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