THALLOPHYTA: FUNGI 



149 



MeruUus lacrymans is the dry-rot fungus, a species attacking woodwork 

 and structural timbers. It often causes great destruction to wooden 

 buildings. 



5. Agaricaceae. This is the large family of gill fungi, a group to which 

 the common mushrooms and toadstools belong. The basidiocarp may be 

 bracket-like but more commonly is umbrella-hke. It is usually fleshy, 

 rarely leathery in texture. In this family the hymenium covers blade- 

 like radiating plates known as gills. Of the numerous genera, a few 

 common ones are Coprinus, Agaricus, Amanita, Lepiota, Hypholoma, 

 Russula, and Marasmius. The largest genus is Marasmius, with about 

 450 species. 



Fig. 122. Coprinus micaceus. A, cross section through a few of the gills, X 10; B, enlarged 

 portion of same, showing four basidia arising from surface of gill, each with four stalked 

 basidiospores, X750. 



Agaricus and Other Mushrooms. The common field mushroom 

 {Agaricus canipestris) grows in lawns, fields, and along roadsides. It is 

 the principal species used for food and practically the only one that is 

 cultivated. The mycelium lives on organic matter in the soil. The 

 fleshy basidiocarp arises just below the surface as a "button" composed 

 of interwoven hyphae. Soon a stalk or stipe and a cap-like pileus become 

 differentiated. The gills, which develop on the lower side of the pileus, 

 are covered by a membrane called the velum. This extends from the 

 margin of the pileus to the stipe, becoming ruptured as the pileus expands. 

 In Agaricus and many other mushrooms a portion of the velum remains 

 attached to the stipe, forming an annulus around it (Fig. 121). In 

 Amanita, a genus of poisonous mushrooms, the young basidiocarp is 

 completely enclosed by an outer membrane that ruptures as the stipe 

 elongates, forming a cup or sheath, called the volva, at the base of the stipe. 



The hymenium of the Agaricaceae, covering the surface of the gills, 

 consists of innumerable basidia, each of which typically bears four basidio- 

 spores on slender sterigmata (Fig. 122). The cells of the vegetative 

 mycelium are typically binucleate and there are two nuclei in the young 



