The tale of an invisible spore that grows into a bounty of nature
I begin with a description of the life cycle of a mushroom to give us an overview.
First, the spore; the form the mushroom takes to disperse across unimaginable distances and times. Spores are in many ways similar to seeds, both being essential for reproduction. However, they also differ so much that scientists use this, together with other facts, to distinguish fungi (the correct scientific word for mushrooms) from plants.
A seed produces a plantlet with a root, a stem and leaves, which will then directly grow into a mature specimen. From a spore, however, only a fragile filament called a hypha (hyphae in plural) will emerge. The hypha will first have to adapt to the food source in its environment, then look for a mate, eventually producing what we call mushrooms. The mushroom will finally close the cycle by releasing its spores.
Fungi are made entirely of this one basic structure: the hypha. Hyphae are single-cell threads which grow into a network called the mycelium which resembles a white mould; such as you see growing on cheeses like Camembert. Within the mycelium, and also with adjacent plants, there are exchanges of nutrients and information about the environment.
We can think of the mycelium as a network comprising populations of hyphae. Each hypha emerged from a different spore, and each of those has its own sex. For the mushrooms, the concept of sex is far more complicated than our black and white mindset prepare us for, I will devote a future article to fungi sex types. When a hypha finds another that matches; the two pair up. During the mating, the two hyphae exchange their cellular nuclei. This new type of hyphae is said to be dikaryotic because of its two nuclei. Scientists speak of primary and secondary mycelium.
Only secondary mycelium develop into what we call the mushroom. Dikaryotic hyphae get thicker like ropes twisted at both ends so that the secondary mycelium appears denser. A combination of environmental factors (temperature, moisture, and evaporation rate) will then trigger the actual mushroom to form. Scientists refer to this as fructification as it will produce the carpophores. The word carpophore means bringing the carpellum, the female reproductive organs: a term which is a legacy from the time when scientist considered fungi to be a type of plant.
So, the carpophores are the prize for the mushroom forager. They are the product of a transformation phase in the life cycle of a mushroom. It is the carpophore that produces spores and reinitiates the wheel of life. The mycelium, by contrast, is virtually immortal. As long as the environment is stable, and food is available, it can grow indefinitely without ever producing a single mushroom.
The mycelium clone itself, unseen, pervades any suitable substrate. It may live underground or within a plant; some live in dung, others in an insect’s exoskeleton. As hyphae have germinated from spores which landed on a suitable substrate, they will have travelled inward, much as human cultures colonise new territories. The scarcity of food and the lack of free space will then push the mycelium to emerge from the substrate. At its external surface, the mycelium transforms, and tiny immature carpophore called primordia appear.
The archetypal mushroom has a stalk enlarged at the base and a hat with gills in its underside. Notable exceptions are the Boletus which instead have a porous hat underside. However, the diversity of shapes, colours and textures in the mushroom word equals that of imagination. From the gills, clouds of spores will float into the air to disperse and initiate a cycle somewhere else.
So to recap: for most of the time, the mushroom mycelium lives within a substrate resembling a white mould. It is made of hyphae, which are threads branching into the mycelial network. When conditions are right, the mycelium produces the carpophores, the edible part. Carpophores upon releasing the spores reinitiate the life cycle. There are many variants of this basic cycle, and mycologists use these differences to separate mushrooms into taxonomic groups.
Clearly then, mushrooms are quite different from plants. They don’t have roots, trunks and leaves, but only hyphae. Mushrooms disperse using spores instead of seeds. Crucially, they can’t make their food by photosynthesis as plants do. Mushrooms, instead, synthesise substrate-specific enzymes to digest food before assimilating it. Fascinating. I will discuss this more in the next article. Mushrooms are metabolic wizards with an underappreciated ability to produce valuable molecules.