Fungi – a separate kingdom
Fungi are a very common and diverse group of organisms that includes mushrooms and puffballs, yeasts and moulds. They are found everywhere – in all habitats on land, and also in fresh water and the ocean. They were once grouped in the plant kingdom, but unlike plants they cannot make their own food through photosynthesis, and they digest their food externally before absorbing nutrients
Mushroom or toadstool?
There is no clear distinction between mushrooms and toadstools. Toadstool is a common but vague term for some kinds of mushroom, usually a poisonous or inedible one. Toads were once thought to be very poisonous, but ‘toadstool’ is said to come from the German tod-stuhl, meaning death-chair – referring to the the fatal effects of mushroom poisoning, and to the shape of the fruiting body.
Fungi are more closely related to animals than to plants – for example, they are mainly composed of chitin, which forms the exoskeleton (covering) of insects and other arthropods. However, they are now considered distinct enough to warrant their own major grouping – the fungi kingdom.
Structure and size
When asked to name some fungi, people usually think of mushrooms, toadstools and puffballs. But these are just the reproductive parts of certain types of fungi. Most fungi lie underground or in wood or leaves as a spreading network of fine tube-like filaments called hyphae. As the fungus grows, these branch and interweave, seeking nutrients for growth. Eventually they form a cobwebby mat known as a mycelium. This is the body of the fungus.
Some fungi are microscopic and single-celled (for example, yeasts). At the other extreme, a North American relative of New Zealand’s bootlace mushroom Armillaria novaezelandiae is reported to be among the largest and perhaps most ancient organisms in the world, with an underground mycelium spreading over hundreds of hectares.
How do fungi live?
Fungi do not have chlorophyll and so cannot make their own food by photosynthesis. Instead, they gain nutrients from living or dead material around them, including soil and wood. There are five fungal groups, each with a specific survival strategy:
- Saprobic fungi are decomposers – the most common group of fungi. They feed on dead organic matter, digesting it externally and breaking down complex chemicals into simple molecules. They play a vital role in all ecosystems, because nutrients are released and recycled for other organisms.
- Mycorrhizal fungi live underground in a give-and-take relationship with the roots of most plants. The fungus gets food produced by the plant’s leaves, while the spreading threads of the fungus help the plant roots gain water and nutrients from the soil.
- Parasitic fungi feed on living organisms. This can cause serious damage to plants and animals, including humans (athlete’s foot and ringworm are fungal diseases).
- Some fungi co-exist with algae, forming the organisms we call lichens. The fungus receives sugar, which the alga makes through photosynthesis. In turn, the alga is protected from the outside, and receives water and minerals from the fungus.
- Beneficial fungi known as endophytes live within healthy plants, giving the plant advantages such as being less palatable to insects, or more drought-resistant.
Most fungi reproduce by spores. These are simple structures, usually a single cell with a protective coating. Each spore is microscopic, and as a mass they look like dust or powder.
A fungus produces vast numbers of spores, which are spread in various ways – for instance by wind, raindrops or animals. When a spore falls in a suitable site it germinates into a hypha – the first of the threads that make up the body of a fungus.