Life Cycle

The term life cycle describes the series of predictable changes that an organism goes through until it is mature enough to reproduce. Knowledge of the major stages or changes that all species undergo during their lives is essential to the study of the life sciences. Studying an organism from birth to sexual maturity is an ideal way to learn what is most important and essential to its life and continuance.

For some species, a complete life cycle is only fifteen days, while for others it can be decades. However, during the normal life cycle of every organism, growth and reproduction always take place. Between birth and sexual maturity, some species go through a long sequence of basic changes over time while others appear to make a direct trip. For example, although mammals are relatively complex animals, their life cycle is fairly straightforward. Mammals begin to develop from a fertilized egg and once born, they simply continue to develop or grow. There is certainly much variation between mammals, since a human baby takes about eighteen months to learn to walk, while a horse will stand up almost immediately at birth and romp in a day. Childhood for mammals also varies in length. Humans enter puberty (the stage at which they begin to mature sexually) in their early teens, while a dog may be ready to have a puppy before it is a year old. Despite these differences, the life cycle basics are nearly the same for all of the higher animals (birth, growth and sexual maturation, fertilization, birth).

INCOMPLETE METAMORPHOSIS

While some lower organisms have simpler life cycles, there are many animals and plants with life cycles that are not so straightforward. Some animals go through complex life cycles in which they physically become an entirely different type of individual. In other cases, a period of asexual reproduction (without the union of sperm and egg) is followed by a period of sexual reproduction. For example, a grasshopper has a three-stage life cycle called incomplete metamorphosis. After an adult female grasshopper lays an egg and buries it, the egg develops and eventually hatches. What emerges from the shell is called a nymph. At this stage in its life, the nymph may look like a miniature adult but it has no wings and no working reproductive organs. As the nymph grows, it periodically sheds its outer skin or molts, and with every molt it becomes more of an adult. When it sheds its skin for the fifth and final time, it has become an adult grasshopper and is ready to mate and reproduce.

COMPLETE METAMORPHOSIS

Other insects, like a moth or butterfly, go through a much more complicated process called complete metamorphosis. After an adult female moth lays its eggs and they develop and mature, what hatches looks like

a worm and is called a larva. A larva is the caterpillar stage in a moth's life cycle. As a caterpillar, the larva is nothing more than an eating machine, and its body is built to help it consume as much food as possible. It has a long body with three pairs of true legs. It also has a large head with strong jaws that allow it to feed on plants. Many caterpillars have some form of camouflage or coloring that allows them to blend in with the plants they eat. Others may have bright warning colors and irritating hairs that keep predators away. After a series of molts, or outer skin shedding, the caterpillar produces an outer covering around itself called a cocoon and attaches itself and the cocoon to a branch. Inside these coverings, most of the larva's cells are broken down and begin to reform as a pupa. As the pupa develops inside, it is reformed and transformed into an adult insect, and a moth or butterfly emerges. As an adult, the insect is soon ready to reproduce and its life cycle is complete.

ALTERNATION OF GENERATIONS

Discovering the details of an organism's life cycle can sometimes be essential to understanding its true nature. For centuries, no one was able to discover how ferns reproduced. It was long thought that since a fern was a green plant, it had to produce seeds (and therefore reproduce sexually with male and female sex cells). Yet finding a fern's seeds proved impossible. Botanists (people who specialize in the study of plants) were only able to solve this problem by closely studying a fern's life cycle. It was finally discovered that ferns, as well as other plants like mosses, reproduce by spores and not seeds. Also, it was found that a fern has a sexual stage that alternates with an asexual stage that produces spores. This process of going through two different plant forms in one life cycle is called the alternation of generations.

A fern's life cycle begins when a mature fern plant produces spores inside little cases, which are attached to the underside of the fronds (leaves). Called sori (singular, sorus), these dark brown dots are sometimes mistaken for bugs or disease spots. When the spores mature, their cases split open and the tiny, light spores are sometimes carried great distances from the parent plant by the wind. When it lands in an inviting place, the spore develops into a tiny green plant called a gametophyte and produces sperm and egg. This is the sexual stage of the fern. When sperm and egg unite during rains or with dew, a fertilized egg forms and the asexual stage of its life begins. The egg develops into a new individual spore-producing fern plant, which will begin the cycle all over again.

[See alsoLarva; Metamorphosis ]