Fungi are eukaryotic, usually multicellular,
non-motile, heterotrophic organisms,
which as a group comprise the Kingdom Fungi.
ChytridiomycotaCommonly known as chytrids, these fungi produce zoospores (tiny, single-celled bodies produced in the swollen tips of hyphae) that are capable of moving on their own through liquid by simple flagella. Members of the phylum Chytridiomycota are the most primitive of fungi. In older classifications, they were placed in the class Phycomycetes under the subdivision Myxomycophyta.
In Mucor, the tips of side-threads are partitioned off and those of opposite strains join together. The nuclei join in pairs and a hard wall forms around them. This is the zygospore. When the zygospore germinates, after meiosis, it puts out a single thread which bears zoospores at its tip. These are distributed and form new threads. Most of the life of these fungi is spent in the haploid state (i.e. the nuclei have only one set of chromosomes). Only the sexual zygospores have two sets. This is in contrast to the higher plants and animals whose body cells have two sets of chromosomes. This is because in the higher organisms the reduction of the chromosome number takes place just before sex-cells are formed.
GlomeromycotaAlso known as the arbuscular mycorrhizal fungi, members of this phylum form an ancient group with fossil remains dating back 400 million years. Only one species has been observed forming zygospores; all other species only reproduce asexually.
There are usually two different strains of hyphae which will produce fruiting bodies only when they meet. The hyphae join but the nuclei remain separate. The hyphae continue to grow and produce more cells, each with two nuclei. These hyphae and branches of the original ones form the fruiting body. The spore-chambers (asci) develop at the tips of the hyphae which have two nuclei. In a cell, at or near the tip, the nuclei fuse and then divide into eight. Each new nucleus takes on some cytoplasm and forms a spore. The spores are released when the spore-chamber opens. Spores of both strains are produced, and later grow into new hyphae.
BasidiomycotaTo this phylum belong the most familiar fungi – the mushrooms and toadstools. Their early life is much like that of the previous group – they exist as fine branching hyphae on dead leaves, manure, etc. When threads of opposite type meet, they again produce fruiting bodies of closely matted hyphae, some of which have two nuclei in their cells. The fruiting body of the mushroom develops underground as a small knot of hyphae. Its structure is almost fully formed before it appears above ground, and then, by absorbing large amounts of water, it grows up into the air and opens out into the typical umbrella-shape. On the underside of the cap there are many radiating gills which bear the spores. The hyphae bear club-shaped cells at the tip and in these cells (the basidia) the two nuclei fuse. They then divide by meiosis and the four new nuclei pass into four tiny swellings on the outer end of the cell. These are the tiny spores (basidiospores). They fall when ripe and are distributed by wind.
In the cultivated mushroom, only two spores are formed at the tip of each cell so that each spore has two nuclei. There is no need for two different threads to meet before producing fruiting bodies in this case, because the threads already have two nuclei.
Some other fungi of this group have numerous pores on the underside instead of gills. The spores are produced on the linings of the pores in these cases. Many of the bracket fungi on trees are of this type.
Related entries• lichen
• slime mold
Related category• BOTANY
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