CAUSE AND NATURE OF CROWN-GALL 



37 



upon the phellogen meristein, which forms the cork, from 

 whence it rapidly passes by means of intercellular spaces 

 through the cortical parenchyma to the true cambium zone 

 beneath. This view is entertained from the fact that the sur- 

 face of the gall is never protected by cork or true epidermis, 

 but the large parenchyma cells, with their enormous intercellu- 

 lar spaces, which form the outer portion of the gall, are directly 

 exposed to the soil and the numerous saprophytic fungi usually 

 found in decaying vegetable matter. 



Fig. 16. — Same as Fig, 15, with the exception that the section is through the center 

 of the gall and showing the central region of the outbursting cambium (X 20). 



Again, when the gall first begins its development there is a 

 pushing outward of a small area of the true cambium, which is 

 transformed into large hypertrophied parenchyma cells, as 

 shown in Figs. 15 and 16, which represent transverse sections 

 across a small root having a fair-sized gall over a region of 

 previous injury. 



In its youngest stages the tissue of the gall is a mass of paren- 

 chyma with numerous minute areas of rapidly dividing meristem 

 scattered through it. The areas of meristematic tissue are cen- 

 ters of growth (Fig. 17. ), and probably originate in the out- 

 bursting of the normal cambium, stimulated by the growth of 

 the parasite. As the galls become older these centers of growth 



