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 structure, reproducing, or storing the energy for later use.

      .A 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 travels.  Hibernating 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 supplies.

     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 potatoes, underground.  They may have tap roots that can be enlarged, like carrots.   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.

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Mobility

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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 movement.      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.      Seeds 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 maturity.

We 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.       Because animals 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.

Mobility, 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.

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Growth

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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 be stunted.      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 chemical life processes and many plants become dormant, waiting for the return of spring.  However, some plants die. leaving seeds that will germinate when warmer weather comes again.

Plants 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.

Animals, on the other hand, are designed for efficient movement, and so cannot modify their forms very much. The bodies of many animals are formed in the embryonic stage: their forms change by growing, but no new parts are added once the basic parts are formed.  In the higher animals a part of the body that is lost will not grow back.      Some animals, 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 stature.

     It is interesting to see how the sizes of animals change over time.   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 horses are 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, low-nutrient grass.

     There have also been examples of animals becoming smaller.  This happens over time when a group of large animals finds itself dealing with inflexible limits on resources, for example by being trapped on a large island.  Horses have downsized to ponies, and a group of mammoths that began as larger than present day elephants gradually adapted 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.