If the outer surface of the root dries off, the cell pouches shrink and, 

 in the outer layers, are entirely separated from one another. Then a tan- 

 colored, powdery mass forms which may be wiped away wdth the finger. 

 Even the lamellae of plate cork (t) which are present at the edge in thick 

 layers (of equal size, under normal conditions) and, gradually dying back 

 from the outside, fall away at the place of the tan disease, are also drawn 

 into the process of loosening. These split off because some of the middle 

 layers round off their cells and show a tendency to assume the structure of 

 cork as will be described more fully later under the cherr}^ 



Fig-. 24. 



Cross-section through a diseased spot in an apple root. (Tan disease.) 

 (Orig.) 



If the outgrowth of the bark at the edge of the tan canker and the emp- 

 tying of the cell have reached maturity, the well-known hourglass arrange- 

 ment of plate cork layers occurs (f) which cut off the hypertrophied bark 

 parenchyma, finally becoming cork, and it becomes an element of the bark 

 scales. The cell elongation meantime advances laterally and further toward 

 the inside. Thus at w we see the beginnings of this since the bark cells, 

 normally elongated tangentially, are becoming square in cross-section and in- 

 crease in number by division in order to round off more toward the diseased 

 side, to become more open by enlargement of the intercellular spaces (r) 

 and finally to pass over into the radial elongation which increases to pouch- 



