DEVELOPMENT OF PLANTS 229 



seen in the timbers of mines and under the bark of decaying trees. 

 In certain species the mycelium is phosphorescent and the cause 

 of the pale light that sometimes appears upon moist decaying 

 wood, the so-called fox wood. The mycelium bears at various 

 places the complex fleshy or woody body commonly known as 

 the mushroom or toadstool. If the mycelium grows in a regular 

 manner, radiating outward in all directions, then the mushrooms 

 will have a similar arrangement. The older central portions of 

 the mycelium will finally die off, while the newer portions con- 

 tinue to radiate outward and produce the mushrooms, which con- 

 sequently appear in a more or less regular circle. In this way, 

 the fairy rings, which are often held in superstitious awe, come 

 about. 



The Mushroom or Toadstool. — The common umbrella type 

 of mushroom consists of a stalk or stipe and a cap or pileus, 

 on the underside of which are located radiating plates or gills 

 (Fig. 167, A). This structure originates on a strand of the 

 mycelium as a very small ball composed of a mass of interwoven 

 hyphae (Fig. 166, 3). Soon, however, the hyphae of this mass 

 begin to grow in a very regular manner. At first the growth 

 results in the formation of a cavity in the upper part of the ball 

 that extends completely around it (Fig. 166, 1). Hyphae now 

 grow down into this cavity in regular lines, forming the radiat- 

 ing 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 become 

 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 



