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TPO26- Survival of Plants and Animals in
Desert Conditions
1
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6
Survival of Plants and Animals in
Desert Conditions
The harsh conditions
in deserts are intolerable for most plants and
animals. Despite these
conditions,
however, many varieties of plants and animals have
adapted to deserts in a number of
ways.
Most plant tissues die if their water content
falls too low: the nutrients that feed plants are
transmitted
by
water;
water
is
a
raw
material
in
the
vital
process
of
photosynthesis;
and
water
regulates the
temperature of a plant by its ability to absorb
heat and because water vapor lost to the
atmosphere
through
the
leaves
helps
to
lower
plant
temperatures.
Water
controls
the
volume
of
plant
matter produced. The distribution of plants within
different areas of desert is also controlled
by water. Some areas, because of their
soil texture, topographical position, or distance
from rivers
or groundwater, have
virtually no water available to plants, whereas
others do.
The
nature of plant life in deserts is also highly
dependent on the fact that they have to adapt
to the prevailing aridity. There are
two general classes of vegetation: long-lived
perennials, which
may be succulent
(water-storing) and are often dwarfed and woody,
and annuals or ephemerals,
which have a
short life cycle and may form a fairly dense stand
immediately after rain.
The
ephemeral
plants
evade
drought. Given
a
year
of
favorable
precipitation,
such
plants
will
develop vigorously and produce large numbers of
flowers and fruit. This replenishes the seed
content
of
the
desert
soil.
The
seeds
then
lie
dormant
until
the
next
wet
year,
when
the
desert
blooms again.
The perennial vegetation adjusts to the
aridity by means of various avoidance mechanisms.
Most
desert
plants
are
probably
best
classified
as
xerophytes.
They
possess
drought-resisting
adaptations: loss of water through the
leaves is reduced by means of dense hairs covering
waxy
leaf
surfaces,
by
the
closure
of
pores
during
the
hottest
times
to
reduce
water
loss,
and
by
the
rolling
up
or
shedding
of
leaves
at
the
beginning
of
the
dry
season.
Some
xerophytes,
the
succulents (including cacti), store
water in their structures. Another way of
countering drought is
to have a limited
amount of mass above ground and to have extensive
root networks below ground.
It is not
unusual for the roots of some desert perennials to
extend downward more than ten meters.
Some plants are woody in type
—
an adaptation designed to
prevent collapse of the plant tissue
when water stress produces wilting.
Another class of desert plant is the phreatophyte.
These have
adapted
to
the
environment
by
the
development
of
long
taproots
that
penetrate
downward
until
they approach the assured water supply
provided by groundwater. Among these plants are
the date
palm,
tamarisk,
and
mesquite.
They
commonly
grow
near
stream
channels,
springs,
or
on
the
margins of lakes.
Animals also have to adapt to desert
conditions, and they may do it through two forms
of
behavioral adaptation: they either
escape or retreat. Escape involves such actions as
aestivation, a
condition of prolonged
dormancy, or torpor, during which animals reduce
their metabolic rate and
body
temperature during the hot season or during very
dry spells.
Seasonal
migration is another form of escape, especially
for large mammals or birds. The
term
retreat is applied to the short-term escape
behavior of desert animals, and it usually assumes
TPO26- Survival of Plants and Animals
in Desert Conditions
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6
the pattern of a daily rhythm. Birds
shelter in nests, rock overhangs, trees, and dense
shrubs to
avoid the hottest hours of
the day, while mammals like the kangaroo rat
burrow underground.
Some
animals have behavioral, physiological, and
morphological (structural) adaptations
that enable them to withstand extreme
conditions. For example, the ostrich has plumage
that is so
constructed that the
feathers are long but not too dense. When
conditions are hot, the ostrich erects
them on its back, thus increasing the
thickness of the barrier between solar radiation
and the skin.
The
sparse
distribution
of
the
feathers,
however,
also
allows
considerable
lateral
air
movement
over the skin
surface, thereby permitting further heat loss by
convection. Furthermore, the birds
orient
themselves
carefully
with
regard
to
the
Sun
and
gently
flap
their
wings
to
increase
convection cooling.
（
672
WORDS
）
**********
**************************************************
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Paragraph 1:
The harsh conditions in deserts are
intolerable for most plants and animals.
Despite these conditions, however, many
varieties of plants and animals have adapted to
deserts in
a number of ways. Most plant
tissues die if their water content falls too low:
the nutrients that feed
plants are
transmitted by water; water is a raw material in
the vital process of photosynthesis; and
water regulates the temperature of a
plant by its ability to absorb heat and because
water vapor lost
to the atmosphere
through the leaves helps to lower plant
temperatures. Water controls the volume
of
plant
matter
produced.
The
distribution
of
plants
within
different
areas
of
desert
is
also
controlled by water.
Some areas, because of their soil texture,
topographical position, or distance
from rivers or groundwater, have
virtually no water available to plants, whereas
others do.
Q1:
According to paragraph 1, water provides all of
the following essential functions for plants
EXCEPT
A.
improving plants’ ability to absorb sunlight
B. preventing plants from
becoming overheated
C.
transporting nutrients
D. serving as a
raw material for photosynthesis
Paragraph 2:
The nature of
plant life in deserts is also highly dependent on
the fact that
they have to adapt to the
prevailing aridity. There are two general classes
of vegetation: long-lived
perennials,
which may be succulent (water-storing) and are
often dwarfed and woody, and annuals
or
ephemerals, which have a short life cycle and may
form a fairly dense stand immediately after
rain.
Paragraph
3:
The ephemeral plants evade drought.
Given a year of favorable precipitation,
such
plants
will
develop
vigorously
and
produce
large
numbers
of
flowers
and
fruit.
This
replenishes the seed content of the
desert soil. The seeds then lie dormant until the
next wet year,
when the desert blooms
again.
Q2: Paragraph 3
suggests that during a dry year ephemerals
A. produce even more seeds than in a
wet year
B. do not sprout
from their seeds
TPO26-
Survival of Plants and Animals in Desert
Conditions
3
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6
C. bloom much
later than in a wet year
D.
are more plentiful than perennials
Q3: Howis paragraph 2 related to
paragraph 3?
A. Paragraph 2 provides a
general description of desert plants, and
paragraph 3 provides a
scientific
explanation for these observations.
B.
Paragraph
2
divides
desert
plants
into
two
categories,
and
paragraph
3
provides
further
information about one of these
categories.
C. Paragraph 2 proposes one
way of dividing desert plants into categories, and
paragraph 3
explains one problem with
this method of classification.
D.
Paragraph 2 discusses two categories of desert
plants, and paragraph 3 introduces a third
category of plants.
Q4: In saying th
at
ephemerals will develop
“
vigorously
author means that
their development will be
A. sudden
B. early
C. gradual
D. strong and healthy
Paragraph 4:
The perennial
vegetation adjusts to the aridity by means of
various avoidance
mechanisms.
Most
desert
plants
are
probably
best
classified
as
xerophytes.
They
possess
drought-resisting
adaptations: loss of water through the leaves is
reduced by means of dense hairs
covering waxy leaf surfaces, by the
closure of pores during the hottest times to
reduce water loss,
and by the rolling
up or shedding of leaves at the beginning of the
dry season. Some xerophytes,
the
succulents (including cacti), store water in their
structures. Another way of countering drought
is
to
have
a
limited
amount
of
mass
above
ground
and
to
have
extensive
root
networks
below
ground. It is not
unusual for the roots of some desert perennials to
extend downward more than ten
meters.
Some plants are woody in type
—
an adaptation designed to
prevent collapse of the plant
tissue
when water stress produces wilting. Another class
of desert plant is the phreatophyte. These
have
adapted
to
the
environment
by
the
development
of
long
taproots
that
penetrate
downward
until they approach
the assured water supply provided by groundwater.
Among these plants are the
date palm,
tamarisk, and mesquite. They commonly grow near
stream channels, springs, or on the
margins of lakes.
Q5: The word
“
countering
” in the passage
is closest in meaning to
A.
eliminating
B. making use
of
C. acting against
D. experiencing
Q6: According to paragraph 4, some
desert plants with root systems that are
extraordinarily well
developed have
A. relatively little growth aboveground