Spore

A spore is an extremely tiny, specialized package of cells used by some organisms during reproduction. Plants use spores as they do seeds, for reproduction; while certain algae, fungi, bacteria, and protozoans use spores to help disperse themselves widely and to protect themselves from unfavorable conditions. Spores vary in size but all are microscopic and usually contain a single cell.

In botany (the study of plants), spores are regarded as reproductive cells that are capable of developing into a new individual plant, either directly or after fusion with another spore. Plants that do not flower, like mosses and ferns, do not grow from a seed that was created sexually by male pollen combining with a female ovule. Instead, they reproduce by means of spores. The plant that develops from a spore does not resemble the parent plant, since this plant is in the first phase of a life cycle called the "alternation of generations." This means that in the life cycle of a plant, two generations exist alternately, one after the other. For example, for centuries no one knew how ferns actually reproduced. Most 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 it was impossible to find a fern's seeds until botanists studied the entire life cycle of a fern. Finally, they discovered that ferns reproduce with spores. They also discovered that a fern has a sexual stage (producing sperm and egg) that alternates with an asexual stage (producing spores). In the life cycle of a fern, a mature fern plant develops little brown spore cases called sporangia that are attached to the underside of their leaves, or fronds. When the spores are ripe, the cases split open in the dry air and the dustlike spores are carried away by the wind. When the spore lands where there are damp conditions, it germinates (begins to sprout) and grows into a very small, heart-shaped plant. This tiny plant begins to mature and develops sex organs which, after fertilization (the process in which an egg cell and a sperm cell unite to form one cell) occurs, grows into a plant we recognize as a fern.

Organisms besides plants use spores for other purposes. Certain kinds of algae, bacteria, fungi, and protozoans all form what are described as survival spores. Besides the ability of spores to be easily and widely dispersed, they also have the ability to survive under harsh conditions. For fungi that are not able to move, spores are an ideal way to spread themselves around, and when conditions are right, fungi produce dispersal spores that function like seeds and germinate quickly under proper conditions. They grow, mature, and produce more spores out of which more fungi will grow. But when the environment becomes unfavorable—too hot or cold or too dry—fungi produce survival spores that may live for years before germinating. Certain types of bacteria form spores for protective reasons. Bacterial spores, which are bacterial cells that have gone dormant (resting) and developed a thick, hard wall, can usually survive even in hot, boiling water. Protozoans—most of which are parasites that live in or on other animals—also form protective spores by a type of cell division. The protozoan that causes malaria is injected into a healthy person by a mosquito carrying its spores. Although the spores cannot move, they multiply by cell division in the person's liver and then enter the blood cells. Soon the blood cells burst and release the spores into the liquid part of the blood. When a mosquito stings this infected person and takes its blood, it also takes the spores and eventually deposits them into yet another healthy person. For plants and other organisms, spores have proven to be a successful means of establishing themselves in new environments as well as a way of surviving unfavorable conditions.

[See alsoAlgae; Botany; Bacteria; Fungi; Plant Reproduction; Plants; Protozoans ]