42 DISEASES OF TREES 



fungus-body which consists of these thick-walled hyphse 

 becomes almost as hard as stone.* On the other hand, there are 

 instances of the entire walls or only their outer, less frequently 

 their inner parts being converted into a mass of slime, and 

 under certain conditions the walls, when treated with iodine, 

 become as blue as starch would under similar circumstances. 

 This occurs, for instance, in the mycelium of Hysterium, and the 

 apices of the asci of Rosellinia quercina. 



At first the hyphae are almost always colourless, but in later 

 life the walls very often assume a light or dark brown colour. 

 In rarer instances other colours are produced, e.g. the blue green 

 of Peziza cernginosa, which causes the so-called green-rot in 

 the dead wood of the oak, beech, and spruce. Sometimes the 

 coloration is confined to the outer or to the inner layers of the 

 cell-wall. The mycelium, which increases acrogenously and pro- 

 duces lateral branches, generally remains in a filamentous condi- 

 tion that is to say, the mycelial filaments remain isolated, or, at 

 most, coalesce only at the points where they cross each other. 



A mycelium which vegetates on the outside of leaves, fruits, 

 &c., as is the case for instance with the mildews (Erysiphe], 

 is said to be epiphytic. When it vegetates in the inside of 

 plants it is called endophytic. In this case it either grows 

 from cell to cell by piercing the walls (intracellular), or it 

 advances between the cells (intercellular). In the latter case 

 it behaves, as a rule, like most epiphytes, sending out short 

 branches, known as sucker-tubercles or haustoria, into the interior 

 of the cells, in order to extract the nutrient contents. 



When the opportunity is presented for the filamentous 

 mycelium to develop vigorously outside the nourishing 

 substratum as happens most frequently in the case of the 

 wood-inhabiting Hymenomycetes it forms a skin-like layer, 

 which often attains large proportions. In other cases it may fill 

 cracks or other cavities in the stems of trees. Such layers, 

 crusts, and masses of fungal growth are best known in the case 

 of Polyporussulphureus, P. vaporarius, P. borealis, Hydnum diver- 

 ) Trametes Pint, Merulius lacrymans, &c. 



* [Such indurated masses of fungus-mycelium are usually termed 

 in reference to their hardness : they commonly serve as storehouses in the 

 sense indicated on p. 40. ED.] 



