What is cactus?
A: A cactus is a member of the plant family Cactaceae.
Cacti occur in a wide range of what?
A: Shapes and sizes.
Although some species live in quite humid environments,
most cacti live in what kind of habitats?
A: Habitats subject to at least some drought.
Many live in extremely dry environments, even being
found where?
A: In the Atacama Desert, one of the driest places on Earth.
Because of this, cacti show many adaptations to do
what?
A: Conserve water.
For example, almost all cacti are succulents, meaning
they have what?
A: They have thickened, fleshy parts adapted to store water.
Most species of cacti have lost true leaves, retaining
only what?
A: Spines, which are highly modified leaves.
As well as defending against herbivores, spines help
prevent water loss by doing what?
A: Reducing air flow close to the cactus and providing some shade.
In the absence of true leaves, what do cacti's enlarged
stems carry out?
A: Photosynthesis.
Cacti are native to the Americas, ranging from
Patagonia in the south to parts of western what?
A: Canada in the north—except for Rhipsalis baccifera, which also grows in
Africa and Sri Lanka.
Cactus spines are produced from specialized structures
called what?
A: Areoles, a kind of highly reduced branch.
As well as spines, areoles give rise to what?
A: Flowers, which are usually tubular and multi-petaled.
Many cacti have short growing seasons and long
dormancies and can react quickly to what?
A: To any rainfall, helped by an extensive but relatively shallow root
system that quickly absorbs any water reaching the ground surface.
Cactus stems are often ribbed or fluted, which allows
them to do what?
A: To expand and contract easily for quick water absorption after
rain,
followed by retention over long drought periods.
Like other succulent plants, most cacti employ a
special mechanism called "crassulacean acid metabolism" (CAM) as part of
what?
A: Photosynthesis.
Transpiration, during which carbon dioxide enters the
plant and water escapes, does not take place when?
A: during the day at the same time as photosynthesis, but instead occurs at
night.
The plant stores the carbon dioxide it takes in as
what?
A: Malic acid, retaining it until daylight returns, and only then using it
in photosynthesis.
Because transpiration takes place during the cooler,
more humid night hours, water loss is what?
A: Significantly reduced.
Many smaller cacti have globe-shaped stems, combining
the highest possible volume for what?
A: For water storage with the lowest possible surface area for water loss
from transpiration.
What is the tallest free-standing cactus?
A: Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft).
What is the smallest?
A: Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at
maturity.
A fully grown saguaro is said to be able to absorb how
much water during a rainstorm?
A: As much as 200 U.S. gallons (760 l; 170 imp gal.
In most cacti, the areoles produce new spines or
flowers only for how long?
A: A few years and then become inactive.
Many cacti have roots that spread out widely, but only
penetrate what?
A: A short distance into the soil.
In most cacti, the stem acts as the main organ for
what?
A: Storing water.
Some cacti have in addition large what?
A: Taproots.
No known fossils of cacti exist to throw light on
what?
A: Their evolutionary history.
Cactus flowers are pollinated by what?
A: Insects, birds, and bats.
What are the most common pollinators of cacti?
A: Bees.
What type of birds are significant pollinators of
cacti?
A: Hummingbirds.