From Seed to Seed:
Plant Science for K-8 Educators
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From Seed to Seed: |
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Activity 9: Wood Cookies Grades: K-4 Associated Lesson Topics:
National Standards: Planting the Seed... Since trees don't celebrate birthdays, how do we know how old they are? How old do you think the section of tree you have is? How did you arrive at that guess? What did you count to get that number? Teacher Information: A "wood cookie" is simply a small slice of a tree trunk. Once students
have guessed their cookies' ages, encourage them-if they haven't already
done so-to count the "annual rings." The most obvious tree rings are composed
of a year's worth of xylem cells. Phloem cells
Necessary Materials:
Procedure:
Harvesting the Crop... Do all of the rings look the same? Are some larger or smaller than others? Is there any pattern to the change in ring size? What do you think could have affected the size of the rings? Do you think that the only purpose of the cells that make up these rings is to tell us how old the tree is? What else do you think they do? Throughout the growing season, xylem cells become progressively smaller. Although several environmental factors may be responsible for this (light, temperature, length of growing season), the presence of water is the most common explanation. The large amount of water present in the spring (rainfall, snow-melt) produces much larger cells than in the fall, when water availability is at a minimum. Similarly, the width of these annual rings gives us an indication of how much rainfall there has been during these years. Years with low amounts of rainfall produce narrow rings, whereas years with high rainfall produce wide rings. This relationship can sometimes pose a problem because abrupt changes in rainfall during the course of a year may produce false annual rings, resulting in an inaccurate estimate for the age of a given tree. Sources
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produce rings, but they are much more difficult to see and therefore are
not usually used to age a tree. Within each ring, the large cells toward
the center of the trunk are those produced in the spring (early wood;
see diagram). The xylem cells toward the outer side of the ring (that
make up the "band" separating one year of growth from the next) are compressed
cells produced in the fall (late wood).