In biology, a spore is a small unit used for reproduction in some plants, algae, fungi, and protozoa. Spores help these organisms spread and survive, even in bad conditions for a long time. Spores are part of the life cycles of many living things. Scientists believe spores first appeared during the mid-late Ordovician period as a way for early land plants to survive.

Bacterial spores are not used for reproduction. Instead, they are strong structures that help bacteria survive in bad conditions. Myxozoan spores release tiny, infectious cells called "amoebulae" into their hosts to cause disease. These spores also reproduce inside their hosts by joining two cell parts in a structure called a plasmodium, which forms from the amoebula.

In plants, spores are usually single-celled and have one set of chromosomes. They are made through a special type of cell division called meiosis in a structure called the sporangium, which is part of a plant that has two sets of chromosomes. In rare cases, some algae and fungi can make spores with two sets of chromosomes. When conditions are good, a spore can grow into a new organism through another type of cell division, forming a multicellular structure called a gametophyte. This structure later produces gametes, which join to form a zygote. The zygote then grows into a new plant. This process is called alternation of generations.

In seed plants, spores are made inside the plant. Large spores, called megaspores, and small spores, called microspores, help form complex structures that become seeds and pollen grains, which are used to spread the plant’s offspring.

Definition

The word "spore" comes from the Greek word spora, which means "seed" or "sowing," and is related to the Greek words sporos ("sowing") and speirein ("to sow").

In everyday language, the main difference between a "spore" and a "gamete" is that a spore can grow on its own to form a sporeling, while a gamete must join with another gamete to create a zygote before it can develop further.

The key difference between spores and seeds as ways to spread plants is that spores are single cells and are the first cell of the gametophyte stage. Seeds, however, contain a developing embryo, which is the multicellular sporophyte of the next generation. This embryo forms when the male gamete from the pollen tube joins the female gamete inside the ovule. Spores grow into haploid gametophytes, while seeds grow into diploid sporophytes.

Classification of spore-producing organisms

Vascular plant spores are always haploid. Vascular plants are either homosporous (also called isosporous) or heterosporous. Homosporous plants make spores that are the same size and type.

Heterosporous plants, like seed plants, spikemosses, quillworts, and some ferns, create two types of spores with different sizes. The larger spore, called a megaspore, acts like a "female" spore, and the smaller spore, called a microspore, acts like a "male." These spores usually develop inside separate structures called sporangia. A megasporangium makes megaspores, and a microsporangium makes microspores. In flowering plants, these structures are found in the carpel (for megaspores) and anthers (for microspores).

Fungi often make spores during both sexual and asexual reproduction. Spores are usually haploid and grow into fully developed haploid individuals through cell division. Some spores, like urediniospores and teliospores in rusts, are dikaryotic. Dikaryotic cells form when two haploid gamete cells join. In dikaryotic cells, the two haploid nuclei fuse in a process called karyogamy, creating a diploid cell. This diploid cell then undergoes meiosis to produce haploid spores.

Classification of spores

Spores can be grouped in different ways, such as by the part of the organism that produces them, their purpose, when they form during the life cycle, and whether they can move.

Below is a table that shows how spores are grouped, their names, features that help identify them, examples, and pictures of different spore types.

External anatomy

Under high magnification, spores often have detailed patterns or decorations on their outer surfaces. Scientists use specific terms to describe these features. Some patterns mark areas where the spore's tough outer layer can be broken during germination. Spores are grouped based on the number and location of these marks and openings. Alete spores have no lines. Monolete spores have one thin line (called a laesura), showing where two spores once touched and later separated. Trilete spores have three thin lines radiating from a central point, indicating that four spores were once joined together in a shape like a pyramid. A larger opening shaped like a groove may be called a colpus. The number of colpi helps classify plant groups. Eudicots have spores with three colpi.

Spores found in groups of four, enclosed in a covering, are the oldest known evidence of land plants, dating to the mid-Ordovician period (around 470 million years ago). No large plant fossils from this time have been found yet. Individual trilete spores similar to those of modern cryptogamic plants first appeared in the fossil record at the end of the Ordovician period.

Dispersal

In fungi, both asexual and sexual spores, called sporangiospores, are often spread by being forcefully pushed out of their reproductive structures. This process helps spores leave the structures and travel long distances through the air. Many fungi have special mechanical and physical features, such as proteins called hydrophobins, that help with spore release. These features include the structure of the ascus, which holds ascospores, and the buildup of special chemicals in the ascus that cause the spores to be released explosively into the air.

The release of single spores, called ballistospores, happens when a tiny water droplet forms on the spore. When this droplet touches the spore, it launches the spore into the air with a very fast acceleration. Some fungi use other methods, like relying on outside forces such as wind, as seen in puffballs. Other fungi attract insects, like flies, by using bright colors and strong smells to help spread their spores, as seen in stinkhorns.

In Common Smoothcap moss (Atrichum undulatum), studies have shown that the vibration of the sporophyte, which is the part of the plant that holds the spores, plays a key role in releasing spores.

For spore-shedding vascular plants, such as ferns, wind spreads very light spores over long distances. Spores have an advantage over seeds because they need less energy and materials to produce. However, spores are more likely to be eaten by fungi and bacteria than seeds because they contain little food. Spores are also less likely to be eaten by animals.

In the spikemoss Selaginella lepidophylla, spores are partly spread by a special type of diaspore, which is a seed-like structure that rolls away like a tumbleweed.

Origin

Spores have been discovered in microfossils from the mid-late Ordovician period. Scientists have two main ideas about when spores first appeared: either before or after land plants existed. One widely studied idea is that spores helped early land plants, like embryophytes, spread and survive in dry environments. This idea is supported by the thick walls found in cryptospores. These walls likely protected plant offspring from harsh weather conditions. A more recent theory suggests that spores may have developed earlier, forming due to mistakes during a process called meiosis in algae, which is thought to be an ancestor of land plants.

Whether spores appeared before or after land plants, they have helped scientists study topics such as paleontology and plant relationships. Cryptospores, the spores found in microfossils, are well preserved because of the materials they are in and their widespread presence during their time. These microfossils are especially useful for studying Earth's early history because large fossils, like plants, are rarely found or preserved. Both cryptospores and modern spores have varied shapes that provide clues about past environmental conditions and how plant species are related.

Gallery

• Spores from the moss Bartramia ithyphylla (seen under a microscope at 400x magnification)
• Opened fern sporangia (seen under a microscope; no spores are visible)
• Spores and elaters from a horsetail (Equisetum, seen under a microscope)
• Fossil plant spores (Scylaspora) from ancient rock layers in Sweden dating to the Silurian period
• Mold spores and visible cells from fruit mold (seen under a microscope at 2000x magnification)
• Clusters of spores inside sporangia of the slime mold Reticularia olivacea, found in pine forests of eastern Ukraine
• Inside surface of the peridium of the slime mold Tubifera dudkae with spores