DEVELOPMENT OF PLANTS 243 



extends completely around it (Fig. 166, i). Hyphae now grow 

 down into this cavity in regular lines, forming the radiating 

 plates or gills noted in Fig. 167. This development divides the 

 ball of hyphae into an upper part or pileus and a basal region, 

 the stalk or stipe. As this growth proceeds the mass of hyphae 

 extending from the margin of the pileus to the stipe becomes 

 drawn out and ultimately form a rather thin membrane known 

 as the velum (Fig. 166, 2, vl). This entire growth is often 

 enveloped by a sheath-like mass of hyphae, the volva (Fig. 167, 

 C, D). At this stage of development, the young mushroom is 

 rather spherical in shape and so small that it is quite concealed 

 in the ground or by surrounding vegetation. When conditions 

 are favorable for further growth, as after a rain, each cell of this 

 miniature fungus absorbs moisture and rapidly expands, thus 

 causing the mushroom to spring up as by magic and reach its 

 full growth in a few hours. As the stipe elongates, the pileus 

 spreads out like an umbrella, rupturing the veil, a part of which 

 clings to the stalk (Fig. 167, A, a) in the form of a ring, the 

 annulus, and a part may also hang in the form of ragged fila- 

 ments from the edge of the pileus. If a volva is formed, this 

 is also ruptured by the elongation of the stipe, forming a cup at 

 the base of the stipe, and usually portions also remain attached to 

 the top of the pileus as scales and patches (Fig. 167, A, D). 

 The structure of the mushroom is very simple. The stipe con- 

 sists of a mass of nearly parallel hyphae that show little modifi- 

 cation save at the surface of the stem, where they are sometimes 

 more compactly arranged (Fig. 168, A, B). By cutting across 

 the gills, so that we can look into the ends of them, it will be 

 seen that the hyphae extend down the center of the gills and also 

 continually radiate out on either side, forming a compact layer 

 of rather elongated cells on the surface of the gills, known as the 

 hymenium (Fig. 168, C). A magnified view of a portion of this 

 hymenium show's that it is composed of paraphyses and basidia 

 (Fig. 168, D). The basidia are not divided as in the smuts and 

 rusts, but the spores are formed in the same manner at the end 

 of two or four small tubes that grow out from the apex of the 

 basidia. It should be stated that a series of intermediate forms 



