Teacher's Pages
ACTIVITY 9 - "Dr. Van Helmont Is Soil Food For Plants"
ACTIVITY 9 - STUDENT'S PAGE
(It may be helpful to view the student page alongside this teacher
page.)
On the students' pages, students' ideas about soil as
food for plants are challenged as they
think about an experiment done by the Belgian Jan Van Helmont in
1642. In this experiment he showed that plants do not use food from the soil to support their
growth.
Advance Preparation: If you feel inclined to dress up as Dr. Van
Helmont for a role play activity, start collecting things to use for your costume. The idea is to give
the impression of someone who lived long ago -- so don't worry too much about historical accuracy.
Something akin to colonial male dress seems to work well in communicating the message of time distance
(knickers, white knee highs, jacket with long tail in back, scarf around neck, a beret?, rimless reading glasses).
Materials:
Copies of the student pages for each student.
A Possible Teacher Narrative
FRAME
"Today we are going to think about our hypothesis that soil
is food for plants. Write down in your journals what you think right now. Is soil food for plants? Why do you
think yes or no? What evidence are you using to support your position?"
Discuss ideas that students wrote down.
ACTIVITY
Read or tell about Traveling back in time. Get the students in a
time traveling mood.
ACTIVITY
The questions on this page are preparing students to make a
prediction about Van Helmont's experiment on the next page.
Additional Background for Teachers:
Using the History of Van Helmont's time to Be Sensitive to
Diverse Students
Van Helmont was a Flemish physician and physiologist. The Flemish
were people of Germanic descent who inhabited Flanders, which is today the northern part of
Belgium. In the 1600's this area was sandwiched between the new and economically powerful Dutch
Republic to the north and France to the south. Parts of this area were under Dutch control; others under
Spanish.
Show students the countries of Belgium and the Netherlands on a
map or globe. Ask them if they know anything about this area of the world and the people who live
there. Have you ever heard of Belgium or the Netherlands? They may have heard of the Belgian horses
Belgian sheepdogs, Dutch shoes and tulips, etc. Ask if any them have family histories tracing back
to Belgium or the Netherlands.
The Dutch Republic of the 1600's was economically strong. The
Dutch in this time period were active explorers, claiming lands and building colonies and trading
stations in Africa and the Indies (the Dutch East India Company). The young republic's commercial supremacy
was firmly established. Wealthy homes in London or Paris were full of goods provided by Dutch
traders and sailors , and Flemish tapestries covered the walls of their homes. The art of painting
flourished. Every town of any size had its community of professional artists.
Science and medicine also flourished, with money available to
support the work of professional
scientists and physicians. The microscope was invented during the
first decade of the seventeenth century, and its power was realized in the middle of the century
when a Dutch civil servant, Antoni van Leeuwenhoek decided to develop the magnifying glasses used by
drapers to inspect cloth. In so doing, he created an instrument powerful enough to discern red blood
cells and bacteria. Leeuwenhoek became a famous scientist of his time. He had no university
education but compensated for that with a sharp mathematical mind, great manual dexterity and keen
eyesight. He was so admired that he received kings as admiring guests.
What was life like for other people at that time period? Were all
people as privileged as Dr. Van Helmont and van Leeuwenhoek? These scientists had the luxury of time and
support to do experiments like this. But there were still many poor in the Netherlands. Van Helmont
was among the privileged class of people who could devote their time to science or art.
And what was life like in America at that time? This was the time
when the Pilgrims and Puritans and the Jamestown settlers were just getting established in the New
World. Interestingly, the Pilgrim's story has a Dutch connection. The Pilgrims lived in the Netherlands for
awhile to escape the persecution they felt in England at the time. However, they became upset with their
children learning to speak Dutch and being influenced by Dutch ways of life. Eventually, they left the
Netherlands, too, to find religious freedom in the New World.
Dutch explorers were active in claiming lands and colonies in the
New World. The colony of New Netherland was founded by the Dutch West India Company, which
exercised a monopoly over Dutch trade in the New World. The Dutch built trading posts along the
Atlantic coast and up the Hudson and Connecticut rivers, frequently clashing with Indians and English
settlers in their efforts to acquire furs, timber, and other natural resources. In 1655, they annexed New
Sweden (later to become the state of Delaware. The main Dutch settlement was New Amsterdam at the
south tip of Manhattan Island. The Dutch, however, had trouble attracting settlers since few could
be tempted to leave their rich and tolerant mother country to start anew in a hostile land.
And the Dutch were early on connected to the slave trade in the
New World. In 1619, a tragic social revolution began with the arrival in Jamestown of twenty slaves
on a Dutch privateer. At first, the Africans were employed under the same terms as white indentured servants.
Later in the century, they were denied all rights.
ACTIVITY
Have students read about Van Helmont's experiment and make their
predictions.
Student Responses
Students who believe that plants get some or all of their food
from the soil should predict that the weight of the tree will go up and the weight of the soil will go down.
ACTIVITY
Read and discuss the results of Van Helmont's experiment and his
conclusions.
The contrast between the student's predictions and what actually
happened in the experiment should be emphasized. Make sure that students realize that the experiment
provides evidence that plants do not take in food from the soil. You might ask students a question that
gets them thinking about the biologists' definition of food: Do you think the soil contains energy that
plants can use to live and grow?
A Visit with Dr. Van Helmont?
If you feel inspired, dress up (or have a colleague dress up) as
Dr. Van Helmont. Tell the class that we are going to time travel back to the year 1647 and talk to the
scientist who did this experiment. Students will be asking Dr. Van Helmont questions about his experiment and his
conclusions. They might ask him:
How did you do your measuring?
Why did you do this experiment?
Do the people of your day listen to you and your findings?
Then Dr. Van Helmont can ask the students questions like:
In your experiments, in the future, what have you found out? Did
you find any more evidence to support my conclusion that soil is not food for plants?
Does my experiment make sense to you?
Do you agree with my conclusions about soil?
Do you agree with me that my experiment proves water is food for
plants?
Expected Student Responses
Students may begin to wonder at this point, if plants do not get
their food from the soil, then where do they get it from? This is an important first step in getting students
prepared to find the explanation of photosynthesis sensible.
Students may ask about the tiny bit of weight lost by the soil.
The weight lost consisted of minerals that plants absorbed through their roots. Plants need these minerals
in small amounts for a variety of functions in the plant. They are needed to make the enzymes that regulate
photosynthesis, for example. But this idea is not likely to have much meaning for students at this point. It
might make more sense to some students after photosynthesis has been explained.
Some students will argue that the tree did get its food from the
soil, because minerals have no weight. These students are not using conservation of matter as a
principle in their thinking. It does not bother them that "something" (the weight of the tree) is coming
from "nothing" (the weightlessness of minerals in the soil).
Some students may ask, "Do plants poop?" These students
are thinking that soil could be food for the tree, but that the weight of the soil does not change much
because plants "poop" out waste that adds weight back to the soil. An interesting aspect of this way of
thinking is that there is some attempt to consider the need for conservation of matter. This would be an
interesting question to return to after the students have learned about the "waste products" of
photosynthesis.
Addressing the Nutrients Issue
Students may have questions about what minerals and fertilizers
do for the plant if they are not "food." Why do people spend so much money on fertilizers and minerals? Don't
they help plants grow?
With some students it is better to set the nutrient issue aside
and keep the focus on the energy issue. Your students, however, may be ready to consider a more complicated
definition of food:
Food is a substance that gives both nutrients and energy to a
living thing. A NUTRIENT is a
mineral that organisms need to be healthy, but nutrients don't
provide energy that living
things need to be active. Examples of nutrients are nitrogen,
phosphorous, potassium. So
for something to be food it has to have ENERGY, in addition to
nutrients. It is the energy
that enables living things to be active.
In the Van Helmont experiment, the tree did get some nutrients
from the soil, but these are not providing energy for the tree to grow. In addition, the small weight of the
minerals taken in by the tree could not account for the massive weight gain of the tree. The weight gain
comes largely from matter that is built from carbon dioxide and water in the photosynthesis process.
Thus, the argument that many make that most of the weight of a tree comes from the carbon dioxide in the
air. (See the Private Universe Teacher Workshop video, "Lesson Pulled from Thin Air").
Activity Ten will continue to address the food energy issue.
Addressing the Water Issue
Most students are reluctant to abandon the idea that water is
food for the plants. Telling them that water does not have energy in it is not very convincing to them. They
argue that maybe it doesn't have energy for people, but it does for plants. There is no one single activity
that will convince students otherwise. The strategy employed in this unit is to repeatedly raise questions
about the water, so that students are at least questioning their original certainty that water is food. This
questioning stance towards water will enable them to hear photosynthesis as a way to solve the puzzle they are
experiencing: "Water must be food for the plants because they cannot live without it, but water does
not provide energy to living things so water cannot be the food. I'm confused."
Confusion is a good sign at this point!