36 CAUSE AND NATURE OF CROWN-GALL 



each succeeding year, until finally it so weakens the stem that 

 the tree breaks off. I have observed trees from nine inches to 

 a foot or more in diameter upon which galls had begun to de- 

 velop on opposite sides of the crown in early life, and which 

 were so injured that a stick could be thrust completely through 

 the tree from side to side. If a gall be removed during the first 

 season of its growth, it will not only immediately begin to re- 

 appear around the margin of the wound, but over the central 

 portion as well (Fig. 14.) On account of the greatly accel- 

 erated growth of some portions of the gall over others, many 



Fig. 15. — Photo-micrograph showing a transverse section through an almond root 

 with gall attached. The section is from a point a little to one side of the center of the 

 gall and shows the margin of the outbursting cambium (X 20). 



surface areas, where the growth has been arrested, become in- 

 folded ; hence a section through a normal gall will almost 

 invariably show small patches of broken and irregular brownish 

 cells that originally were on the surface, but have been left be- 

 hind and completely covered by the rapid growth of the sur- 

 rounding tissue. 



The almond seedling possesses a well-developed root system, 

 comprising a strong tap-root, from which numerous laterals 

 arise. The root is protected by a rather thick layer of cork. 

 The first effect of the organism causing the disease is probably 



