At 12:21 PM -0400 5/12/97, Robert C. Hodson wrote:
--snip--
>One of the plants was jewelweed (orange touch-me-not), Impatiens capensis.
>Cut stems were placed in aqueous toluidine blue O solution.
--snip--
All sorts of variations are possible --- coating selected
>leaves with vaseline, effect of recutting stem under water to remove air
>blockage, etc.
Bob,
I have done this with kidney bean seedlings for years.
I usually use 1% Eosin Y (brilliant red). I give them
the simple stand-it-in-the-bottle exposure to the idea
and then ask them to use/request-and-use materials
available to test the concepts of transpiration in plants.
Sophistication then varies with their experience and
creativity. A few questions from me can usually get
them thinking and then asking their own questions and
seeking answers.
It is a beautiful low-tech lab with lots of excellent
science. Fans, lights, temperature & dark chambers,
plastic bags and twist ties, etc. are "room supplies"
that can help. Add a Li-Cor meter and thermometer and
photon flux density dose responses can be generated.
Also don't forget the cut-off-all-the-blades trial.
A split-stem (one in blue and one in red dye) was enlightening
for one student (I used Acid Blue 25 but I might try your
idea of Tol. Blue next time).
Also, don't forget the roots! If students carefully
excavate a young bean seedling with an intact root system
(NO BREAKS OF ANY KIND) and stand it in a bottle with
a cut-stem control, you see the effect of the endodermis
on the transpiration stream! As Eosin Y is not a soil
mineral, there are no transport carriers and so the only
uptake is up to the endodermis (apoplastic!) and so there
is little if any dye going up the intact shoot. Freehand
sections of primary root (secondaries are too thin for my
students to section well) verify it.
At the other end of the cut shoot, if you observe dye accumulating
in the mesophyll between veins (takes more than a few minutes)
you can ask the students about that. Is that evidence for
evaporative pull on the transpiration stream? The dye is
clearly darker between veins than within veins. What does
that tell you about soil mineral concentrations in leaves?
There is a lot that can be learned from this very simple
system. A couple flats of kidney beans in vermiculite
can go a long way. It deals with roots, stems, leaves,
and especially guard-cell function!
Students are always amazed at how rapidly transport occurs
in plants. It is a rather "invisible" process and this
project gives it some visibility.
By the way, I keep my dye in Nalgene 125 mL wide-mouth bottles
and once a year run it through a cheesecloth filter. Otherwise
it is an on-the-shelf exercise using almost no supplies besides
the plant material. I have used the dye for years, just adding
some every few years to restore lost volume.
ross
_______________________________________________________________
Ross Koning | koning at ecsu.ctstateu.edu
Biology Department | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA | fax: 860-465-4479
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