TOADSTOOLS AND MUSHROOMS (HYMENOMYCETES) 385 
The order is divided scientifically into a number of sub- 
groups according to the method of exposing the hymenium. In 
the largest and most important group of Hymenomycetes, the 
hyraenium covers the surface of thin radiating plates called gills. 
These Fungi are known as the Agarics or Gill Fungi. To the 
Gill Fungi belong most Toadstools and the Field Mushroom 
(Agaricus campestris) which is extensively cultivated for market. 
Fig. 338. — Stages in the development of the Mushroom, Agaricus cam- 
pestris. 1, ground line; m, underground portion of mycelium; s, stipe; 
p, pileus; g, gills; a, annulus.  X 3. 
On account of their structural complexity the Agarics are re- 
garded as highly developed Fungi. They develop as shown in 
Figure 338. 
Before developing the sporophore, the mycelium becomes well 
established in decaying organic matter and this may require 
considerable time. In the development of a sporophore, there 
first appears on the surface of the substratum a small spherical 
body called a button which has a skin-like covering within which 
the sporophore is forming. This body elongates very rapidly if 
