Fungi

Species in this phylum reproduce sexually by forming spores on top of club-shaped structures called basidia. The club fungi are believed to be closely related to the sac fungi. Both groups have cells which are separated by septa (walls), and both have a dikaryotic phase in their life cycle; a phase with two haploid nuclei per cell. The septum of the club fungi is somewhat different from those of sac fungi and is referred to as a dolipore septum. The dolipore septum has a bagel-shaped pore in its center.

The club fungi reproduce asexually by producing asexual spores or by fragmentation of mycelium.

The sexual reproduction phase of the club fungi involves three developmental stages of the mycelium. In the primary stage, a haploid spore germinates and grows a germ tube, which develops into mycelium. The mycelium initially contains a single haploid nucleus. Then, its haploid nucleus divides and septa form between the nuclei.

A secondary mycelium forms upon conjugation of two sexually compatible hyphae. The secondary mycelium is dikaryotic, in that it has two haploid nuclei, one from each parent. As the dikaryotic mycelium grows, the cells divide and more septa are formed between the new cells.

Each of the new cells in the secondary mycelium has one haploid nucleus from each parent. This is assured by clamp connections, specialized structures unique to the club fungi. These are loop-like hyphae which connect the cytoplasm of adjacent cells and through which nuclei move during cell division. In particular, during cell division, one nucleus divides directly into the newly formed cell; the other nucleus divides inside the clamp connection and the two daughter nuclei migrate through the clamp connection in opposite directions to the two daughter cells.

The tertiary mycelium is simply an organized mass of secondary mycelium. It is a morphologically complex tissue and forms structures such as the typically mushroom-shaped basidiocarps commonly seen in nature.

Sexual reproduction of the club fungi begins upon fusion of two primary hyphae to form a club-shaped structure, known as a basidium. Second, the two haploid nuclei inside the basidium fuse together to form a diploid zygote. Third, the zygote undergoes meiosis to form two haploid nuclei. Fourth, these two haploid nuclei undergo mitosis to form a total of four haploid nuclei. These four nuclei then migrate into projections, which form on the tip of the basidium. These projections then develop into four separate haploid spores, each with a single nucleus.

In the species of club fungi which are large and fleshy, such as the mushrooms, a mass of basidia form a structure called a basidiocarp. The spores on the basidia are released from the underside of the fleshy gills of the mushroom. The color and shape of the basidiocarp, as well as the color of the spores are often diagnostic for species identification.

This large phylum includes species which are known as mushrooms, toadstools, earthstars, stinkhorns, puffballs, jelly fungi, coral fungi, and many other interesting common names. Some species, such as the rusts and smuts, are pathogens which attack agricultural grains. Other species, such as the fly agaric (Agaricus muscaria) and some species in the genus Psilocybe, produce chemical hallucinogens and have been used by numerous cultures in their religious ceremonies. Another species, Agaricus bisporus, is the common edible mushroom found in supermarkets.

An important aspect of the club fungi is the great diversity of alkaloids and other toxic and psychogenic chemicals produced by some species. For example, Amanita virosa, a mushroom colloquially known as "death angel," is so deadly poisonous that a small bite can kill a person. A related mushroom is Amanita muscaria, known as "fly agaric," which is hallucinogenic. Over the millennia, numerous cultures have eaten the fly agaric as part of their religious ceremonies. For example, R. Gordon Wasson has shown that Amanita muscaria is the hallucinogenic plant referred to as "Soma" throughout Rg Veda, the ancient religious text. According to Rg Veda, the ancient Aryans who invaded India about four millennia ago ingested "Soma" as a euphoriant.

While mushrooms are the best-known club fungi, many other club fungi grow underground as mycorrhizae. Mycorrhizae result from a symbiosis between a plant root and a fungus. In mycorrhizae, the fungus typically supplies nitrogen-containing compounds to the plant, and the plant supplies carbohydrates and other organic compounds to the fungus. Mycorrhizae are very important for the growth of orchids. One reason many orchids are difficult to grow is because they require particular fungal species to form mycorrhizae on their roots.

A recent report investigated a subterranean club fungus, Armillaria bulbosa, which is a pathogen on tree roots. The investigators used molecular biology techniques to demonstrate that a single subterranean "indi vidual" of this species in Northern Michigan was spread out over 37 acres (15 ha) and weighed an estimated 22,000 lb (10,000 kg). Based on the estimated growth rate of this species, of about 0.7 ft (0.2 m) per year, this individual was about 1,500 years old.