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.