for the transport of plants
also rely on water for their internal transport system. They have
two main tissues that are responsible for the movement of substances
around the plant. One is a tissue called em. This is responsible
for moving water and dissolved minerals from the roots to the parts
of the plant above ground. The water flows up the tiny tubes called
xylem vessels, which are rather like a bundle of drinking straws.
When you drink using a straw, water is drawn up the straw because
you suck at the top. Plants draw up water from the ground in a similar
manner. As water evaporates from the surfaces of the leaves, more
water is drawn upward from the root and up the stem to replace it.
This creates a continual flow of water from the roots to the leaves.
The evaporation of water from the leaves is called transpiration.
have tiny pores, called stomata. Most of these are found on the
under surface of the leaf. The stomata are designed to allow gases
to enter and leave the plant for photosynthesis (the process of
making food from carbon dioxide and water using light energy) and
respiration (the process in which glucose and other food materials
are broken down to release energy). When the stomata are open, water
vapor can also escape. Water loss is less at night because the stomata
are closed. Many leaves have a shiny, wax-like, waterproof covering
on the upper surface of their leaves, known as a cuticle. This covering
helps to reduce water loss because plants with a waxy cuticle can
only lose water through the stomata on the lower surfaces of each
adaptions of plants
plants have to be able to survive for months, and sometimes even
years, without a supply of water. Plants that show special adaptations
to conserve or store water are called xerophytes. Cacti and succulents
are the most common examples. They have small leaves, so less water
evaporates from their surfaces. In extreme cases the leaf has been
reduced to a spine. The leaves are usually covered with a thick,
waxy cuticle to reduce water loss. Many species of cacti are covered
in a layer of white hairs. The white color reflects heat away from
the plant, while the hairs trap a thin layer of air around the plant,
making it more difficult for water to evaporate. Some plants, such
as the giant fig, have evolved incredibly deep roots that are able
to reach down to the water table many feet below ground. Others
have a shallow but wide root system. This is designed so that, when
it rains, the roots can absorb water from as large an area as possible.
Many cacti are shaped like a barrel so that liquid can be stored
within the stems. Often this liquid is quite acid to the taste,
but one or two cacti contain sweet, fresh water that is safe to
and succulents are not the only plants that live in deserts. Annual
plants (plants that live for just one year) live there too, sometimes
surviving only for a few weeks, just long enough to produce seeds
before dying. The landscape in a desert is transformed after rain,
for the annual plants have to complete their life cycles very quickly.
As soon as the rain falls, the seeds in the ground germinate and
within a few weeks they flower and produce seeds. These seeds may
have to stay dormant in the ground for several years until it rains
again when they, in turn, can germinate.
Namib Desert in southwest Africa is different from other deserts.
Although it rarely rains, the desert is close to the coast and is
sometimes covered by fog. On several nights each year, the fog moves
over the desert and, as it does so, tiny droplets of water condense
in the cool air and fall to the ground. Many animals and plants
have developed behavior patterns to make use of this water supply.
Darkling beetles, which have very long legs, clamber to the top
of sand dunes and align themselves to face the coast. The beetle
will raise its abdomen and, as the fog moves past, droplets of water
condense on it and roll down into its open mouth. The Namib Desert
is also home to an unusual plant called welwitschia. It does not
resemble a conventional desert plant because rather than having
small leaves, welwitschia has huge leaves, each being several yards
long. Running beneath the upper surface of the leaf are absorbent
fibers. These fibers are specially adapted to absorb any moisture
that condenses on the surface.