1. Cycads
   Succulents
2. Annuals
   Perennials

Trees aren't annuals, they're perennials. That means they'll keep coming back, season after season, until they've reached the end of their lifespans.

1. Woody
   Branches at least 10 feet long
2. Diamond-shaped leaves
   Long-lived

All trees are woody creatures. The qualities of that wood varies greatly by species, location and health.

1. 4 feet
   8 feet
2. 20 feet
   100 feet

To be sure, there are plenty of shorter trees in the world. But most trees grow to a height of at least 20 feet, and often much, much taller.

1. Protection
   Water circulation
2. Photosynthesis
   Reproduction

Some trees have rough bark, others have smooth, skin-like bark. But all bark is the same in that it's meant to offer protection from the environment.

1. Aspen
   Oak
2. Pine
   Douglas fir

In Utah, there is a colony of aspens called Pando, the Trembling Giant. The colony has about 47,000 trees (more than 6,600 tons) regarded as one organism … perhaps the second-largest organism on the planet (after a huge Oregon fungal colony).

1. Yellow bark
   Small diameter
2. Sharp thorns
   Leafy crown

Trees generally have long branches that reach to the skies. These branches terminate in a leafy crown that makes trees so distinctive.

1. Their leaves fall off.
   They produce the toughest bark.
2. Their wood is very soft.
   They have flammable leaves.

Each year, deciduous trees lose their leaves at the end of the growing season. Then, the following year, those leaves begin growing back, starting with buds.

1. 280 years old
   720 years old
2. 1,280 years old
   4,700 years old

In California's White mountains you'll find Methuselah, perhaps the world's oldest tree. Scientists estimate this tree is around 4,700 years old.

1. Douglas fir
   White elm
2. Red oak
   Bristlecone pine

Methuselah, a tree in California, is a bristlecone pine that's more than 4,700 years old. The tree's location is kept secret to prevent attention-seeking vandals from destroying it.

1. Cork
   Timbers
2. Oak
   Conifer

Evergreens are also conifers because they produce cones. Cones fall to the ground and scatter the plant's seeds, ensuring future generations.

1. Quality of branches
   Number of leaves
2. Growth rings
   Scarring on the bark

Every year, trees typically grow a bit in the wet season, and then cell production slows during dry or cold periods. The growth rings of these cells are useful in determining the tree's age.

1. General Sherman
   Old Gabe
2. Trump Tower
   Tolkien Tree

In the Giant Forest of Sequoia National Park of California, you'll find the General Sherman tree, a huge giant sequoia. From base to tippy top, the tree is nearly 275 feet tall. It is -- by mass -- the world's biggest tree.

1. Remove pollution from the air
   Create shade
2. Provide medicine
   All of the above

Trees are incredibly useful to humans. Not only do they provide precious shade, but they filter our air, provide certain types of medicine, and much more.

1. Diseases
   Climate changes
2. Old age
   Pesticides

Unlike most creatures, trees don't die of old age. They succumb to many other threats, though, from diseases to environmental changes and more.

1. About 2,300
   About 4,800
2. About 7,200
   About 60,000

Trees grow in almost all of Earth's climates and regions, from deserts to mountaintops. There are about 60,000 different species.

1. Dead cells
   Seed casings
2. Phloem
   Phlegm

Tree bark is vital to the plant's health, offering protection from all manner of hazards. The bark is made up of dead cells that have desiccated on the plant's exterior.

1. Color
   Photosynthesis
2. Fertilization
   Defense

Trees have all sorts of defense mechanisms. Volatile phenolic compounds are one way the plants ward off pests -- the chemicals may call in predators that keep invaders at bay.

1. Heartwood
   Focal point
2. Wagonwood
   Fenceposts

At the tree trunk's core is the heartwood. The heartwood is typically darker in color than the rest of the truck, and it's important in providing stiffness to the trunk.

1. Broad leaves
   Deciduous
2. Succulents
   Evergreens

Some types of trees, like pines and firs, are known as evergreens. Their iconic green needles remain on the tree's branches for years, unlike deciduous leaves, which are shed annually.

1. 280 feet
   720 feet
2. 380 feet
   1,300 feet

In California, there exists a tree named Hyperion, a 380-foot tall redwood. Like other prized trees, the location of this one is secret so that no one will be tempted to damage it.

1. Two
   Three
2. Six
   Seven

Pine trees are robust creatures capable of withstanding many environments. The only continent where they're not found is Antarctica.

1. 50 to 100
   1,000
2. 10,000
   100,000

As homeowners know, large oaks can generate copious numbers of acorns. A big tree might hurl 10,000 -- or more -- acorns in one season.

1. They are used to make pasta.
   They are either male or female.
2. They are totally fireproof.
   They grow new trees 99 percent of the time.

The next time you see a pine tree, you can start guessing -- is it dropping boy or girl cones? All cones carry just one sex in hopes of reproducing the species.

1. 6 feet in diameter
   38 feet in diameter
2. 696 feet in diameter
   192 feet in diameter

In Mexico you'll find the Arbol del Tule, a Montezuma cypress. This enormous tree is more than 38 feet in diameter … perhaps making your first aparment look small by comparison.

1. By tempting loggers
   By offering reflected light
2. By creating a distraction to predators
   By providing nutrients

In forests, not all trees are the same. Central "mother" trees often use fungal networks in the soil to send excess nutrients to smaller trees … thereby strengthening the forest as a whole.

1. In Canada
   In a single country
2. In Ghana
   Near Seattle

Trees are common, but species aren't always widespread. In fact, a little more than half of all known tree species grow in a single country, demonstrating how important it is for each nation to conserve its resources.

1. Venom creation
   Water circulation
2. Photosynthesis
   Transpiration

Plants need water, and that water must move somehow. Xylem cells have particularly thick walls that make them appropriate for their purpose -- water transportation.

1. 30 liters
   10 gallons
2. 30 gallons
   100 gallons

Trees require a lot of water; the bigger the tree, the more water it needs. A huge oak might chug 100 gallons of water in a single day.

1. Not until recently
   Here from the beginning
2. Since shortly after Earth formed
   In only the past 50,000 years

Trees are new kids on the block. Earth is about 4.5 billion years old. Trees didn't appear until about 400 million years ago.

1. Agriculture
   Dutch Elm disease
2. Parasites
   Herbicides

The Earth has far fewer trees than it did before humans began cultivating crops. Agriculture and its associated land usage has cut tree numbers by about half since 12,000 years ago.

1. In just 2 inches
   In the top 18 inches
2. In the 10-foot range
   About 20 feet down

Many trees put down deep roots. But in most cases, tree roots are found no more than 18 inches deep.

1. About 2 percent
   About 20 percent
2. Roughly 70 percent
   Nearly 90 percent

The Amazon rainforest is vital to oxygen production on Earth. Each year, this 2.1 million square mile area generates about 20 percent of the planet's oxygen.

1. Temperature changes
   Rainfall
2. Soil conditions
   Fungi levels

In tropical and subtropical regions, it's not seasons that trigger leaves to fall. Instead, it's rainfall (or lack thereof) that starts this process each year.

1. In horizontal strips
   In ragged bunches at the crown
2. In one clump
   It doesn't shed bark.

You'll have to forgive the white birch for the messes it makes. It leaves ragged horizontal strips of bark as it move through its life cycle.

1. 10 years earlier
   100 years earlier
2. 5,000 years earlier
   150 million years earlier

In a discovery that still floors researchers, in 1994, the Wollemia (a cone-bearing evergreen) was found living in Australia. The catch? Scientists were sure it had gone extinct 150 million years earlier.