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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