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rising tree sap

John Hewitson jfh30 at cam.ac.uk
Tue Apr 12 12:10:34 EST 2005

I remember starting a thread on this in 1996!


followed by:-

(actually appears to be posted before my initial email! In those days we 
weren't very clever on time-stamping emails.)




- don't expect any "good solid knowledge of what governs the flow upward 
of tree sap in the spring", but I found it interesting!


Dr. John F. Hewitson
Science and Plants for Schools (SAPS)
Homerton College
Cambridge CB2 2PH UK
Tel: +44 (0)1223 507168
Fax: +44 (0)1223 215004
web site: http://www-saps.plantsci.cam.ac.uk

On 12 Apr 2005, at 4:01 pm, Kathleen Archer wrote:

     Dear Plant Ed Folks,
     I have tapped my maple tree in the yard to collect sap for maple 
syrup, and
     in doing this I noticed a couple of things.
     First, the volume of sap is astonishing - the fluid practically 
gushes out.
     Second, the flow rate is closely linked to sunshine. On cloudy 
days, the
     sap flow is strikingly reduced, as is the flow at night.

     The second observation calls into question something I have been 
told by
     others, namely that the driving force of upward sap flow is root
     pressure. I don't see how this can be. Root pressure is what causes
     guttation droplets to form on the leaves of plants, and this is most
     typically seen in the early morning. That suggests to me that root
     pressure should be operating at night. Further, I don't see how cloudy
     days or sunshine could affect root pressure.

     Naturally, since I am lecturing now about water transport in xylem, and
     phloem transport, students are questioning me about maple sap. I'm
     guessing we may not know as much about how this works as we would
     like. But if anyone has good solid knowledge of what governs the flow
     upward of tree sap in the spring, I would be grateful if you would 
share it
     with the group.

     By the way, tapping a single tree for several weeks yielded 4 - 5 
cups of
     truly delicious maple syrup!
     Kathleen Archer

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