One of the most essential conditions for the development of these minute fungi is the presence of a good degree of moisture. So well known is this, that to many minds moisture and mould bear to each other nothing less than the relation of cause and effect. A warm atmosphere is also required. In winter the housewife exercises fewer precautions to keep these intruders from her viands than during the warm summer weather. Besides organic matter, moisture, and warmth, a free access of oxygen must be added as an essential condition for the perfect development of moulds.
When the season comes and the soil is ready, the farmer knows he must sow the seed, or he cannot hope to reap a harvest. So it is with the moulds: to the conditions for growth there must be added the germs of life, or no mould will be produced. How this sowing is accomplished will be better seen after some of the species are considered more in detail.
Our common bread is a substance which offers special inducements for the growth of various moulds, and, in order to study them, a slice was taken and placed on a zinc rack on a dinner-plate and covered with a glass bell-jar lined with filtering-paper which dipped into some water in the bottom of the plate, producing thus a moist atmosphere by the evaporation from its extensive surface. This culture
(Fig. 1), placed in a warm room, secured all the important conditions for the production of a crop of mould. On the bread thus situated a mould made its appearance in about thirty-six hours, and proved to be one of the most common of the bread-moulds (Mucor stolonifer), shown in Fig. 2. When first noticeable, the surface of the bread is covered with a cobweb-like mass of fine white threads, called mycelium, which run in all directions through the tissue of the bread, and perform the work of absorbing nourishment. Soon other and larger threads begin to rise into the air, their tips enlarge, the protoplasmic contents of the threads passing up into the ends, which finally assume a spherical shape. At first these large round heads are of a white