david weindel wrote: > To what extent are >tropical rains like a disinfectant wash? My daughter has assured me that >distilled water acts as a disinfectant, killing thin membraned/skinned >parasitic creatures, osmotic pressure explodes them.
I fear that she is wrong, or else mass death would spread every time it rained. Organisms have to go through immense extremes, and living with water is just one of these. A damp bit of tissue paper exerts a water potential of around 3-5 bar (atmospheres) and dry soil around 50 bar. A leaf has to be in equilibrium with these pressures if it is to extract water, and indeed a dessicated xerophyte may well drop to -30 bar. (The minus sign is a convention, meaning suction rather than pressure.) I once measured the water potential of air dry velamen on an Australian orchid root and got a figure of -40 bar. A leaf from the pseudobulb that this fed was at around -12 bar. Wet velamen was at about -3 bar, and so there was a 9 atmosphere pressure differential drawing water into the pseudobulb when the root got itself wet. One of the abiding mysteries is how a column of water in a plant's xylem avoids cavitation when it is under a 10 bar per metre pressure gradient, and exists at 15-20 bar below atmospheric pressure. Bulk water would, of course, boil; by xylem and phloem do not. I suspect some form of crystalline ordering, although the water itself whips along at anything up to 2 metres per second (in what, at any rate.) A raindrop falling on dry soil changes the osmotic circumstances for 10^9 organisms there within by around 50 bar in a fraction of a second. It is astounding that they survive, but they do. As a by the way, microcurrents in the air subject insects such as gnats to accelerations of 10-100G from brief periods, and they seem able to withstand this as well! _____________________________________ Oliver Sparrow Tel: UK (0)20 7736 9716 www.chforum.org www.treknepal.org www.datafreeze.com
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