How Do Plants Grow: Gaining, Transforming and
Losing Plant Mass
1. What are the materials that contributed to the radish seedlings mass? How did the plant get
those things?
Look at this young tree planted in a bucket of soil. As the tree grows it
gains weight. Think about whether the soil is food for the plant.
2. Do you think the weight of this tree came mostly from materials the
plant took from the soil?
YES
NO
3. Predict whether the weight of the soil in the pot will “increase”,
“decrease”, or stay the “same” as the plant grows:
In the following paragraph, van Helmont describes his
experiment.
“I took an earthen pot and in it placed 200 pounds of earth which
had been dried out in an oven. This I moistened with rain water, and in it planted a shoot
of willow which weighed five pounds. When five years had passed the tree which grew
from it weighed 169 pounds and about three ounces. The earthen pot was wetted
whenever it was necessary with rain or distilled water only. It was very large, and was
sunk in the ground, and had a tin plated iron lid with many holes punched in it, which
covered the edge of the pot to keep air-borne dust from mixing with the earth. I did not
keep track of the weight of the leaves which fell in each of the four autumns. Finally, I
dried out the earth in the pot once more, and found the same 200 pounds, less about 2
ounces. Thus, 164 pounds of wood, bark, and roots had arisen from water alone." (Howe
1965)
4. Copy the table and write down the changes in weight of the tree and the soil.
WEIGHT CHANGE OF TREE
WEIGHT CHANGE OF SOIL
5. How would you explain the results that von Helmont found? Where does the
majority of a plant’s mass come from if not the soil?
6. Why did the soil lose some mass? What components of the soil might now be
somewhere else?
7. Although von Helmont was able to show that plants didn’t simply take mass from the soil
for all of their growth, he believed that instead the plant’s material was somehow composed
of water, the only thing that he had added to the bucket other than soil. Why is that idea
incomplete?
8. What process describing plant growth was unknown to him and other scientists of the
time that we now take for granted?
9. What is the main product of this process that contributes to plant mass?
10. As you consider any plant, though, it is obvious that although it has both water and this
product, it is more complex than either of those things. Can you think of any other
molecules that make up a plant’s dry material, and where within the plant or its cells those
molecules might be found?
11. What was van Helmont's reason for conducting the experiment? In other words, what
question did he set out to answer?
12. What was van Helmont's hypothesis?
13. Briefly restate in your own words the experiment performed by van Helmont.
14. List at least five variables that might affect this experiment. Which variable(s) is(are)
controlled? Which is the experimental variable(s)?
15. Identify the experimental group and control group in this experiment.
16. Seedlings use energy to grow structures like stems and roots before plants develop
leaves. What do you think is the source of this energy?
17. The +light, + water seedlings were constantly under light. Do you think these plants
were carrying out cellular respiration? Explain your answer.
18. To summarize all that we’ve discussed, please copy and complete the following table:
Gaining Mass
Transforming Mass
Losing Mass
Which process is
responsible?
What is the effect on
CO2 around the plant?
What is the main
product(s) of the
process?
19. Read page 107 in your textbook. How do Joseph Priestly’s, Saussure’s and von Mayer’s
results modify van Helmon’t conclusions?