CHAP, xiii.] Fungoid Diseases of Trees 301 



ments of the mycelium only occur in a limited number of 

 fungi. In Agaricus melleus they assume the forms of Rhizo- 

 morpha fragilis var. subterranea so long as they are blackish- 

 brown and lie outside the bark ; but they become Rhiz. frag. 

 var. subcorticalis when the mycelial filaments develop below or 

 within the bark. The sporophores of the former, appearing here 

 and there above the soil, are less numerous, and less apt to 

 attain maturity, than those of the latter variety, which make 

 their appearance in thick clusters near the neck of the root. 

 The blackish-brown filaments of the former are generally round, 

 and ramify throughout the soil like fine rootlets ; whilst those of 

 the latter are whitish and ribbon-like. 



Concerning this fungus, Professor Vines makes the following 

 remarks * : 



' The most remarkable sclerotia are those of Agaricus melleus, a Fungus 

 which is very destructive to timber. The mycelium gives rise to dark- 

 coloured compact strands of hyphae, of the pseudo-parenchymatous 

 structure characteristic of sclerotia ; but they are peculiar in possessing 

 continued special growth, and by this means they soon become long 

 filaments, known as Rhizomorpha. It is in this way that the Fungus 

 spreads from tree to tree : the Rhizomorpha-filaments grow under- 

 ground from the roots of an infected tree to those of a healthy tree 

 (usually a Conifer) ; it penetrates into them and spreads in the tissues 

 external to the wood in the form of a white fan-shaped mycelium. The 

 compound gonidiophores (Agaricus melleus) are borne either on the 

 subterranean Rhizomorpha-filaments, or on the parasitic mycelium ; in 

 either case the gonidiophores come to the surface.' 



Young plants, when attacked, usually die off during the first 

 year of the attack. A microscopic examination of them will 

 then show that all the cambial layer and the resin-ducts have 

 been invaded by the mycelium, robbed of their contents, and 

 gutted to form empty hollows, the starchy matter in the cells 

 being transformed into turpentine, which causes the resinous 

 outflow on to the soil. The receptacles, mushrooms, or toad- 

 stools (pilei), make their appearance in October, being developed 



1 A Students' Text-book of Botany, 1894, p. 314. 



