212 



like outgrowths. By this advance of the process of over-elongation 

 into constantly younger bark parenchyma layers, the activity of the root is 

 finally exhausted at the place of the tan disease. 



The injury is not so intensive in the aerial axes Sometimes in larger 

 trunks the phenomenon is not noticed until the bark is closely examined. It 

 is then found that some bark scales stand out raggedly. If these are re- 

 moved, which may be done very easily, it is observed tliat the outermost 

 layers of the succulent bark tissue form irregular blister-like swellings which 

 rupture later and decompose into dust-like 

 masses which may be wiped away in dry 

 weather. Figure 25 shows the fresh bark sur- 

 face of an apple tree which has been laid bare 

 by the removal of the outer bark scales. 



On this greenish brown, juicy surface 

 hemispherical or elongated warty excrescences 

 (a) appear very clearly. Figure 26 shows a 

 cross-section through such a boil-like swelling, 

 in which, however, the wood, cambium and 

 M ^ J I \ ^ ^ youngest inner bark have not been drawn. 



I \ ' ,- ^ . We recognize at the first glance the corres- 



^^ '^ "-■ pondence in structure with that of the tan spot 



of the root. At the lower part of the figure 

 we find the bark parenchyma with three hard 

 bast bundles of a normal arrangement and 

 position, but close above these hard bast bun- 

 dles is noticeable a change in position since 

 the tangentially elongated bark cells, rich in 

 chlorophyll, begin to increase in length radially 

 (r), to divide and to be arranged in parallel 

 lines broken by large intercellular spaces (i). 

 The fact that this change in tissue must have 

 taken place very early, at the time of pushing 

 out from the cambium, is evident because the 

 permanent tissue of the collenchyma (cl) has 

 developed only one layer within the tissue of 

 the excrescence. The chief part of the swelling has come from the peri- 

 pheral layers which have developed into cushions (w) of elongated, finally 

 pouched cells (s), which have raised the plate-cork cell layers and finally 

 spHt them. 



In explaining this phenomenon we must not forget that these tan places 

 arise underneath the old bark scales, and, with a formation of full cork, finally 

 become bark scales by suberization. Thus we find that the organization of the 

 bark into constricted and constricting cell layers, as they alternate in the 

 bark, has taken place in the young bark tissue, for we find that, in young 

 fresh bark tissue, cross bands of plate-like cells, varying in structure and the 



Fig. 25. Piece of the bark 

 from the trunk of an apple 

 tree with the tan disease. 



a the calluses of the tan disease, f> frag- 

 ments of the dry bark scales covering 

 the whole. (Grig.) 



