Fungi unlike other groups of microorganisms possess or have unique structures which distinguish them from other microbial cells. Morphologically, fungi exist in any of the following structural forms:
Mycelium is the long branching structures of fungal hyphae (Figure 1). Mycelia are specialized structures used by fungal cells to absorb nutrients from their environment or supporting medium. They are known to penetrate the substrate on which they form to acquire nutrients by absorption. Two types of mycelia are exhibited by fungal organisms viz: vegetative mycelia and aerial mycelia. Vegetative mycelia act as the root of a fungal organism, and they are known to penetrate the substrate on which the fungus is growing. They help to absorb nutrient and water for the growing fungal cell. Another type of mycelia which do not penetrate nutrient substrate but grow above it is the aerial mycelia. Aerial mycelia are critical for asexual reproduction in imperfect fungi, and they usually bear or carry conidia or fungal spores. They serve as important reproductive structures in fungi.
Yeasts are the unicellular forms of fungi. They usually reproduce by budding, an asexual reproduction pattern. Budding is a type of asexual reproduction in which the daughter cell emanates or develops from the parent cell. In budding, the daughter cell develops entirely from the mother or parent cell as a localized outgrowth. Structurally, yeast is oval or spherical in form (Figure 2).
Hyphae is the tube-like extension of a fungal cell. They are usually rigid or thick in form. A mass of hyphae is required for the formation of mycelia. Hyphae formation by a fungal cell can be septate i.e. with cross-walls (Figure 3) or non-septate i.e. without cross-walls (Figure 4). Fungi produce two types of hyphae: aerial hyphae and vegetative hyphae.
Aerial hyphae are branching structures of fungi that do not penetrate the supporting medium but instead they project above the surface of the mycelia. They usually bear the reproductive structures of the filamentous fungi (i.e. moulds).
Vegetative hyphae can also be called the substrate hyphae. They are the hyphae that penetrate the supporting medium on which the fungus is growing. Substrate or vegetative hyphae absorb nutrients from the supporting medium necessary for fungal growth. Generally, mycelia are a mass of hyphae.
Pseudomycelium or Pseudohypha is the less-rigid hyphae formed by some fungi (Figure 5). They are different from the true-hyphae which are known to be very rigid.
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