A small tuft was then carefully taken from the apple, and placed on a glass slide, and its area measured with the micrometer, and the number of fruit-stalks it contained determined. The surface was 1100 of an inch square, and the number of stalks 800. As the square inch is a familiar unit of measurement, the estimate made for that small extent of surface gives 3,200,000,000 spores. It is a well-known fact that not only inches, but feet, and even acres of this mould are produced, and the number of spores for daily use must be perfectly appalling. They are light, airy, and invisible to the naked eye and therefore escape our notice; were they female mosquitoes, we would realize their nearness and number. With such multitudes of germs produced we need not wonder that the sowing for moulds should be natural and complete. The light which the microscope throws upon this subject makes it unnecessary to resort to "spontaneous generation" to account for the almost certain growth of mould when the proper conditions are combined.
Fig. 9.—Peziza Fucheliana. De Bary.
When a slice of bread which has been thoroughly overrun by the Penicillium is left under the bell-jar, another mould almost invariably comes, covering the blue surface with a rose-colored coat. The asexual spores and their method of formation are given in Fig. 8. It will be observed that the fruit-stalks are unbranched, the spores when mature always double, and arranged on the stalks in whorls. The development is from below upward: a showing an old stalk with the youngest spores nearly ripe, and some of the older ones gone from their attachments; at b is a younger stalk, where the older whorls are complete and the upper ones small and indistinguishable; c is a more enlarged view of a cluster of ripe spores. After this mould, which is of slow growth, has run its course, the bread seldom produces any other forms, and for further study a new culture must be made.
