44 



REACTION or HOST TO PARASITIC ATTACK. 



to increase in the actual wood elements. This is the case in 

 twigs of silver fir witches' brooms, in young swellings of juniper 

 attacked by Gyvinosporangium, and in the thickened twigs of 

 Albizzia resulting from Uromyces Tepperianii^ (^ig- 181). There 

 may be, however, a distinctly increased growth of the wood. 

 Thus, with attacks of Gymnosporangium frequenting juniper, 

 especially G. sdbinae, there is often a marked thickening of 

 branches due to increase in the xylem-elements. Again, one 

 finds cankers due to Aecidium, elatimtm, accompanied by stem- 

 swellings with a diameter twice or three times that of the 

 normal, and in which the bark and bast form but a thin layer 

 in proportion to the part made up by the wood. Exceptionally 

 striking are the gigantic woody knots formed on the Japanese 



Pinus densifiora, and P. 

 Thunbergii affected by Peri- 

 dermium giganteum (Fig. 

 15). 



Wakker found that mu- 

 cilage canals of Ehamnus 

 Frangula affected by Aeci- 

 dium were not so well 

 developed as in normal 

 twigs. 



Eesin-canals are often 

 irregularly formed and ab- 



Fin. 15.— Wood-swelling on Pinus dmsijlora, attacked normally multiplied in COU- 

 at this place by Peridermium giganteum. J natural size. "^ 



(r. Tubeuf phot.) seouence of parasites. The 



(On PiwiM rAwre&er^iistilllargerexamplesinayoccur.) ■*■ ^ 



resin-canals of the spruce 

 were found by Hartig to be so numerous in plants attacked by 

 Agaricus ruelleus that an abnormal quantity of resin is produced 

 in the wood, and flows from the diseased roots ; hence has 

 arisen the name " resin-glut " or " resin-flux " by which the 

 disease has long been known. A particularly noticeable flux of 

 resin takes place from pine-bark in presence of Peridermium 

 pini ; the mycelium grows in the medullary rays and resin- 

 canals, causing an excretion of resin from all living parenchyma 

 in the wood, so that both bast and wood become completely 

 impregnated with resin, and thin sections of wood transmit a 

 rose-coloured light. 



