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The Smallest Room

A very old joke…

– What’s the smallest room in the world?

– A mushroom!

But what is a mushroom?

Mushrooms are neither plants (kingdom Plantae) nor animals (kingdom Animalia). They are fungi. What we typically recognise as a mushroom is the fruiting body of certain types of fungus. Mushrooms “live on land, in the water, in the air, and even in and on plants and animals. They vary widely in size and form, from the microscopically small to the largest organisms on Earth (at several square miles large)” (Keating, 2017). However, we commonly recognise them when they form above ground (e.g., on soil, in leaf litter, within blankets of moss) or on their food source (e.g., fallen tree branches, trunks of living trees).

Plants typically contain chlorophyll, a chemical used to convert sunlight into sugars that are used for energy. Mushrooms do not possess chlorophyll and so they get their energy by extracting nutrients from plants. There are broadly four methods to do this:

METHOD 1: Grow on living trees/plants and take out nutrients directly. Fungi that live this way are known as parasites. Invading trees, shrubs, and similar, this way can lead to the death of the host plant.

Colour photo of shaggy scalycap
The opportunistic parasite shaggy scalycap (Pholiota squarrosa) growing at base of an oak tree in the Breamish Valley

METHOD 2: Grow on dead or decaying matter such as fallen branches, leaves, tree stumps, and similar: extracting necessary nutrients from the dead/decaying organic matter. Fungi utilising this method are known as saprophytes.

Colour photo of sulphur tuft
Saprophytic sulphur tuft (Hypholoma fasciculare) growing on decaying tree stump in Branton Lakes Nature Reserve

METHOD 3: Grow on and among the roots of living trees/shrubs. This category of fungi forms a mutually beneficial relationship with the host plant: the fungus extracts particular nutrients from the plant but also provides minerals and other chemicals that are indispensable for the healthy growth of the plant’s roots. Such a relationship is called symbiotic and fungi that form symbiotic relationships are called mycorrhiza.

Colour photo of common earthballs
The mycorrhizal fungus common earthball (Scleroderma citrinum) growing in Beanley Wood

METHOD 4: Grow inside living plants (like a parasite) but do not cause any disease in the host plant (unlike parasites). Fungi that live this way are called endophytes. While feeding on the host plant, they release chemicals that give the host an unpleasant taste or make it poisonous, thereby preventing herbivores from eating the plant (Biology Dictionary, 2018). There are very few endophytic fungi that produce mushrooms, so the likelihood of seeing any is low (Nate, 2020).

Graphic image with the words 'seen any endophytes recently?'


Fungi produce a dense network of extremely fine thread-like filaments (hyphae) through which they absorb nutrients and from which the fruiting bodies (mushrooms) grow. This is the mycelium. Each mushroom contains fungal spores. These are “microscopic biological particles that allow fungi to be reproduced, serving a similar purpose to that of seeds in the plant world” (University of Worcester).

What is a toadstool?

There is no scientific difference between a ‘mushroom’ and a ‘toadstool’ – they are the same thing. However, ‘toadstool’ is sometimes used to refer to a fruiting body with a stem and a cap (e.g., common inkcap) and/or to fruiting bodies that are poisonous (e.g., fly agaric).

Colour photo of a common inkcap
A ‘toadstool’ with a stem and cap: common inkcap (Coprinopsis atramentaria) in my lawn in Powburn
Colour photo of fly agaric
A highly toxic ‘toadstool’: fly agaric (Amanita muscaria) in Beanley Wood

In contrast, ‘mushroom’ may be used to refer to fruiting bodies that are edible. (Fisher, 2022).

Colour photo of a field mushroom
Edible field mushroom (Agaricus campestris) growing along roadside at Branton [But see ‘Warning!’ below]


Eating mushrooms

Unless you are 100% certain that you can identify edible mushrooms then do not eat them!

Every year, numerous people must seek medical advice after eating mushrooms that have made them ill.

…Some varieties (of mushrooms) which grow wild in the United Kingdom are poisonous and can make foragers ill when consumed: some types can even be fatal. Foragers should remember that the poisons in some of the most dangerous wild mushrooms are generally not destroyed by cooking.

Gov UK (2014)

And if you are taking children out and about with you, please teach them to ‘Look but NEVER eat!’

Photo of young girl pointing at a mushroom, overlaid with the words 'Look but never eat!'
Beware: many wild mushrooms are toxic!

NOTE: This website and its contents are for information and entertainment only. Do not rely on information contained anywhere on the website to make decisions about eating mushrooms. Always seek expert advice before consuming mushrooms.

Further information

Photo of amethyst deceiver mushroom on brown leaf
Mushrooms in the Valley


Biology Dictionary (2020) ‘Endophytic Fungi’ [WWW] Accessed 10 October 2022.

Fisher, S. (2022) ‘What is a toadstool?’ [WWW] Accessed 10 October 2022.

Gov UK (2014) ‘Take care when picking mushrooms, poisons experts warn’ [WWW] Accessed 10 October 2014.

Keating, H. (2017) ‘Types of mushroom in the UK: common identification guide’ [WWW] Accessed 10 October 2022.

Nate, M. (2020) ‘The Four Categories of Mushrooms Explained’ [WWW] Accessed 10 October 2022.

University of Worcester (no date) ‘What are fungal spores?’ [WWW] Accessed 10 October 2022.

‘What is a mushroom?’ [WWW] Accessed 10 October 2022.

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