I remember, 40 years ago, of placing celery petiole in a little food
coloring and observing some time later the ascent of dye solution in
vascular bundles. With this in mind, in this past week's investigation of
angiosperm structure/function I had my Intro Bio honors students try dye
uptake with some of the plants I had collected that morning from my yard.
One of the challenge questions was, in effect: Attack or defend the
following statement: "Each leaf receives a vascular bundle dedicated to
that leaf; therefore if a plant has 10 vascular bundles in the stem near
the soil it can have at most 10 leaves above that point". Trivial
statement to botanist, but not for students who have just learned about
circulation systems in animals and the dedication of arteries and veins to
and from organs, and who have not been told that vascular bundles can
branch.
One of the plants was jewelweed (orange touch-me-not), Impatiens capensis.
Cut stems were placed in aqueous toluidine blue O solution. I was not
expecting to observe upward movement of dye at about 1 cm per second, or
to be able to so easily see the bundles and their branching pattern with
the naked eye! Thin sections (hand microtome) revealed dye in tissue
around functional tracheary elements (evidence of lateral movement), and
that not all tracheary elements were functional (air blockage?). Wow!
This material will be the subject of a full low-tech lab investigation (3
hr) next year. All sorts of variations are possible --- coating selected
leaves with vaseline, effect of recutting stem under water to remove air
blockage, etc.
I wonder if any of you have worked with this or other material and had a
similar enjoyable learning experience. Have I reinvented the wheel?
Bob Hodson
Robert C. Hodson
Department of Biology
University of Delaware
Newark, DE 19716
(302)831-8440
