I remember starting a thread on this in 1996!
(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)
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
in doing this I noticed a couple of things.
First, the volume of sap is astonishing - the fluid practically
Second, the flow rate is closely linked to sunshine. On cloudy
sap flow is strikingly reduced, as is the flow at night.
The second observation calls into question something I have been
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
with the group.
By the way, tapping a single tree for several weeks yielded 4 - 5
truly delicious maple syrup!