problem. Wikipedia says that cattle and rabbits can apparently eat it without problems. I would say that some birds must also be able to eat the berries. They must have evolved that capability, through lots of animals dying from poisoning, before the immunity was achieved. So the plant was obviously better off remaining toxic, even though animals were getting sick and dying from eating the berries. So you have a plant that's very very toxic, but has attractive looking berries with a sweet taste. It's full of contradictions, but it's survived and flourished.