The spore is the reproductive organ of the mushroom. It differs from the seed of the flowering plant in being destitute of an apparent embryo. A seed contains a plantlet which develops as such. A spore is a minute cell containing a nucleus or living germ, the reproductive cell germ called by some authors the germinating granule. This in turn throws out a highly elongated process consisting of a series of thread-like cells branching longitudinally and laterally, at length bifurcating and anastomosing the mass, forming the vegetative process known as mycelium or mushroom spawn. On this mycelium, at intervals, appear knob-like bodies, called tubercles, from which the mushrooms spring and from which they derive their nourishment. See Fig. 5, Plate A. Where the conditions have been unfavorable this mycelium has been known to grow for years without bearing fruit. Mushroom spores are very variable in size, shape, and color, but are generally constant at maturity in the same genus. Their shape, almost always spherical in the young plant, becomes ovate, ellipsoidal, fusiform, reniform, smooth, stellate, sometimes tuberculate, or remains globose. This feature, varying thus with the age of the plant, should be studied in the mature plant. MYCELIUM. De Leveille has thus defined mycelium: "Filaments at first simple, then more or less complicated, resulting from the vegetation of the spores and serving as roots to the mushroom." The mycelium of mushrooms or the mushroom spawn is usually white, but is also found yellow, and even red. It is distinguished by some writers as nematoid, fibrous, hymenoid, scleroid or tuberculous, and malacoid. The nematoid mycelium is the most common. Creeping along on the surface of the earth, penetrating it to a greater or less depth, developing in manure among the debris of leaves or decayed branches, always protected from the light, it presently consists of very delicate filamentous cells more or less loosely interwoven, divided, anastomosing in every direction and often of considerable extent. Its presence is sometimes difficult to detect without the use of the microscope, either on account of its delicacy or because of its being intermingled with the organic tissues in which it has developed. Sometimes mycelium unites in bundles more or less thick and branched. This has been called the fibrous mycelium. Where the filaments intercross closely, are felted, and inclined to form a membrane, it is hymenoid mycelium. Where the filaments are so small and close that they form very compact bodies, constituting those solid irregular products called sclerotium, it is scleroid or tuberculous mycelium. With malacoid mycelium we have nothing to do in this paper. It is a soft, pulpy, fleshy mycelium. Systematists have divided the Agaricini into groups according to the color of their spores. These groups are defined as follows by various authors: According to--