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Maximum depth, pressure, capilary action, and light

>> Aren't most vascular plants fairly dependent on capillary action >>at
one point or another? You tell me the internal pressure and
>> I'll tell you the corresponding depth in freshwater...
>> - -Y-
>Ah!!! We have a partial winner. You got the why: Hydrostatic >pressure.
>But how deep do you need to go? Meters or feet.
>Everyone always says light.

>Tom Barr

And in this case everyone is right.  Most vascular plants do depend on
capillary action and difference in water potential, but that's because
most plants are terrestrial.  As we all know, plants need CO2, and have
to open stomata to get it, drying out the interior spaces of the leaves.
This requires that water is pulled from somewhere, either ambient fogs
in some cases or from the roots.  To haul water up from the ground
terrestrial plants rely on capillary action and the lower water
potential of the drying leaf to pull water up from the ground.  However,
if you are underwater, this is not a concern, for obvious reasons.  
It wouldn't be getting crushed that would be a problem either.  Vascular
plants certainly have the capability to withstand great pressures, so
from an engineering stand point, this would not be a concern. For
example, the hydrostatic pressure at 10 m is around a third the pressure
exerted on the basal wood of a redwood with 100m of wood above it (with
a density of roughly 0.35 that of water).  I strongly suspect that we
don't see many deep water vascular plants because the ambient light
levels are so low.

Doug Karpa-Wilson