(http://esseacourses.strategies.org)
Crops Investigation #6
Goal:
Students will understand the xylem system and the mechanism of transpiration.
Objectives:
Students will be able to:
- state the functions of roots
- locate the xylem in roots, stems, and leaves
- explain the purpose of transpiration
- explain the process of transpiration
- explain how minerals are absorbed and transported by the plant
Standards Met:
Overview:
There are three portions within this series of activities. First, students
examine the location of xylem in roots, stems, and leaves through dissection
and staining. Next, the students discuss the mechanism of transpiration. Extensions
can be used to provide activities. Finally, mineral transport is discussed.
Materials:
- carrots (cut into disks)
- stems (flowers with a stem like carnations or Queen Ann's lace work well)
- celery stalks
- leaves (anything with obvious veins, like tree leaves or mature spinach)
- food coloring (dark colors work best; don't use yellow or green) (prepare a
concentrated solution for the students to use)
- sharp razor blades
- test tubes and racks
Resources:
Actions:
Prior to the lesson:
- The class has already talked about the major plant organs (roots, stems,
leaves).
During the lesson:
- Use a series of questions to move the students to an understanding of the
three main topics for today.
- "What are the functions of roots?" As you have this discussion, two functions
that should be noted are to provide water and minerals for the plant.
- "What do plants need the water for?"
As you have this discussion, three functions
of water should be noted. One is for plant growth. Water is needed to keep the
cells turgid and to increase of cell (and therefore, plant) size. Second is
for photosynthesis. Water is one of the reactants for photosynthesis because
it provides electrons to the photosystem chlorophylls that lost electrons when
they absorbed light energy. Third, water is needed for transporting
minerals and other dissolved materials through the xylem by means of
transpiration.
- "How do you get water from the roots to the
leaves?" This discussion should include an explanation of the physical xylem transport system. The xylem system
is a continuous series of tubes connecting all organs of the plant. The xylem
is located in different places in different plant organs. In many roots, the
xylem is located in the vascular cylinder in the middle of the root. Give each
student a carrot disk. Carefully bite into the carrot so that the outer ring
of tissue is broken away from the inner circle. First, have the students taste
the outer ring. "How does it taste?" Now, taste the inner circle.
"How does it taste?" "Why do you think that these two parts taste different?" Discuss
the names of these two regions. Make a drawing of the root cross section in
your science notebook. Label the names of these sections. Record your data.
Explain why the two regions taste different.
- In stems, the xylem is located in vascular bundles that are scattered throughout
the stem. Sometimes you can see the vascular bundles if you cut a stem in cross
section. Since they are hard to see, you can use a dye to selectively color
only the xylem cells. If you put a dye (food coloring) in water and allow the
plant stem to transport the water, then only the xylem cells will be stained.
- Students should place a small amount of food coloring solution (1cm is plenty)
in a test tube.
- Make a fresh cut across the end of the plant stem using a sharp
razor blade. Immediately, place the stem in the test tube.
- Allow time for transpiration
to pull the water up the stem, then cut the stem in cross section to see the
xylem cells among the other cells. (note: The time necessary will depend on
the plant used. You can leave the stem in the test tube overnight and make the
observations in the morning. If you leave it overnight, the flower petals will
be colored.) Use dissecting microscopes, if they are available, for this examination.
- Students should make a drawing in their science notebook of the stem cross section
and label the location of the xylem.
- In leaves, the xylem is located in the vascular bundles of the veins. The
students should examine the size and variation in veins of the leaves provided.
Celery stalks are modified petioles (part of the leaf). You can place celery
stalks in food coloring, as you did for the stems. The veins in the celery are
so prominent that you can peel the vascular bundles out of the stalk. Xylem movement
in celery is very fast, so you will need only about 20 minutes for the food
coloring to go about 6 inches. Students should make drawings of the leaves observed
and the location of veins/xylem.
- After the students have an understanding of the physical xylem transport
system, discuss the mechanism of xylem transport: transpiration. "Now that we
understand the transport system that the water moves through, what makes the
water go from the roots to the leaves? What is the mechanism for the
movement?"
There are three key things to point out. The first is that transpiration is
based on concentration gradients of water. Water is always moving to where there
is less water. The second is that water is a special molecule because, since
it is polar, it experiences adhesion and cohesion. Thus, it sticks to itself
and it sticks to other molecules with great force. The third is that water moves
based on pressure. When water evaporates from the leaf, this pulls water through
the leaf, which pulls water up through the stem, which pulls water up from the
roots, which pulls water into the plant from the soil.
- Use the online resource to review transpiration (transport system and mechanism).
- "What about minerals? How do they get from the soil into the plant? How
do they get moving around inside of the plant?" There are three important points
of discussion. The first is that minerals must be dissolved in the soil water
to be taken up by the roots. The second is that minerals are selectively taken
up by the root. For a mineral to be get inside of the root, there must be a
transporter for that mineral in the plant cell. Finally, minerals give in the
xylem just like water does (because they are dissolved in water).
Extensions:
- The rate of transpiration can easily be determined during either (or both)
of the food coloring experiments. To do this, you measure the distance that
the dye traveled in the time period that was provided. (warning: This experiment
requires that the students remove the stem (or petiole) before the dye reaches
that top of the plant segment.) Starting from the end that went into the dye,
have the students cut the stem (or petiole) into 1 cm long sections until the
dye can no longer be seen. The number of sections is the distance traveled.
Divide the distance traveled by the period of time that the organ was in the
dye.
- Students can design experiments to examine factors that affect the rate of
transpiration. Such factors might include: light, temperature, wind, length
of the stem, diameter of the xylem, or humidity.
Assessment:
During the exploration, the students make and label drawings. They
also record observations. If an extension is done, there are calculations for
Extension 1 and an experimental design and/or lab report for Extension 2.
Reflection:
Please write a reflection after you teach this lesson to help us make improvements. Reflections should be e-mailed to j.okeefe@moreheadstate.edu and c.wymer@moreheadstate.edu. If you also complete student reflections, please include these as well.