So why do we have to wait for a change in weather for these guys to come out of hiding? Furthermore how do mushrooms or fruiting body of the fungus form?
Well as stated, mushrooms are the fruiting body of the fungus. The mushroom is not the organism itself, it is part of the organism, just as an apple is part of the tree. Not all fungi form these structures. Think of a bread mold. It would be pretty crazy to pull a moldy loaf of bread out of the cupboard and see a mushroom trying to break out of the bag. When are are talking about mushrooms we are talking about basidiomycetes and acsomycetes. These fungal phylum differ in how they product their spores. Basidiomycetes have their spores on the outside of a cell known as a basidium, This looks like a club with small roundish microscopic objects attached at the end. Ascomycetes produce spores that are encased inside of an ascus or long microscopic sack. This is where our story begins - the fungal spore. The spore is released from the basidium or ascus to a new substrate (soil, wood, plant material). The spore germinates or begins to grow by sending out a germ tube. This "tube" begins to form hyphae (thin threads). Fungi can undergo sexual reproduction where DNA from these hyphae are brought into one hypha (there are lots of steps I am skipping here. If you want a few more details you can check out this diagram Basidiomycete Life Cycle). These hyphae grow and grow forming mycelia (many hyphae together) through out the soil, duff layer, or wood gathering nutrients via excretion of enzymes. After enough nutrients are gathered and the environment is damp (but not flooded) and cool (not cold) the fungus can bring hyphae together into larger bunches, to form what we know as the mushroom. First a "button" is formed where the cap and stalk are inside an "egg." As the stalk grows it pushes the cap through the egg. This egg is known as the universal veil. The remnants of this veil can be seen on Amanitas.
Button stage with universal veil breaking a part.
Mushrooms can be found on growing a various different substrates: wood, soil, duff, or even dung! This is due to the fact that fungi have different feeding strategies that all boil down to how they acquire carbon. Some are more adapted to being able to digest complex carbohydrates from dead woody materials such as cellulose and lignin. These guys produce enzymes that allow them to break down these compounds for energy. We will typically find them growing on logs. These guys are known as saprotrophic fungi.
Other fungi can obtain carbon from trees via a symbiotic relationship (see earlier post of mycorrhizae). Specifically, those associated with trees like the one shown below are ectomycorrhizal fungi (EMF). They have hyphae (see explanation above) that grow from the base of the fruiting body into tree root tips where the exchange of carbon for nutrients occurs. However, some EMF's have been shown saprotrophic capabilities when carbon from a host is limited.
Suillus spp. (One of the Slippery Jacks)
Finally, a third type of feeding strategy are the pathogenic fungi. These fungi cause disease for living plants and animals. These absorb nutrients directly from the cells of the host, but unlike EMF's do not give back to the plant. Pathogenic fungi typically weaken and eventually will kill the plant they colonize.
As Thanksgiving is approaching, I will leave you with well wishes and Turkey Tails (Trametes spp.). Take a look around while hiking (or walking very slowly) post gorge-fest this Thursday. You may notice a whole other world of fall colors you might have normally missed in the past!
*Although I have had a lot of classes/training in mycology, I am not a mycologist. Fungal identification is based on my best educated guess from books/web resources. All fungi were observed at Big Basin Redwood State Park in California. If you have any additions or corrections, please feel free to add them in the comments section. Thanks!