Animals and plants use different life strategies.
- Plants make food: animals consume it.
and animals store
- Animals and plants move in different ways.
- Plants and animals grow differently.
- Plants have more freedom of form than animals.
Plants are primary
producers: they create their food from carbon dioxide and
water by using the energy in sunlight. They take root in a sunny
spot, spread out their leaves (or cells with chloroplasts), and
get to work.
For animals, the problem of
staying alive is more complex. Animals are consumers.
They must find something to eat to get energy.
All animals are dependent on
the food created by plants. Some animals eat plants, and some
eat other animals. In either case, they are taking in a concentrated
form of energy. In order to have a steady supply of food, animals
must either locate themselves where there is an unfailing supply
of food or find ways to move to where the food is.
Sometimes plants and animals
are able to capture more
energy than they need for
basic living. They can make use
of this energy in several
ways, building more bodily
reproducing, or storing the
energy for later use.
developing animal adds bodily
structure by growing larger, a
mature one by, for example,
growing a warmer fur coat for
the winter or perhaps adding
muscle mass. Plants could
enlarge their root systems
and/or grow more twigs,
leaves, and branches.
Another use for the energy
could be reproduction.
Reproduction is expensive in
terms of energy costs. Plants tend to
produce more flowers and seeds
when resources are abundant,
and animals may produce larger
litters (if they have several
babies at once), or more
vigorous young with a better
survival rate. Egg
layers will lay more eggs, and
birds will be able to feed and
raise more hatchlings.
Extra energy may also be
stored for later use.
Animals store extra energy in
the form of fat, which they
can carry with them on their
animals store fat in the
autumn and then go into a
state where the rate of their
metabolisms is reduced, and so
they survive the winter.
Migrating animals store fat
for use on their long
journeys. Storing fat is
very useful for animals, who
may not have access to
plentiful, reliable food
Some animals store energy to
feed their young.
Animals that hatch from eggs,
in water or on land, include
food for their babies in the
yolk, and additional supplies
and water in the material that
surrounds the yolk.
These food supplies provide
energy for the developing
animal as it builds its body
within the shelter of the egg.
Plants don't move around, so,
in a way, storing energy is
easier for them. They
may store tubers, like
They may have tap roots that
can be enlarged, like
Underground energy stores
generally have growth buds on
them, so that a new plant can
sprout and grow rapidly with
this extra food.
Another way that plants store
food is in their seeds.
When we eat nuts, peas, or
beans we are eating the
energy-rich cotyledons that
could nourish a developing
plant. When we eat corn,
wheat, and rice, we are again
eating seeds. This is
energy that plants have
captured and did not use just
being basic plants.
Living things do what they must to find
food. Although we think of plants as being immobile, plants grow
toward the light, and can slowly turn their leaves to take advantage
of whatever light is available. The cells on one side of a stem
divide, becoming two cells and making that side longer, so then
the stem bends. Vines use a similar process to twine around a
support. Using this method of uneven growth, plants can move
their leaves and flowers.
Some plants can move a little
in response to stimuli from the environment. Sensitive plants
can move their leaves in response to touch, and Venus fly traps
will close their leaves around insects. However, plants do not
have muscles like animals -- they use other mechanisms to achieve
Plants cannot move to new places,
but they do need mobility for reproduction. Plants need to scatter their pollen so that the next
generation of plants will be strong. They also need to scatter
their seeds. If new plants grow next to their parents, all of
them must compete for the same resources, and that is not efficient.
If conditions change, the rooted plants may die out where they
are. Plants need to have access to new places to grow, and to
test the limits of their ranges.
need to travel. If seeds are scattered, they may take root
in new places where their kind has not grown before. Seeds dropped
in unsuitable places do not grow up into plants, but plants make
many seeds, and their species can survive if only a few of their
seeds grow to
are used to seeing animals move. Animals are dependent on their
food sources, and must be able to go to new pastures to graze,
or move to new territories to find mates or prey.
can migrate to favorable places they can take their young with
them, either in their mobile bodies during pregnancy, or with
the protection of the parents while the young are immature.
however, requires energy. Animals, who must use some of their
energy for mobility, obviously need more energy to survive than
plants. The chemical reactions that allow for movement require
complex structures -- hearts to pump fluids and special organs
to oxygenate the blood. These organs require energy for maintenance
and performance. It is more energy-expensive to be an animal
than a plant.
Growing takes energy. A living
organism must have energy to maintain itself alive: it must have
some energy left over in order to grow. If there is not enough
energy for growth during a growing period, the living thing may
Plants grow when temperatures
are favorable. In spring, rising temperatures and longer days
allow plants to grow and
produce additional energy for flowering.
In summer, when days are at their longest and the most light
energy is available, plants continue to grow and use more energy
to make their seeds. In autumn, as days grow shorter, plants
store any excess energy in their tissues, or in bulbs, or underground
tubers, and prepare to spend the winter being dormant. They shed
their leaves, so that only slender branches are exposed to winter
storms and heavy snow.
In winter, low
temperatures slow down
processes and many
plants become dormant,
waiting for the return
However, some plants die.
leaving seeds that will germinate when warmer weather comes
retain the power to grow additional limbs, stalks, and leaves
until the waning of their lives. This is necessary because they
may be damaged by animals or fire. In a year when there is not
enough water plants will grow less than in a more favorable year,
but they will grow a little if they can.
on the other hand, are designed for efficient movement, and so
cannot modify their forms very much.
of many animals are formed in the embryonic stage: their forms
change by growing, but no new parts are
added once the basic
formed. In the higher animals a
part of the body that is lost will not grow
such as fishes, turtles and some other reptiles, continue to
grow throughout their
lives, and some can
even regrow limbs.
Mammals and birds, however, reach adulthood and then stop
growing. Growth is affected by the amount of food available,
and an animal or person whose growth is stunted by food shortages
during the growing years will never reach his or her full potential
It is interesting to
see how the sizes of
animals change over
Scientists have found
fossils that show how
the horse evolved from
an animal about the
size of a medium sized
dog to the size that
today. Many of
the large grazing
animals have evolved
from smaller animals,
getting the advantages
of longer strides for
running and larger
digestive systems to
deal with tough,
There have also been
examples of animals
happens over time when
a group of large
animals finds itself
inflexible limits on
resources, for example
by being trapped on a
Horses have downsized
to ponies, and a group
of mammoths that began
as larger than present
until they were only
three feet tall.
Why do animals stop growing?
I'm guessing, but I think it has something to do with the surface area to volume ratio. Each
species has an average body size that works well in its environment,
and the body's organs, such as the lungs and heart, are designed
to work best at that size and weight. Growing larger could
lead to a loss of efficiency. This is especially important for
warm-blooded animals, whose food needs are high. Also, when the
animals become mature, the extra energy that was used in growing
can be redirected toward reproduction and the care of their young. Size
is inherited: evolution would select animals that grew to an
optimally efficient size and then stopped growing.
Members of an animal species
all conform to the same body plan: structural differences between
them are minor. Plants, although recognizable by their leaf forms
and flowers, vary considerably in the placement of their branches
and twigs. Why is this?
Plants spend their lives in
the place where they began to grow: they need to adapt to the
very specific conditions of that place. They will grow so as
to get as much light as possible, and to make good use of the
water available to them. Being able to grow into a shape appropriate
to their circumstances is advantageous to them, and is a good
use of their energy resources. Although members of each species
tend towards a characteristic shape, size, and growth habit,
the individual plant is free to improvise within the context
of the location.
| Animals are not
so free. The Inuit people say that "the slim legs of the
deer are sculpted by the sharp tooth of the wolf." Animals
are constantly being selected on the basis of how well they fit
into their ecological niches, and innovations must improve the
ability of the individual to survive. Each individual is already
the result of a long selection process, and body form is a part
of that fitness.
living beings are shaped by balancing the costs of their structures
against the benefits of maintaining those structures. The power
of movement demands efficiency, strength, and lightness. The
bodies of animals tend to approximate the ideal forms for their
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