POLY POR ALES 



439 



from the substrate, turn the hymenium downwards and are notably dorsi- 

 ventral (Goebel, 1902; Burt, 1918). They may even become stipitate if 

 growing on the upper surface of a log. They are generally gregarious and 

 imbricate on trees, causing an asphyxiation of the wood. 



Another species, Stereum purpureum on fruit trees, raises the epider- 

 mis of the leaves from the palisade layers in vesicles, which fill with 

 air, obscuring the chlorophyll, hence the name "silver-leaf disease" 





Fig. 283. — Cora pavonia. 1. Thalli on tree trunk. 2. Section of fruiting thallus. 

 The dark, spherical structures are gonidia. The reproductive layer is formed of papillae. 

 (1 X %\ 2 X 33; after Johow, 1884.) 



(Brooks, 1911, 1913). A third species, S. frustulosum (Thelephora 

 perdrix), causes the partridge-wood disease of oak. 



Hymenochaete noxia, an extremely polymorphous and plurivorous 

 fungus, is a root parasite in the tropics seriously damaging Hevea, Theo- 

 broma Cacao, Thea and dadap. 



Of special biological interest is the tropical Stereum which forms the 

 lichen Cora pavonia with the alga Chroococcus as gonidia (Fig. 283, 1). 



