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Figure 80.
The poisonous Amanita muscaria is native to North America's temperate and boreal regions
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Growth and Reproduction 
A 
thallus 
is a fungus's vegetative body, which can be unicellular or multicellular. Some fungi 
are dimorphic, meaning they can change from unicellular to multicellular depending on their 
environment. Unicellular fungi are generally referred to as 
yeasts
. Unicellular fungi include 
Saccharomyces cerevisiae (baker's yeast) and Candida species (the agents of thrush, a common fungal 
infection). 
The majority of fungi are multicellular organisms. They have two morphological stages: 
vegetative and reproductive. The vegetative stage is distinguished by a tangle of fine thread-like 
structures known as hyphae (singular, 
hypha
), whereas the reproductive stage can be more 
conspicuous. A mass of hyphae is called a 
mycelium
(Figure 81). It can grow on the surface of a 
surface, in soil or decaying material, in a liquid, and even in or on living tissue. Although individual 
hypha must be examined under a microscope, a fungus's mycelium can be very large, with some 
species truly being "the fungus humongous." The Armillaria ostoyae (honey mushroom) is the largest 
organism on Earth, spanning over 2,000 acres of underground soil in eastern Oregon; it is thought to 
be at least 2,400 years old. 
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credit: Christine Majul)

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Figure 81.
Neotestudina rosati mycelium has the potential to be pathogenic to humans. The fungus enters the 
body through a cut or scrape and causes a mycetoma, a chronic subcutaneous infection
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Most fungal hyphae are divided into distinct cells by end walls known as septa (singular, 
septum). Tiny holes in the septa of most fungi divisions (like plants, fungal phyla are called divisions 
by tradition) allow for the rapid flow of nutrients and small molecules from cell to cell along the 
hyphae. They are known as perforated septa. Septa do not separate the hyphae in bread molds (which 
belong to the division Zygomycota). They are made up of large cells with numerous nuclei, an 
arrangement known as coenocytic hyphae
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