PRINCIPAL GROUPS OF PLANTS. 57 



this way exclusively for more than two or three years. The my- 

 celium is really the plant body, and the part which rises above the 

 surface and is commonly regarded as the toadstool or mushroom 

 (Figs. 33 to 37) is a fruit branch, or spore-producing organ. 

 When these branches first make their appearance they are in the 

 form of small solid bodies known as " buttons " (Fig. 33, I-V). 



FIG. 33. Agaricus campestris, the common edible mushroom, showing at A on tha 

 left mycelium (m) and development of buttons or young mushrooms; I to V, longitudinal 

 sections showing successive stages in development of fruit body; m, mycelium; st, stipe; 

 h, portion between veil (v) and spore-bearing portion (1). 



The illustration to the right (A, B, C) shows the structure of the hymenium in different 

 degrees of magnification: A, section through portion of pileus showing five of the gills; 

 B, section of a gill somewhat magnified; C. section of gill still more magnified and showing 

 sterile cells or paraphyses (q), and the fertile cells or basidia (s), from each of which arise 

 two basidiospores. After Sachs. 



As growth proceeds these bodies differentiate into a stalk-like 

 portion known as the stipe (Fig. 33, st), which is directly con- 

 nected with the mycelium, and an umbrella-like portion borne at 

 the summit of the stalk, called a pileus, which at first is closed 

 down over the stalk, but later expands or opens more or less 

 widely according to ttje species. On the under surface of the 

 pileus, known as the hymenium, the spores are borne (Fig. 33, 



