Essentially, a fungus is a collection of long thin tubes known as hyphae.10 "Long" might be a fraction of a millimeter, or many metres; "thin" is typically around 5 ^m, i.e. five thousandths of a millimetre. A fungal hypha is not explicitly or obviously partitioned into cells. Many hyphae appear under the microscope as transparent colourless tubes with nuclei dotted along them. There is only the merest hint of a septum in the space between one nucleus and the next.

Part of the reason for this structural plan might be that most fungal cell walls are made of chitin, the tough, resistant covering material found in arthropods, including beetles and crabs. (Anyone who has tried to kill a cockroach by treading on it knows how strong chitin can be.) Chitin is not conducive to cell-cell communication. If a hypha were divided explicitly into cells by chitinous walls, then each cell would be forced to act as an

20 Specialised parts such as reproductive units are produced from these when necessary, but they are not durable. For instance, mushrooms appear overnight, but soon collapse and disintegrate again.

independent unit. The fungus would be not so much an organism as a colony of organisms.

Rather than reiterate now-familiar arguments, we shall simply state here that fungi do fit the characterisation of "livingness". The stimuli to which they respond are usually limited to moisture, nutrients, temperature and sometimes light. These stimuli can activate or inactivate genes, affecting e.g. growth, which can be explosively rapid (as in the case of fungal reproductive bodies; mushrooms appear overnight). They can also affect the internal state. For example, the presence of nutrient induces the hypha to secrete digestive enzymes and absorb the digestion products - a striking similarity between fungi and bacteria.

Fungi eat either living or dead organisms or organic waste. They are not primary producers but are either parasites, which inhabit the host that provides the nutrients, or detritivores, causing decomposition of dead organisms or excretory products. Detritivores are ecologically indispensable; they are instrumental in recycling resources within ecosystems. Parasitic fungi lack some of the components they need for independent, host-free existence, as all parasites do, but they are alive: they exhibit a continual dialogue among gene expression, stimulus-response and internal state.

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