One of my students asks where the "old sap" goes. What
should I tell him?
Sapwood carries water and food (nutrients) up and down the tree. The old
sapwood becomes heartwood.
Sap flows up to the leaves in the spring, and in the autumn, the sap flows
downward to the roots to store the nutrients there.
Anthony Brach Ph.D.
This explanation should start with the function of sap in the tree, that it
is the water that conducts dissolved minerals and other growth products up
and down between leaves and other parts of the plant. Sap is there all the
time the tree is actively photosynthesizing and growing, and never really
gets old and "goes" anywhere. At the end of the growing season the tree
stops taking up water and thus sap is no longer present
The simplest and most straight forward reply would be that sap moves
more like a circulatory system so there is really no "old sap". By
contrast, water moves in a single direction--usually from the roots to
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Update: June 2012