Spores are usually haploid and unicellular and are produced by meiosis in the sporangium by the sporophyte. Once conditions are favorable, the spore can develop into a new organism using mitotic division, producing a multicellular gametophyte, which eventually goes on to produce gametes.

Two gametes fuse to create a new sporophyte. This cycle is known as alternation of generations, but a better term is "biological life cycle", as there may be more than one phase and so it cannot be a direct alternation. Haploid spores produced by mitosis (known as mitospores) are used by many fungi for asexual reproduction.

Many ferns, especially those adapted to dry conditions, produce diploid spores. This form of asexual reproduction is called apogamy. It is a form of apomixis.

Spores are the units of asexual reproduction, because a single spore develops into a new organism. By contrast, gametes are the units of sexual reproduction, as two gametes need to fuse to create a new organism.

Definition

The term spore derives from the ancient Greek word σπορα ("spora"), meaning a seed.

In common parlance, the difference between a "spore" and a "gamete" (both together called gonites) is that a spore will germinate and develop into a sporeling, while a gamete needs to combine with another gamete before developing further. However, the terms are somewhat interchangeable when referring to gametes.

A chief difference between spores and seeds as dispersal units is that spores have little food storage compared with seeds, and thus require more favorable conditions in order to successfully germinate. (This is not without its exceptions, however: many orchid seeds, although multicellular, are microscopic and lack endosperm, and spores of some fungi in the Glomeromycota commonly exceed 300µm in diameter.) Seeds, therefore, are more resistant to harsh conditions and require less energy to start mitosis. Spores are produced in large numbers to increase the chance of a spore surviving in a number of notable examples.

Classification

Spores can be classified in several ways such as:

By spore-producing structure

• Sporangiospores: spores produced by a sporangium in many fungi such as zygomycetes.
• Zygospores: spores produced by a zygosporangium, characteristic of zygomycetes.
• Ascospores: spores produced by an ascus, characteristic of ascomycetes.
• Basidiospores: spores produced by a basidium, characteristic of basidiomycetes.
• Aeciospores: spores produced by a aecium in some fungi such as rusts or smuts.
• Urediospores: spores produced by a uredinium in some fungi such as rusts or smuts.
• Teliospores: spores produced by a telium in some fungi such as rusts or smuts.
• Oospores: spores produced by a oogonium, characteristic of oomycetes.
• Carpospores: spores produced by a carposporophyte, characteristic of red algae.
• Tetraspores: spores produced by a tetrasporophyte, characteristic of red algae.