These form long filaments or hyphae, which become agglomerated into a mycelium or mass of compact tufts. They multiply not only by gemmation, but by the formation of spores.

These moulds vary considerably according to the soil in which they grow, and the amount of oxygen present. Thus, if the spores of the common white mould, Mucor mucedo, are sown in a liquid containing sugar and exposed to the air, they grow on the surface, forming branched hyphae without septa, and the liquid absorbs oxygen. But if the mycelium be immersed, or the oxygen withdrawn, septa develop in the hyphae, and they break up into segments which multiply by budding, forming a kind of yeast with large cells, and, like the true yeast, decomposing sugar into alcohol and carbonic acid.

They may be trained to thrive on substances on which they do not usually grow by gradually altering the composition of the soil. Thus, the commonest of all moulds, Penicillium glaucum, although it does not usually grow on blood, may be trained to do so by transplanting it from bread to peptone, and then to blood.

Heat destroys these fungi, but a much higher temperature is required to kill the spores than the perfect plant, and in order to destroy the spores a temperature of 110°-115° C, kept up for an hour, is requisite.

The mould-fungi cause some local diseases in the body, and especially skin diseases such as favus, tinea tonsurans, tinea versicolor, tinea sycosis, onychomycosis, and the madura-foot or fungus-foot of India. They also occur in the fur of the tongue.