Dec. 6. 191S Honeycomb Heart-Rot of Oaks 423 



There is a brownish discoloration of the heartwood on the outer edges 

 of the affected area. This character is also common to several other 

 heart-rotting fungi. 



When a living tree having the rot caused by 5. subpileatum is first 

 split open, there is a very distinct odor of old honeycomb. In some 

 white oaks the old pockets have blackish deposits on the walls which 

 make this rot resemble even more strongly an old, blackened honeycomb. 



MICROSCOPIC CHARACTERS 



A microscopic examination of the diseased wood in the initial stage 

 of a pocket shows small groups of partially delignified wood fibers scat- 

 tered in the neighborhood of the large vessels. Delignification in these 

 wood fibers begins with the inner layer or the tertiary lamella of each 

 fiber and proceeds outward toward the primary or middle lamella. The 

 middle lamella is then attacked and rapidly dissolved, thus freeing each 

 cell from its neighbor. 



The walls of the small medullary rays are more slowly delignified than 

 the wood fibers, while the walls of the large vessels resist delignification 

 much longer than either the wood fibers or small medullary rays. The 

 tyloses in the large vessels are the last to be delignified. They contain 

 many small, irregular holes, apparently made by the passage of fungus 

 hyphae through them. Delignification is not very pronounced in the 

 cells of the radially placed rows of small vessels of the summer wood. 



The pits of the vessels and the cells do not seem to be enlarged by the 

 action of the fungus until the last stages are reached, if at all. 



FUNGOUS MYCELIUM 



In the earliest stages of the rot the enzyms seem to precede the fungous 

 hyphae, especially in the region of the wood fibers. In the larger vessels 

 a few colorless very small hyphae can be seen in the region adjacent to the 

 area first delignified. As delignification advances, the threads in the 

 vessels increase in number, and during the period of cellulose absorption 

 the vessel from which the delignification started often becomes stuffed 

 with small, intricately branched, colorless hyphae. 



In the center of the pockets are often seen small, white, threadlike 

 bodies. On examination these prove to be (i) the remnants of the 

 delignified walls of the vessels and especially of the tyloses, which often 

 persist even after all of the walls of the vessels have been absorbed, and 

 (2) fungous tissue, which is composed of large (lO/x), longitudinal, hyaline, 

 thin-walled hyphs and many smaller hyphae, all interwoven into a rodlike 

 mass. 



In many of the pockets where much of the cellulose has been absorbed, 

 dense white fluffy masses of mycelium either nearly fill or in some instances 

 only line the cavities. This mycelium is composed of small, branched, 

 colorless, thick-walled hyphae, some of which have granular or tuberculate 

 walls. If the pockets border on checks or windshakes, the fluffy masses 

 of mycelium are a reddish brown in place of white and often form a more 



