16 BULLETIN 871, U. S. DEPARTMENT OF AGRICULTURE. 



certain time produce conditions unfavorable for their further develop- 

 ment and are forced to seek another field. 



In the wood the hyphae are hyaline, varying in diameter from 

 0.8 to 3.3 p. but being most commonly 0.8 to 1.7 n, branching and 

 rebranching into the finest threads, anastomosing, sparsely septate, 

 rarely constricted at the septa, and sometimes having clamp connec- 

 tions. They never become so abundant as to fill the tracheids 

 completely. Usually the hyphas pass from the lumen of one tracheid 

 into that of an adjoining tracheid and then extend up or down 

 the lumen, but occasionally a single hypha may cross several 

 tracheids in a radial or tangential direction without extending up or 

 down their lumens or giving off any branches. The holes in the 

 walls of the tracheids made by the hyphae are very small, particularly 

 so since the hyphae are often sharply constricted w r hen passing through 

 the walls. Rarely the hypha? are irregular in shape. 



The hyphae composing the cobweblike and feltlike masses of 

 mycelium in the badly decayed wood (see p. 13) are usually 

 hyaline, but sometimes have granular contents. They vary in diam- 

 eter from 0.8 to 40 fx, are richly branched, more commonly septate 

 than the hyphae found in the wood cells, and sometimes constricted 

 at the septa. No clamp connections were found. They frequently 

 anastomose. They were often very irregular in shape, and globose 

 or spindle-shaped swellings were frequent. 



OTHER FORMS OF DECAY. 



Besides the typical decay already described, two other very 

 characteristic forms were foimd. One of these is characterized by 

 small spots or pockets of brown decayed wood varying in width from 

 0.5 to 2 mm. (0.02 to 0.08 inch) and in length from 1 to 4 mm. 

 (0.04 to 0.16 inch), with the long axis running vertically in the wood. 

 In some cases larger decayed spots are formed by the joining of two 

 or more smaller ones. The tiny decayed spots are separated by 

 apparently sound wood. As for the structure of the decayed' wood 

 and its reactions with various reagents, these agree exactly with the 

 typical form of dry-rot (see p. 14), and this decay is very probably 

 an abnormal form of the typical decay caused by Polyjwrus amarus. 



The other form of decay consists of very small white spots (measure- 

 ments as given above) in which the wood has been reduced to cellulose, 

 separated by apparently sound wood. The structure of the decayed 

 wood is practically as described by Hartig (8, p. 53-54; 9, p. 36-37) 

 for decay caused by the ring-scale fungus (Trametes yini), and the rot 

 under consideration is undoubtedly caused by this fungus, since, 

 through the courtesy of Dr. James K. Weir, the writer has been 

 privileged to examine sporophores of Trametes pini with the typical 





