34 DISEASES OF DECIDUOUS FOREST TEEES. 



Trees are chiefly grown for the wood which they may produce, and, 

 with the possible exception of shade trees, anything which destroys 

 the heartwood of the tree causes serious loss. These losses increase 

 with the age of the tree, for the older the tree becomes the more wood 

 is destroyed. Ultimately all affected trees are blown over, it being 

 only a question of time before this happens. Fomes igniarius, in fact, 

 mav be considered one of the chief factors determining the length of 



/ *-) ^j 



life of many of the deciduous forest trees. 



EFFECT OF THE FUNGUS ON THE WOOD STRUCTURE. 



The diseased wood is very sharply bounded from the healthy wood 

 by black layers about one-eighth to one-sixteenth of an inch in width. 

 (PL II.) There may be but a single one or there may be several 

 arranged more or less concentrically. (PL II, fig. 2.) Just outside 

 of these layers there is a layer consisting of from three to six annual 

 rings, which is darker in color than the normal wood because of the 

 infiltration into the same of products of the decomposed wood. When 

 there is but a single black layer the rotted wood extends out to this; 

 when there are several such layers the completely rotted wood may 

 extend out only to the inner layer, while between the series of layers 

 the wood will be found in various stages of partial decomposition. 

 These black layers never exactly follow the annual rings of growth. 

 They are usually very irregular, crossing the rings back and forth. 

 The completely rotted wood is white to light yellowish in color, accord- 

 ing to the species of tree in which the fungus is growing. When 

 rubbed between the fingers it breaks up into fine flakes, but does not 

 powder. It has lost its strength and can no longer be called wood. 



In the completely decayed wood the mycelium- of the fungus is 

 abundant in the large vessels and medullary rays. The walls of the 

 wood cells are very much thinner than in the normal cells, and in many 

 places the middle lamellae are wholly lacking. The addition of chlori- 

 odid of zinc shows that the walls which are thinnest are composed of 

 cellulose. In other words, one of the principal effects of the fungus is 

 the solution of the lignin elements of the cell wall. Extending from 

 the large vessels the fungous hyphae pass freely across the wood cells 

 and between the remnants of the walls, binding them together in a 

 more or less compact mass. In the earlier stages of the disease the 

 hyphae develop most abundantly in the medullary rays, and from these 

 they pass to the wood cells through the pits. As a rule the hyphse 

 do not enter the cells at other points by a direct solution of the walls. 



Shortly after a hypha has passed through a pit the latter enlarges, 

 forming an irregular hole in the walls, and as the solution of the walls 

 continues two or more such holes coalesce, forming a large opening. 

 At this stage the affected cell walls still retain enough of their lignin 



140 



