FUNGI. 29 



destructive activity. Thus a single spore of diminutive 

 size may be the cause of complete destruction of a large 

 mature tree. 



The spores of fruiting bodies which appear in the 

 form of shelves, conchs, or toadstools are usually de- 

 veloped on the underside. This underside or lower 

 surface is nearly always more or less horizontal, which 

 often makes it possible to determine whether a fruit- 

 ing body was formed on the living tree or after the tree 

 had fallen. In this group of fungi the underside of 

 the fruiting body is either porous (PI. IV), spiny (PL 

 XVII), or gill-bearing (PL III). 



FUNGI WITH FRUITING BODIES IN THE FORM OF STOOLS OR 



CRUSTS. 



POROUS FUNGI. 



In porous fungi the underside is pierced by innumer- 

 able holes (pores) which are the openings of tubes. 

 These tubes are nearly always perpendicular, and the 

 spores are developed on the lining of the walls. The 

 pores are sometimes elongated, forming an irregular 

 network. Fungi of this group are often injurious to 

 cut timber, such as railroad ties (PL II). The fruit- 

 ing bodies of the porous fungi are described as shelf, 

 bracket, or hoof-shaped, or as crusts, according to their 

 form. 



SPINY FUNGI. 



The underside of fruiting bodies of this kind of 

 fungus is made up of spines or teeth, on the surface of 



