520 SPECIAL PLANT PATHOLOGY 



much beyond that of the spruce and balsam fir. In the early stages, 

 according to von Schrenk, small white spots appear, which occupy the 

 entire width of an annual ring. Two or more of these spots soon join, 

 at first in a longitudinal direction, then laterally also, so that one or 

 more rings of woods are transformed to cellulose. The rings are thus 

 separated from adjoining ones so that a series of easily separable 

 tangential plates are formed. The line of separation between the 

 rings is always at the point where the summer wood stops and the 

 spring wood of the following year begins. 



The progress of decay is marked by the attack of more and more 

 sound wood fibers which are reduced to loose fibers of cellulose until the 

 wood has disappeared, when black lines appear, scattered irregularly. 

 The tangential plates become ultimately extremely thin and they then 

 consist of the resistant summer wood cells more or less infiltrated with 

 resin. The whole of the former woody cylinder becomes a mass of 

 separate fibers which can be pulled out individually. 



The fruiting organ is found commonly on all of the afi"ected trees. 

 It is readily distinguished from allied forms by the light red-brown 

 color of the hymenial surface, and the regular small round pores. The 

 form of the pileus varies greatly. Sometimes the brackets are large 

 on the larch, lo cm. (4 inches) in width laterally, 7 cm. (2.8 inches) from 

 front to back, and 5 cm. (2 inches) in thickness, and are formed at the 

 ends of old hard stubs and at scattered points on the bark. Some- 

 times sessile sheets are formed inside of the brackets. The basidia, 

 which form the hymenial surface that lines the pores, are smaller at 

 the apex and form from slender, spore-bearing sterigmata. The 

 basidiospores are brown at maturity. 



Lemon {Citrus limonum, Risso.) 



Brown-rot {Pylhiacystis citriophora, R. E. Smith). — The disease is 

 characterized by a copious exudation of gum from the trunk just above 

 the bud union. A certain area of the bark surrounding the part which 

 shows gummosis dies, becomes hard and dry without any evidence of 

 the fungous parasite. It appears especially destructive on the fruits 

 after packing, and is recognized as a brownish, or purplish, discolora- 

 tion of the rind, which is lighter green than on the ripe fruits. It 

 spreads rapidly from fruit to fruit, and is also characterized by its 

 peculiar odor and the presence of small flies attracted to it. The 



