Green giants: tree scientists dare breathtaking heights to solve a mystery surrounding one of nature's tallest skyscrapers.

Walking on the moist, spongy carpeting of the forest floor, Stephen Sillett squeezes through thick patches of trees. Their brown trunks climb high into the sky like magic beanstalks--topping out at a dizzying 30 stories tall. Sillett is exploring Humboldt Redwoods State sunlight, keeping the air cool and the surroundings dim. But as Sillett hoists himself up the rope, sparse rays hit his face. "Sometimes you get these incredible sunbeams that cruise through the forest," he says.

Good thing: Sunlight is vital for tree growth. To get the most sunlight, this redwood has to grow tall enough to poke its top above the shade of neighboring Park--a protected area for coast redwoods, Sequoia sempervirens (SUH-coy-uh SEM-purVIE-renz). His goal: find out what keeps a coast redwood, or even the giant sequoia (shown, far right), from spindling upward forever.

In his quest to solve the mystery, Sillett, a botanist (plant scientist) at Humboldt State University in California, winds deeper into the park. He halts beneath a towering conifer (type of tree that bears cones). Nicknamed "Stratosphere Giant," it is the world's tallest living tree, with a height of 112 meters (370 feet).

Undaunted by its heights, Sillett has no intention of remaining at the tree's base. That's because he knows that the answer to his question lies among the topmost branches. Joined by George Koch, a botanist from Northern Arizona University, Sillett clips on his beltlike harness and a safety helmet. He spies his lifeline--a heavy rope that's looped over a clump of sturdy branches about 60 m (200 ft) off the ground. Holding tightly to the rope, Sillett moves his feet step by step up the tree trunk.

SUN CATCHERS

Near the forest floor, tightly packed trees squeeze out trees. "[Stratosphere Giant] has been competing with its neighbors for light, and it's been racing to the sky," Sillett explains.

The tree's needlelike leaves are its most powerful sun-grabbers. And Sillett is determined to get a sample. He tip-toes out onto a giant tree branch as if he's on a balance beam. "All the action is out toward the edge of the branches where the light is," says Sillett. That's where he snips off a stem of leaves and seals it in a plastic bag. The waxy leaves contain chlorophyll, or the emerald pigments that capture the sun's energy to power photosynthesis. This process tunas carbon dioxide (C[O.sub.2]) and water into food (see Nuts & Bolts, p. 18). Sillett wraps his hands around the rope and swings over to another branch for more leaf collecting.

BY THE BUCKETFUL

Inching upward by leaping from branch to branch takes a toll on Sillett. "Climbing trees is a very physical thing--especially if you're climbing the world's tallest trees," he says. While his ascent is demanding, there's a trickier climb taking place beneath the redwood's bark. Water that's in the soil gets soaked up by the redwood's roots. Then, hair-strand-thick tubes called xylem tissue carry roughly 500 gallons of water daily from the roots to its top leaves.

Making that journey possible: teeny pores, or stomata (STO-mah-tuh), that dot the redwood's leaves. During the day, the pinhole pores open. Heat from sunrays causes water to evaporate (turn from a liquid to a gas) out of the open pores--a process called transpiration. And water molecules tend to stick together. So as water exits needles on the tree's upper branches, more water gets tugged upward through the xylem.

STRETCH AND SNAP

When Sillett reaches the treetop, he looks out over a field of rolling, green hills. "When you get up there, the wind is blowing, and each tree is moving a little bit independently. It's an impressive sight," says Sillett.

Another observation: "At the bottom of these redwood trees the leaves are long, extended needles. At the top, they become tiny scales," Sillett explains (see photo, middle left). Why the change? Leaves swell and grow larger when they are full of water. But leaves at the tops of coast redwoods can get parched. That's because as water molecules inch upward, gravity (force that pulls two objects together) pulls them downward. That causes the water stream to stretch and get thinner. "The water column is like a rubber band being stretched tighter and tighter," says Koch.

And for the "Mount Everest" of the forest world, it takes a huge upward tug to out-compete gravity. Sometimes, that pull power can be greater than water's sticky forces. When that happens--snap!--the water band breaks apart and an air bubble, or embolism (EM-buh-LIH-zum), fills the void. "When air bubbles form inside (the xylem], the tree can't conduct water to its top," says Sillett. So the leaves collapse like popped water balloons.

STILL GROWING

Turns out, water stress keeps redwoods from reaching cloud-brushing heights. "As a tree gets taller, and farther from the soft, it's harder to pull up water," says Koch. Is there a maximum redwood height? To find out, Sillett and Koch calculated the height at which the tree's water band would snap. Answer: 122 m (400 ft). So even this redwood has growing room.

Do these rules hold for the redwood's neighbor? "We think the same water limits should apply to the giant sequoia," says Koch.

As the sun goes down, Sillett rigs his hammock for a treetop slumber. That way, he can take measurements of the redwood's leaves just as their stomata open in the morning. Sillett says, "When you wake up, the birds are singing their predawn chorus, and a fresh breeze is blowing." But it's back to work for these daring scientists.

Nuts & Bolts

Green plants like the coast redwood use natural ingredients to make food through photosynthesis.

1. BEAMING

When sunlight hits leaves, structures called chloroplasts convert it into energy to fuel photosynthesis.

2. SOP UP

The redwood has shallow underground roots that absorb water from the most soil.

3. LEAF DOTS

When a leaf's stoma opens, carbon dioxide enters and water exits.

4. TREE FARE

Inside leaf cells, chemical reactions turn the carbon dioxide and water into sugars the tree needs to grow. The process releases oxygen from the stomata.

OUT ON A LIMB: Stephen Sillett stops on a redwood branch to collect leaf samples.

[ILLUSTRATION OMITTED]

REDWOOD RELATIVE: Photographer James Balog snapped more than 300 photos and pieced them together to create this towering image of "Stagg," a 73.8 m (242 ft)-tall giant sequoia. Compare its size with the scientist (circled).

HANDS-ON SCIENCE

(NO LAB REQUIRED)

PREDICT

Which vegetable do you think has the most chlorophyll (green pigment, or a colored chemical compound, which helps plants absorb sun energy): spinach, green cabbage, red cabbage, or carrots? What about other pigments--carotenoids (yellow-orange to red) and anthocyanin (red-blue to purple)?

YOU NEED

safety goggles * five 20 oz plastic cups * 6 large spinach leaves * permanent marker * isopropyl rubbing alcohol (0.7 1, or 3 cups) * 2 red-cabbage leaves * 2 green-cabbage leaves * 1 large carrot with leaves * vegetable grater (one for the class) * 5 pieces of plastic wrap (each 13 cm by 13 cm, or 5 in. by 5 in.) * scissors * 5 coffee filters * tape * 5 pencils * 5 paper towels * pencil and paper

PROCEDURE

1. Put on your safety goggles.

2. Rip 6 spinach leaves into fingertip-size pieces.

3. Place the shredded leaves in a cup. Using a permanent marker, label the cup with its contents.

4. Pour 140 ml (0.6 cups) of rubbing alcohol into the plastic cup, or enough to cover the clippings.

(Caution: Keep the alcohol away from your mouth, eyes, broken skin, and hot surfaces.)

5. Repeat Steps 2 to 4 for the red cabbage, green cabbage, carrots, and carrot leaves. Use the vegetable grater to shred the carrots.

6. Cover each cup with a plastic-wrap piece. Set the cups aside for 1 hour.

7. Cut 5 coffee-filter strips (2.5 cm by 13 cm, or 1 in. by 5 in.), one from each filter.

8. Tape each strip around the middle of a pencil, so that it hangs perpendicular to the pencil.

9. Remove the cups' plastic wrap.

10. Position each pencil across the top of each cup. Each filter strip should hang inside its cup with only its tip touching the rubbing alcohol. If the strip is too long, cut a sliver from the free end.

11. Place each plastic-wrap piece across the top of each pencil-topped cup. Set the cups aside for 1 hour.

12. Remove each pencil and filter from its cup.

13. Place each filter strip on a paper towel, next to its cup, to dry for 30 minutes.

14. When dry, observe the bands on the filter strips, and record their color and darkness.

CONCLUSIONS

1. Which of the vegetables showed the most chlorophyll? The least?

2. Which of the vegetables showed anthocyanin? Carotenoids?

DID YOU KNOW?

* A giant sequoia called "General Sherman" is the world's largest tree. Located in Sequoia National Park, California, the 2,000-year-old tree has a volume of 1,472 cubic meters (52,000 cubic feet).

* Coast redwoods can live to be 2,000 years old. One reason: Their reddish bark contains tannic acid, a plant chemical that has a bad taste. This chemical keeps hungry pests from munching on the tree's bark.

CRITICAL THINKING:

* Trees take in carbon dioxide for photosynthesis. They also release oxygen in this process. What do you think would happen if most of the world's trees died off? Hint: Carbon dioxide is a greenhouse gas that traps heat in the atmosphere.

CROSS-CURRICULAR CONNECTIONS:

LANGUAGE ARTS: Have students read "The Giving Tree," by Shel Silverstein, Harper Collins Publishers, 1964. Then have each student write a short story about how a tree provides for human needs, from the tree's point of view.

RESOURCES

* Play the Fantastic Forest game at: www.nationalgeographic.com/forest/index.html

* Learn more about photosynthesis at: www.pbs.org/wgbh/nova/methuselah/photosynthesis.html

1. The spinach had the most chlorophyll. The red cabbage and carrots showed no chlorophyll. Both red cabbage and carrots grow below the ground, so they don't need chlorophyll.

2. Spinach, green cabbage, and carrot Leaves showed green bands on the filters; red cabbage showed purple; carrots showed yellow. Red cabbage contains the pigment anthocyanin, and carrots contain carotenoids.

DIRECTIONS: Match the word(s) in the left column with the correct phrase in the right column.

-- 1. botanist          a. process that turns carbon dioxide and water
                           into food
-- 2. conifer           b. air bubble
-- 3. chlorophyll       c. plant scientist
-- 4. photosynthesis    d. plant pigments that help capture the sun's
                           energy
-- 5. xylem             e. turn from a liquid into a gas
-- 6. stomata           f. type of tree that bears cones
-- 7. evaporate         g. teeny pores on leaves
-- 8. embolism          h. tissue that carries water from the tree's
                           roots up to its topmost leaves

1. c 2. f 3. d 4. a 5. h 6. g 7. e 8. b