The Agaricus campestris, or common mushroom, is a type of that vast group; it consists of two distinct parts, the nutritive and reproductive. The nutritive part is the mycelium or mushroom spawn of gardeners, which resembles a mass of white spider’s threads mixed in inextricable confusion, and carries on for a time all the functions of the plant. Mycelia may exist for years without bearing the reproductive part, but fruit never can be produced without spawn. The mushroom itself, which springs from the spawn, is the fruit-bearing part, in which the spores are formed and ripened. It is distinguished by a kind of hat or bonnet called the pileus, supported by a stem. The pileus is lined by a number of gill-shaped plates or lamellæ radiating from a common centre; they are the reproductive organs in which the spores are produced by free cell formation, a process always preceded by a concentration of the matter within the parent cell, which is then divided into as many nuclei as there are to be spores. In the higher fungi, the number of spores thus formed is definite; in the Agarics they are in groups of four placed at the extremity of a stem, springing from the summits of these reproductive gills. Most of the Agarics rise from the ground without any cover; the pileus or cap may show every variety from a smooth polished surface to hairs or shaggy scales, but some of the more highly organized have a general wrapper or volva, which encloses the whole plant, and bursting at last, it leaves some traces behind. In others, the pileus is at first clothed with fibres, which vanish or leave traces on its margin forming a veil or curtain. Some have a membrane attached to the stem, either connected with the volva, or spread under the gills when young, and, when more or less persistent, it is called a ring. The spawn, which is the earliest product of the spore, has a great variety of forms. Sometimes it is filamentous, sometimes tubular, creeping extensively, or concentrated in a felted mass, sparingly developed, or produced in abundance. Besides, it is developed in a great variety of situations often difficult to detect. The fructification is alone evident, under innumerable forms, which are as rapid in their growth as they are for the most part ephemeral in their duration. Some species have been known to acquire several square inches of surface in a single night; the plant, however, is often far advanced before it appears above ground. Some Agarics grow most readily after thunder storms and abundant rains; certain species are always single; others grow in dense aggregations; while in several species there is a tendency to assume a circular arrangement, and that not merely when the spawn is perennial, but when the whole existence of the fungus is confined to a few days or weeks.