The Blacl: Zones Formed hy W ood-Destroijing Fungi 13 



depth of the tissue exposed through wounds. In cross sec- 

 tions of such wounds this is especially noticeable in as much 

 as the discolored tissue is sharply differentiated from the 

 underlying unchanged woody tissue. Microscopic examina- 

 tion shows that this darkening is due to the fact that the cell 

 walls of the wood in question have been discolored by a 

 brownish substance. Especially have the lumina of the ves- 

 sels and the adjoining cells become filled up with a solid mass 

 of brown material, Frank has named this brown material 

 '' wound-gum."'"' In the tissues thus modified, in the vicinity 

 of wounds tyloses arise in most dicotyledonous woods. These 

 are bladder-like outgrowths of the parenchyma cells which 

 project into the vessels and occlude' them quite effectually. 

 We find the vessels partly filled with '" wound-gTim " and 

 partly filled Avith tyloses. 



Eobert Hartig (1878), in his work on the diseases of wood, 

 explained this browning as the first step in the decomposition 

 of the wood or of wound decay. He stated (pages 66 and 

 140) that the dark brown discoloration of the decayed wood 

 is due to the fact that a yellow or broA\mish liquid is contained 

 within the woody elements. This liquid substance, upon dry- 

 ing, collects as a crust upon the cell walls or as a brittle yellow 

 or brownish substance which cracks in various directions and 

 completely fills the cells. He considered this substance to be 

 a humus solution and stated that it consists of the decom- 

 position products of the cell contents which have been dis- 

 solved in the water that leaches in from without and pene- 

 trates farther into the wood. 



*This substance has no connection with the true gum Avhich arises 

 through the secretion of the cell walls of stone fruits and tropical gum 

 trees and Avhich, in every respect, is of a different nature. Similar 

 material has been called " protection gum " and " wood gum " by other 

 authors. Theodor Hartig and certain later authors have used the word 

 " Kernstoffe " which the writer has translated to the term " decompo- 

 sition products " and used throughout this paper to avoid ambiguity 

 and for tlie sake of uniformity. 



