187 



They Show a tendency to the "gnarl formation" de- 



♦ scrihed above in detail.^ In old age, a disinte- 

 gration of these knots taJ^es place, in such a way 

 that a depression is formed in the middle of the 

 swelling. 



The bacteria swellings of the Aleppo pine, fast 

 becoming a manace to wooded districts of the maritime 

 Alps, are still larger than those of the olive and 

 more regularly rounded. Ho disintegration of the 

 central parts takes place later. According to 

 Pfillieux the formation of the knots proceeds from 

 the bark tissue; histologically they also resemble 

 wound- wood. 



As an example from the list of Zoocecidia. I will men- 

 tion the gall of Chermes fagi . the beech wo oly louse, which 

 tiartig'=' has oloseiy investigated. The gall- format ion here 

 begins directly beneath the cork and can advance even to the 

 woody-body; all parenchymatic elements of the bark, includ- 

 ing the tissue of the cambial rays grow out extraordinarily 

 vigorously and divide actively in a tangential direction, so 

 that long, multicellular, regularly radial rows are produced 

 (compare fig. 79) , by which the stone cells and the prosen- 

 chymatic elements of the bark are pushed from the normal po- 

 sition. He found in the bark excresences of Gold-ribes 

 (p. 80) that similar changes may also be produced by hyper- 

 trophy of the parenchymatic bark elements.- The case of the 

 beech-Chermes gall again illustrates very distincly the great 

 correspondence between kataplasmatic galls and callus tissues. 

 The abnormal bark tissues illustrated in figure 79 show the 

 /oAo\ greates similarity with the bark callus excrescences de- 

 (208) scribed above for Populus* In both cases a constancy of 



direction of cell-division and a lack of differentiation of 

 tissues is common. 



Bark excresences. the cells of which display no regular 

 arrangement^ are produced by different Ceutorrhynchus species, 

 especially C suleicollis, the "kohlgallerrusselkafer" (cab- 

 bage weevil) on the roots of various Crucifera (Brassica, 

 Raphanus) . The wood too is developed abnormally abundantly 

 on the side infected. ^ 



^209) Vifitch.es Brooms and Sta^ Head . 



An especial class of galls is formed by the so- 

 called witches-brooms for Thunder-bushes) , branch- 



I. Prillieux, loc. cit. p. 298 — "des faisceaux sinueux de 

 bois traumatique a cellules courtes, qui s' enroulent autour 

 de centres de formation. lis apparaissent ca et la dans la 

 masse du parenchyme, au voisinage des points ou se montrent 

 les colonies de b9,cilles. Ces enroulements de fibres lig- 

 neuses sent tout a fair coraparables k ceux des madrures des 

 bourrelets qui se produisent au bord des plais des arbres et 

 au plancher ligneux qui s' organise dans la moelle, a la 

 base de certaines boutoures." (Compare above p. 180) 



2. Die Buchen-V/ollaus (Chermes fagi Kltb) . Untersuch aus 

 d. fo'rtsbot. Inst. Munchen, 1880, Bd. I, p. 156. ^ 



3. Frank, loc. cit., p. E88. 



