44 



REACTION OF HOST TO PARASITIC ATTACK. 



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

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

 attacked by Gjjninosporcuigiu.m, and in the thickened twigs of 

 Alhizzia resulting from Uromyces Tcpperianus (Fig. 181). There 

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

 Thus, with attacks of Gymnosporangimn frequenting juniper, 

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

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

 finds cankers due to Aecidinm datinum, 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 



Finns dcimflora, and P. 

 fT""^. Thunhcrgii affected by Peri- 



dcrmium gigantcvm (Fig. 

 15).^ 

 -?' ^XT', ^ >' f'^ Wakker found that niu- 

 '*'^^-. rJ*^,0,.^H> eilage canals of Bhamivm 

 J^"^^ ,"' ^ Frangnia affected by Acci- 

 ^'^■,<^ .^ diam were not so well 

 ?' ^ developed as in normal 



twigs. 



liesin-canals are often 

 irregularly formed and ab- 



Fio. 15.— Wood-swelling on Pi)iits(;e/is(lrt()/((, attacked nomiallv multiplied ill COU- 

 at this place by Peridermiuni (jiganteum. J natural size. "^ *■ 



(V. Tubeuf phot.) sequeiice of parasitcs. The 



(On Puiiis r/iitH'^crffustiUlargerexaraples may occur.) ^ ^ 



resin-canals of the spruce 

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

 Agaricus melleus 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 Pcridermium 

 pi/ii ; 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 lifdit. 



I 



