Despite the fungal mode of nutrition figuring prominently in originally defining kingdom Fungi (Chapter 1), the fundamental aspect of cell biology which sets the fungi off from other major kingdoms is the apical growth of hyphae. Extension growth of the hypha is limited to the apex. Growth at the tip is what makes new fungal hypha, but there is more to development than growth at hyphal tips can achieve alone. The vegetative fungal mycelium is an exploratory, invasive organism. Its component hyphae are regulated to grow outwards into new territory and consequently possess controls which ensure that hyphae normally grow away from one another to form the typical ‘colony’ with an outwardly-migrating growing front (see Carlile, 1995). Tissue development requires that different hyphae cooperate in an organised way. For tissue to be formed the invasive outward growth pattern of the vegetative mycelium must be modified so that independent hyphal apices grow towards each other, allowing their hyphae to branch and differentiate in a cooperative fashion.

Since the developing structures (spore-forming fruiting bodies, for instance) are actually produced by the vegetative mycelium, these changes in growth pattern must be localised, and must be a response to regulatory processes which are imposed upon the vegetative mycelium. Another aspect of localisation is that tissue formation demands that the continuous tube of hypha produced by the growing apex is divided up into cells or compartments by the formation of cross-walls (usually called septa).