Structure and Characteristics of Fungi

The fungal thallus – the mycelium – grows from a spore and is made up of very fine threads, invisible to the naked eye, usually branched, intertwined and tangled, round, pigmented or pale, without cross-walls (septa) in the lower fungi and septate in the higher fungi, called hyphae.

Each segment contains from one to a dozen or so nuclei and cytoplasm with ribosomes, mitochondria and other organelles. Most often the mycelium that grows from a spore consists of two functionally different parts: the substrate (vegetative) part, which supplies the fungus with water and nutrients, and the surface (generative) part – the fruiting body, whose biological purpose is to mature and disperse spores.

In some fungi (for example, some puffballs, truffles and endophytes) both stages of mycelial development take place only within the substrate (soil, wood).

From a biological and physiological point of view, both phases of mycelial development are inseparable parts, very important in the life of the fungus. The vegetative mycelium is usually perennial, while the generative one is annual; only in some, for example many polypores, is it perennial.

In the course of evolutionary processes in nature, the mycelium did not appear all at once; it formed very gradually. The mycelium of the first fungi was very primitive – just a naked protoplast (protoplasm and nucleus), without even a membrane. Even today, after many millions of years, parasitic fungi of such structure are still found in the bodies of animals or plants.

Some fungi, for example yeasts, do not form a vegetative mycelium; it is much more convenient for them to live and reproduce as single, budding segments (cells). However, the original unicellular fungal organisms were puny, delicate little creatures. Over time their physiological and biological needs changed and improved, their morphological traits changed, and the fungi changed too, adapting better and better to environmental conditions. For example, looking at a cep or some other cap mushroom or polypore, it is hard even to imagine what a long evolutionary path it has had to travel.

The vegetative mycelium is a sensitive yet vigorous network of very fine hyphae (1–20 µm and thicker). It is estimated that the length of hyphae in 1 cm3 of soil reaches from 40 to 120 m. That the mycelium of some fungal species is annual and that of others perennial is known even to the more observant mushroom picker who year after year hurries to his favourite spots to gather ceps. The mycelium grows at its tips, spreading radially in all directions; the growth rate of individual species varies, for example in cap mushrooms it is 10–20 cm, rarely 30–50 cm per year.

In the forest people see only the fruiting body of the fungus, i.e. its reproductive organ (the generative mycelium), while its other part – the vegetative mycelium – is settled in the substrate (litter, soil, wood). Incidentally, some fungi are homothallic, when a mycelium grows from a single spore and later also produces fruiting bodies, for example dung fungi.

However, most fungi are heterothallic, i.e. fungi that produce fruiting bodies only during the secondary (dikaryotic) mycelium stage, when the mycelium grows from two spores of different potential (male and female), for example ceps, chanterelles, etc. The secondary mycelium is perennial, whereas in heterothallic fungi a mycelium grown from a single spore is short-lived and does not form or produce fruiting body primordia.

The nature and biological purpose of the fruiting body is to form, mature and disperse spores. Thus it is nature's foster child, raised and perfected by nature, and as if in return for this it must mature and disperse as many spores as possible, since this is extraordinarily important in the life of every fungal species. The shape, size and colour of the fruiting bodies of individual species vary greatly, depending on environmental conditions and way of life. Even as a primordium, the fruiting body already has two missions – to mature spores and to disperse them.

The lifespan of a fruiting body, except for some polypores, is very short (from a few hours to a dozen or so days), and people are convinced that mushrooms grow very fast. This is partly true. But what they call a mushroom is only the above-ground part of the fungus – the fruiting body, which under good conditions does indeed grow quickly.

But how does the vegetative mycelium, which is invisible to us, live? How it has overwintered and when it gets around to growing can only be guessed.

It is believed that it wakes up in early spring and starts growing together with all living nature when the soil temperature rises to 5–8 °C, while its intensive life begins at no less than 15–18 °C. Only after a few months do the fruiting body primordia begin to form; being of different ages and at different depths, they begin to form fruiting bodies, which emerge at the surface at different times. Only from this moment, given sufficient warmth and moisture, does a cep or an orange birch bolete or another cap mushroom really grow very quickly, its cap increasing by 1.5 to 5 cm per day. For the vegetative mycelium to recover, grow stronger and gain new potential vital forces, the appropriate amount of warmth, moisture and light is needed; a considerable role is also played by CO2 and the exchange of other gases, even windiness and the whole microclimate.

So a cep, a birch bolete, a chanterelle, a Russula or any other mushroom, like a cucumber, a cabbage or a potato, does not grow in a few days but over several months. When picking mushrooms, let us know this and be more careful: let us not tear up the litter, rummage in the moss or destroy fungal habitats while searching for tiny ceps or chanterelles.

The world of fungi is very diverse; many of them differ not only in their structure but also in the peculiarities of their life.