They also compose
They seem to come out of nowhere. In higher latitudes, fungi appear in autumn, following the rain and cooling temperatures. In the tropics, they pop up at any time — sprouting from fallen timber, pushing through soil, dotting the landscape as brackets and baskets, fingers and parasols, and strange forms that unwrap and unwind and live for a day like mushroom mayflies.
But this is only a tiny part of the story. What we see above the surface is a fungal flower, an ephemeral structure with the sole purpose of reproduction. Below the surface, work goes on unseen.
The mycelium is the part of a fungus that carries out the day to day business. It is made up of fibres (hyphae) that ramify through soil, rotting wood or even animal dung. (Fungi are decomposers, after all; they get their nutrition where they can.) This is where it all happens, regardless of what’s going on — or not — above.
Hyphae secrete enzymes that break down the organic matter in which they grow. Those feeding on wood produce cellulases and lignases that take apart the structural material, often leaving behind something that looks exactly like a fallen tree, but has nothing of its rigidity and resistance. The same enzymes are deployed on herbivore dung, which contains fragments of leaves with undigested cellulose. Waste not, want not.
Some fungi have more sinister dietary requirements. Cordyceps, Ophiocordyceps and their relatives feed on living insects. In some cases, the fungus alters the insect’s behaviour, making it crawl to a location that will benefit spore dispersal. After death, the host’s body remains intact, held together by hyphae. Fruiting bodies emerge like spikes from the soft tissue between the sclerites.
But many plants — perhaps all of them — are indebted to fungi. A symbiotic relationship between the two plays out underground. Hyphae wrap around tree roots and nudge their way inside. Instead of issuing destructive enzymes, they exchange nutrients. Fungi transfer water and minerals, plants send back carbohydrates formed by photosynthesis. As might be expected, the relationships between fungi and plants — mycorrhiza — are complicated. Some fungi enter plant cells, others hang around outside. Orchids and heaths have their own arrangements, as does the Australian fringe lily genus Thysanotus. Whether only one or a range of fungi pair up with a plant species depends on a variety of factors. In Australia, introduced fungi, such as Amanita muscaria, commonly associated with non-native trees, have transferred their attentions to local species.
There is more, of course. Achlorophyllous plants, which lack photosynthetic pigments, exploit mycorrhizal relationships to parasitise fungi that are parasitising other plants. Nutrients are shunted along in one direction. Where fungi are involved, things are never simple. And it all goes on in the darkness beneath our feet, except for that brief period when the rains come and the fungal ‘flowers’ bloom.
Note: To make things even more complicated, some fungi take over plants, suppress their activity, and produce fruiting bodies that resemble flowers. But that’s for another day.