Teacher's Guide
Activity 2 - Grades 6th- 8th
GOAL:
To understandtree growth
VOCABULARY:
pith, heartwood,sapwoodor xylem,
cambium,inner bark or phloem,outer
bark, annualgrowth rings
BACKGROUNb:
A tree is a living organismmadeup of cells. The roots transportwater and nutrientsfrom the soil
up the tree. When the nutrient rich water reachesthe leaves, the tree photosynthesizesusing Carbon
Dioxide and energy from the sun. The tree manufacturesits food (sugaror sap) in its leavesand the sap
travelsback down the tree.
The outer bark protectsthe interior layersof the tree from insects,diseases,injuries, excessheat
andcold and in somecases,fire.
Next to the outer bark is the inner bark or phloem. The inner bark movesthe sapdown the tree.
It lives for only a shorttime andthenbecomespart of the outerbark.
The next layer is a living layer of cells called the cambium. Thesecells make new bark on the
outsideand new wood on the insideof the tree. Every year cambiumaddsa new layer of cells, thickening
the tree.
The new wood is called sapwoodor xylem. The sapwoodmovesthe water,nutrientsand nitrogen
from the roots to the leaves.Eachyear new sapwoodis made. In time, the older layersof sapwoodbecome
heartwood.
Heartwoodis the oldestpart of the tree. It is often darkerthenthe softwood,asit hasstiffenedand
dried out. It is the most desirablewood for lumber. The heartwoodgives the tree strength. As treesage,
the heartwoodcanlose its strength,asthe tree becomesmore susceptibleto insectdamageand disease.
At the centerof the heartwood,is the pith. The pith is wherethe tree startedto grow.
Insidethe sapwoodand heartwoodare the annual growth rings. One light and one dark-colored
ring showoneyear's growth. Wider, light coloredrings are called springwood. Narrow, dark rings arecall
summerwood.Treesgrow fasterin the spring,whenthereis adequatewater and slower in the summer.
The tree rings tell us about environmentalchanges.For example,a tree growing on a steepslope
has wider spacedrings on the side facing down the slope. Narrow rings indicate drought. Wide rings
indicategood growing conditions. In standsof treesthat havebeenthinned,a group of narrow rings will be
followed by wider growth rings. Thinning allows the tree to grow fasterbecauseit hasmore spaceand has
to competewith few treesfor sun,water and nutrients.
STUDENT ACTIVITIES:
Review photosynthesis. Explain that each tree, through the processof photosynthesistakes in
carbondioxide andreleasesoxygen. This processcanbe scientifically statesas:
6C02+ 6H20
C6H1206
+ 602
CarbonDioxide + Water
Sugar+ Oxygen
Discussthe equation. Ask the studentsto hypothesizewhat would happento the equationif there
were a lack of light, water, space,etc. What if there was too much carbon dioxide? Could there be too
much oxygen? Who elseusesthe oxygen? What living things needcarbondioxide?
Ask the students:Do older treesor youngertreesproducemore oxygen? (Youngertreesproduce
more oxygenbecauseof their rapid growth. Christmastree farmsarean excellentsourceof oxygen!)
Test for carbon dioxide in exhaled breath. Limewater will turn from clear to milky when it
encounterscarbon dioxide. Fill one balloon with air from a pump. Have a studentblow up a second
balloon with air from his/her lungs. Dischargethe air from the balloons into two separatecontainersof
limewater.Note which oneturns milky.
Ifpossible, obtain a fireplacelog (SeeMS #4). Sawdown the centerof the wood. Sawacrossthe
wood to the center. Saw diagonally acrossthe upper end and removethe sawnpiece of wood. This will
exposethe parts of the tree. Define the vocabularyusing the log. Look for growth indicators. Was this
tree crowded? Are the rings widely spacedor closetogether?
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EXTENDED ACTIVITIES:
. Test for gasesgiven off by green plants by using elodea plants in an aquarium. Put the
aquariumin the sunlight andnotice the bubblesfonning on the plant. What happenswhenthe
plant is cut off from sunshine? Are there bubbles then? Why not? (The plant can't
photosynthesize.)
. As a group act out the parts of a tree, using RangerRick's, Nature Sco~e.Treesare Terrific
activity, "Build a Tree".
. Conduct an experimentto demonstratethat trees lose water through transpiration,using
RangerRick's, Nature Sco~e.TreesareTerrific activity, "Tell-Tale Transpiration".
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.
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Match tree ring patterns using Ranger Rick's, Nature Sco~. Trees are Terrific activity,
"Readingthe Rings".
Demonstratethe condition neededfor trees to live and grow, using Project Learning Tree's
activity, "Every Tree For Itself'.
Ifpossible, observea fallen stump. Identify the partsof the tree.
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