266 



THE FUNGI 



--cap 



a stalk (stipe)) which in some genera arises from a cup (volva) 

 and is expanded above into the cap (pileus). The under surface 



of the cap bears many thin plates 

 which hang down in a radiating ar- 

 rangement and are called gills. The 

 gills illustrate very well the struc- 

 ture and position of the basidia on 

 a fruiting surface, or hymenium, 

 and cross sections are shown in Fig. 

 238. It will be seen that the basidia 

 are the swollen terminal cells of a 

 compact mesh of hyphae, and that 

 each bears a group of four spores 

 on short stalks or sterigmata. 



The toadstool is really a fructi- 

 fication. It is attached to an ex- 

 tensive mass of mycelium, which 

 is the vegetative portion of the 

 plant. This mycelium generally 

 lives saprophytically in the soil, 

 frequently around buried roots of 

 trees, but there are some para- 

 sitic gill fungi (Plate VI) which 

 cause the decay and final death of 

 valuable timber. The toadstool 

 develops from an. accumulation of 

 hyphse in small structures called 



FIG. 237. A group of mushrooms 

 (Armillaria mellea) 



my, mycelial attachment ; c, cf, c", 

 young stages called buttons; 

 mature mushroom with expanded 

 cap (pileus) shown ahove; st, 

 stem (stipe); g, gills; r, ring. 

 After Hartig, through Bennet 

 and Murray 



buttons (Fig. 237, c, 



The 



cap region with the gills and stalk 

 become differentiated within the 

 button, and finally break out from 

 the surrounding envelope and ex- 

 pand in a few hours to their full size ; hence the expression a 

 " mushroom growth." The remains of the envelope are found in 

 some forms as scales on the top of the cap (see mature mushroom 



