Transpiration: Role of Stems

Leaf Draw Transpiration: Role of Stems
How DO Plants Raise Water from the Ground to their Highest Leaves?
Something's wrong! Root Pressure

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It is not the amount of work one puts into one's research, but the quality of the time you devote. Many of the Nobel-winning experiments took very little time and energy to do. What made them landmarks in science was the fact that they took a look at something from a different perspective. Most of the experiments were elegantly simple. They were condensed right down to the fundamentals of logic: "if..., then...." What is more is that they applied that logic to very momentous questions. You might define such questions as pertaining to characteristics of a large sector of the universe.

The following experiment you are considering can be elegantly simple because it probably deals with a characteristic of all plants; namely, how they move water from the ground to their leaves on the highest branches (transpiration). Remember that some tree leaves are up to 350 feet above the surface of the ground! No one knows exactly how it is done. And thus you are staring at a "big question."

But first let us try to get ourselves into the right mindset for thinking elegantly. Long ago, around 1900, biologists were wondering about cells and all the things inside those cells that their microscopes revealed to them. There were greenish things in plant cells. These were called chloroplasts, which means literally "green bodies." But the biologists of that time didn't know what good they were. But there was a bigger mystery because there was another body that was found in all cells - of plants and animals (all life at that time was divided into only those two kingdoms [my, how have things changed!]). Because it was usually in the middle of the cells, and because physicists were revealing the atom, the biologists got caught up in the vogue and applied the word "nucleus" to that central body in the cell. BUT the biologists had no idea what value a nucleus had!

And here an example of an elegant experiment - one that revealed the value of the nucleus. As soon as Mendel's writings were rediscovered, and a number of scientists began looking at genetics - or heritable traits, as they were then called. All Mendel knew was that they were at least inside his pea seeds, but where? He didn't care because that was not "his" question. But in the 1930's J. Hammerling* did care and wanted to make that "his" question, so he looked around for a "tool organism" - one that possessed certain properties that might help him in his quest for the answer. Well, he saw that there was this type of very large, single-celled alga that had the shape shown to the left. It was nothing like a spherical cell so the nucleus couldn't be in the center. But Mendel's traits had to be stored in there somewhere. But where?

What he decided to do was "divide and conquer" and take pieces of this asymmetrical cell and hope that one part of it could regenerate. If a part could, then, obviously, the "traits" must be in that part and not in the others (unless, of course, the "traits" were scattered all throughout the cell). So he clipped the cell into two parts - the foot, and the head. These were then separated into separate dishes and cultured to see which would grow. Here is his experiment in pictures:

You thus see that only the head lived and regenerated into the whole creature again. Thus the foot contained all the "traits" necessary for the full life-cycle of that organism. And when the biologist took a closer look inside of the foot, well-, you can see for yourself: he saw the nucleus, and therefore assumed that the nucleus was the home of the "traits." If you want to know more about Hammerling's pathfinding work, click on any of the pictures of Acetabularia cells on this page!

Now you might use this same strategy to figure out something important about transpiration - or how water can move in plants. "Divide and conquer!" Take a plant and see which of the separated parts can move water.

But first you should know a little about the history of this question. Some botantists (people who study plants) think that the leaves suck the water up through all the little tubes inside the plant (this they call "lead draw"). Other botanists think that it is the roots that pump it up to the leaves (and this they call "root pressure"). But aren't they missing something? What about the stalk? The tree's trunk? Well, that has been pretty much discounted by botanists because they know the water rises in the xylem, and the xylem is nothing but a collection of dead cells. So how can they do anything? Oh, oh! But they haven't tested it. They just did that terrible no-no scientists shouldn't do: jump to unfounded conclusions! (For more questionable transpiration concepts, click this.) And here is where YOU can jump in and perhaps jump to a "founded" conclusion.

Imagine that you start out with a white flower such as is schematically shown here. Carnations might be a good bet, but talk it over with your local florist first. In this diagram, the stem is tapered for purposes of illustrated the thinner flower end versus the thicker lower end. This taper will continue to be used in the next figures.

You must do this standard procedure because it is a sort of control. It shows that you can make this work in your hands. Place the flower in some water containing floral dye obtained from your florist or floral wholesale house. (Food colorings work also - but make a mix of them.) After awhile, you will notice that the outside edges of the petals start turning the color of the dye. If you can get this to work, you've got it made - the rest is very easy! Elegance, remember?

Next, snip off the flowers and leaves from several stems, and place some in the dye solution with the flower-ends "up", and others upside down. Using a magnifying glass, you may soon see blue dye well up as small beads in the stems that are flower-end up. (Another trick might be to touch a piece of white paper to the end of the stem to see if a colored spot is blotted onto the paper.) The question then is, what do you see at the base of the upside down stems? And the next VERY IMPORTANT question is: what does that mean?

WHAT'S NEXT?

If your stem actually exhibited "one-way" characteristics, then you should expand the proof of the applicability of your hypothesis to a wider range of plants. The flower and stems that you used were from the group of plants known as dicots (two infant leaves sprout from the seeds). Let's look at members of other groups of plants. But remember that in order to see the colors, you should choose plants with white or pale yellow flowers.

See if the one-way phenomenon also works in monocots (one leaf sprouts from seed). These are the grasses, and a good choice might be one of the pale or whitish ornamental bamboos or canes.
See what you can do with twigs taken from a ginko tree. Ginkos are members of a very ancient line of plants, and were around long before either the monocots or dicots.
Ferns
Mosses, etc.

* Hammerling, J. 1953. Nucleo-cytoplasmic relationships in the development of Acetabularia. J. Intern. Rev. Cytol. 2: 475-498. (reported his work done in the 1930's) (Interestingly this was the same great year that Watson and Crick published their double helix paper, and when Jonas Salk started dispensing his anti-polio vaccine.)

HISTORY of question.This article is strongly recommended for background reading, but do not believe everything you read. You are likely to be disproving some of it! Read: Karen Wright; "Antigravity Plumbing;" Discover [magazine] vol 23 (2002), no. 9, pp 20-21. (Return to where you were reading.>

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