Andrew K Fletcher wrote:

To disagree with this theory is to suggest a mechanism for evaporation that
does not have any effect on the density of residual fluids inside the tree.
Please elaborate as I find this impossible to understand. If we can agree on
density changes due to evaporation in transpiring leaves then the rest of the
theory is correct also.

Yes. That is a very interesting point. What I suggested implies that
the chemicals in the sap are transported up the tree by the "pumping
mechanism" which clearly raises the question of why does not the
materials in the sap concentrate at the top of the tree as water
evaporates from the leaves? Good point. I'm not sure what the answer
is! Clearly most plants have various chemicals of various kinds in
the sap. I do not believe there is any evidence that pure water is
pumped up plant stems. I'm not sure what kind of "pumping mechanism"
would pump just the water leaving the chemicals in place. I do presume
that a tree for example has a much larger area (volume) for storing
liquid at the leaves and perhaps processes in the leaves somehow use
the chemicals transported in the sap. But this is all speculation on
my part. I'm not an expert in plant physiology.