From Seed to Seed:
Plant Science for K-8 Educators

Activity 8: How Trees Measure Up (and Out)

Grades: 5-8

Associated Lesson Topics:

• Growth (apical and lateral meristems)
• Role of wood in our lives

National Standards:

• SCIENCE: Standard A, Standard C
• MATH: Measurement Standard, Data Analysis and Probability Standard

Planting the Seed...

How do trees get bigger? Do they grow in only one direction? How can we measure the speed and direction of tree growth? As the tree grows taller, how can you be sure that you are measuring the diameter of the same location on the trunk? How can you make certain that you are measuring the same twig?

Teacher Information:

This activity introduces students to the concept that trees grow in two directions. Primary growth, which occurs in apical meristems, is responsible for a tree growing taller and its branches and roots growing longer. Lateral meristems, on the other hand, produce secondary growth, allowing the tree to grow in diameter.

Over the course of the school year students will measure and chart the change in the diameter of a tree and the length of a twig. We will share a trick for doing this successfully, but challenge your students first to come up with their own method.

Necessary Materials:

• Nontoxic, waterproof paint (bright color).
• Small paintbrush.
• Yarn.
• Yardstick.
• Small ruler.

Procedure:

• Begin this activity as soon as school starts.
• Each student or group of students should choose a deciduous tree. Encourage them to choose young trees. This will be "their" tree for the remainder of the year. Have them draw a map in their journals to record where their tree is located. Once students have carefully observed and described their trees in their journals, they can use a tree identification guide.
• Have students create a data table in their journals. The table should have three columns for the date, diameter of tree, and length of twig.
• Each group should choose a point on their tree and mark it. We recommend using nontoxic, waterproof paint. Use a bright color. Do not use nails or anything that will harm the tree.
• Each group measures and records the diameter of the tree at this point. To measure the diameter, wrap a piece of yarn around the tree. Measure the length of the yarn using a yardstick or ruler-if it's a small tree.
• Meanwhile, have students identify a twig that they will use to measure primary growth. Students should tie a piece of yarn loosely around the base of the twig so they can identify it over time. Have students identify the terminal bud, leaf scars, and terminal bud scale scars (see diagram). Have them locate the previous year's bud scale scar. Measure from this bud scale scar to the tip of the terminal bud.
• Gather data on the tree diameter and twig length each month. Have students graph the data and calculate the change in each parameter every month.

Harvesting the Crop...

At the end of the school year, ask: Did the trees grow more during some months than others? Which month saw the most growth? The least? What were the weather conditions (e.g., heat, sun, rain) during the months that saw a lot of growth versus those that only saw a little? What does this tell us about the activity of the meristems? What factors may have influenced this change in growth? In temperate regions, meristems are dormant during the winter. Therefore, students will notice very little change in the length of their twigs and diameter of their trees during winter months. At the start of each growing season, primary growth is resumed and lateral meristems are reactivated. During these months students will observe the greatest change in tree size.

Since there is quite a bit of room for measurement error in this activity, it also provides an excellent opportunity to explore the scientific inquiry process with your students. Is it possible that you stretched the string on one occasion but not on others? Did you measure from the lower or upper side of the bud scale scar each time? Did your original twig get broken off? Based on the answers to these questions, you and your students can discuss the importance of being consistent when you are conducting scientific research.

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