In the paper by Canny, 1995, A new theory for the ascent of sap --
cohesion supported by tissue pressure, Ann. Bot. 75:343-357, he refers to
support for the compensating pressure theory in reference to insects that
feed on xylem sap. Scholander's diffusion pressure deficit theory
requires up to (actually down to) -20 bars of pressure in the xylem in
order for the cohesion theory to work, and Scholander measured pressure of
this magnitude. Canny, based on the work of Zimmerman, challenges this
theory, citing Zimmerman's measurements of mostly positive pressure at the
tops of tall trees. One point Canny makes is that insects would be unable
to extract xylem sap from xylem at a pressure of -20 bars, having the
ability to generate only 3-4 bars of sucking pressure themselves.
My question, what insects (orders, families, or genera) tap into xylem?
Don't most (all?) tap into leaves, and aren't these mostly (entirely?)
aphids and other Homoptera that tap into phloem? What taps into stem
xylem?
If there are some that tap into stem xylem at the top of a tall plant, I
can visualize the insides of the insect being sucked into the xylem at -20
bars!
This paper is creating considerable discussion among my students and
colleagues at all levels.
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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