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The Fight for Light and Water
Teacher Background
We know that most of a tree trunk is dead. Only a thin layer of cells near the bark, the leaves and
the roots are alive. If plants make food only in their green leaves and this food is used to nourish
the growing regions, how does food get down the trunk to nourish the roots? How does water
absorbed by the roots get up to the treetop to provide hydrogen for photosynthesis? Small-sized
photosynthesizers like single-celled green algae in a pond transport materials easily; simple
diffusion moves food in and waste out. But as body size becomes larger, simple diffusion no
longer suffices. Trees and large plants are composed of many cells and distances are great
between the source of materials and where they are used in the plant. Several different hypotheses
have been proposed to explain how materials move through the bodies of large trees.
Atmospheric pressure, for example, can push water up from the roots to a height of about 32 feet.
But some trees in the temperature rainforest are almost 10 times taller. How does water get up
more than 300 feet in a temperate rainforest spruce tree?
SIDEBAR: Big Trees of the Temperate Rainforest
The largest known spruce in the world is located in the rainforest of Washington state's Olympic
Peninsula. This tree, the Queets Spruce, contains over 12,000 cubic feet of wood in the trunk
alone and is one of the fastest growing trees on Earth. Spruce trees more than 8 feet thick and 200
feet tall are common. Spruce branches are often 30 feet long.
Big leaf maples are abundant as well. They have trunks that are 3-4 ft thick, massive limbs that
spread out 70-80 ft, and giant leaves measuring 6-12 inches across. Branches may be covered
with hanging moss which weighs a ton or more?
A Western Red Cedar named the Nolan Creek Red Cedar, located near Forks on the Olympic
Peninsula, has a trunk 19 ft. in diameter and is 178 ft tall.
Rainforest trees climb fast and keep growing. Some grow to 175 ft in 100 years. The largest trees
are 400-700 years old and are still growing rapidly.
Objective
This set of Activities enables students to experiment with some aspects of the competition for two
key resources in the rainforest, light and water, and to explore trees from roots to leaves, soil to
sunlight, bottom to top.
Vocabulary
phloem, xylem, photosynthesis, respiration, adhesion, cohesion, stoma *plural: stomata),
chlorophyll, transpiration
Engage
Leaves take in carbon dioxide and water, but they also give off great quantities of water drawn up
from roots. A red maple growing in a humid environment evaporates 50 gallons of water per day.
There must be an equally large rate of water absorption at the root tips. The ability of 300 ft tall
trees to push sap and minerals up through their microscopically thin xylem vessels by capillary
action is quite amazing. To understand such an enormous liquid exchange system we need to ask
four questions.
1. Where does water enter? (via the roots)
2. How does water climb trunks? (adhesion and cohesion place water under tension and pull it up)
3. Where does water go? (We can detect it just under the leaf surface.)
4. How does it get out of the waterproof coating of the plant? (through openings called stomata)
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