I am not a plant physiologist, but I am surprised to hear John refer to
"once a source, always a source" as dogma. That hardly seems to apply to
trees. In spring the roots are the source and in summer and autumn the
leaves are the source. The direction of phloem reverses to supply the
buds and young leaves with energy from the roots. I haven't thought much
about this in relation to sun and shade, but early sun leaves become shade
leaves as the tree grows higher, although in deciduous trees it will be
mostly new leaves that take on the new role.
Janice
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Janice M. Glime, Professor
Department of Biological Sciences
Michigan Technological University
Houghton, MI 49931-1295
jmglime at mtu.edu
906-487-2546
FAX 906-487-3167
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