They range in size from the single-celled organism we know as yeast to the largest known living organism on Earth — a 3.5-mile-wide mushroom.

Dubbed “the humongous fungus,” this honey mushroom (Armillaria ostoyae) covers some 2,200 acres in Oregon’s Malheur National Forest.

The only above-ground signs of the humongous fungus are patches of dead trees and the mushrooms that form at the base of infected trees. (See image on left.)

It started out 2,400 years ago as a single spore invisible to the naked eye, then grew to gargantuan proportions by intertwining threads of cells called hyphae.

Under a microscope, hyphae look like a tangled mass of threads or tiny plant roots. This tangled mass is called the fungal mycelium, and is the part of the famous honey mushroom that spreads for miles underground.

If mushrooms and other fungi can get so huge, why mention them on a site about microorganisms?

Visible fungi such as mushrooms are multicellular entities, but their cells are closely connected in a way unlike that of other multicellular organisms.

Plant and animal cells are entirely separated from one another by cell walls (in plants) and cell membranes (in

animals). The dividers between fungal cells, however, often have openings that allow proteins, fluids and even nuclei to flow from one cell to another. A few fungal

species have no cell dividers: just a long, continuous cell dotted by multiple nuclei spread throughout.

The zoospores have no cell wall, are uniflagellated, and may swim for 24 hours on endogenous energy reserves. On contact with a suitable surface (e.g., a nematode cuticle), the zoospore encysts by withdrawing its flagellum and surrounding itself with a thick cell wall and then adhering to the surface. The fungi Arthrobotrys oligospora can capture a nematode when it merely touches the outside of its trap.

Animation

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