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most cork layers have fallen off, blister-like or flat, warty swellings put in an 

 appearance and later these have a diseased wooly outer surface. If the 

 place becomes somewhat dr}-, a reddish yellow to a brownish yellow powder 

 may be brushed off which in color resembles fresh tan bark. This may have 

 given rise to the term "Tan Disease." In introducing the subject of this dis- 

 ease into scientific discussion I have re- 

 tained the name used by practical growers. 

 The same process takes place also in 

 roots and young branches. Young bran- 

 ches with knotty tan pustules may be 

 found in cherries. Up to the present this 

 disease of the bark of the older trunk and 

 roots has been observed most frequently 

 in apples. Plums seldom suffer. Similar 

 processes, resulting in the falling off of 

 larger pieces of bark, have been found in 

 elms and will be treated under growth 

 disturbance due to marshy soils. 



In figure 23 is seen a piece from an 

 apple root, natural size. Its bark has been 

 broken open by cross-tears varying in size, 

 the edges of which have been forced back ; 

 the open places are covered with an ochre 

 powder or (when first taken from the soil) 

 with soft, moist, brown masses. Figure 

 24 represents a cross-section through such 

 a callus place. We find the wood (c is the 

 cambial zone) of a practically normal 

 structure traversed by the medullary rays 

 (m), most of which show no variation 

 whatever. Only in some (m') it is notice- 

 able that in the younger portions they be- 

 gin to broaden, thereby causing a looser 

 construction. This process of loosening, 

 however, finds its evident expression only 

 in the bark where the rows of medullary 

 ray cells, beginning to separate from one 

 another, form loops. While the younger 

 inner bark, with its hard bast cords, still shows no change from a 

 normal structure, the older layers (at left side of the illustration) display 

 an impoverishment of the cell contents and some radial stretching (k'). 

 This excessive elongation of the bark parenchyma becomes greater, the fur- 

 ther toward the outside the cells lie, and it increases within the cork zone in 

 such a way that the cells lying free on the outer surface take on a pouch- 

 like form (s) and are only very loosely united with one another. 



Fig^. 23. Apple root with ruptured 

 tan spots, natural size. (Orig.) 



