THE TREATMENT OF THE SHOOT 165 



matous cells. With the development of the woody cylinder a 

 strengthening of this outer protective layer takes place, by the 

 formation of several layers of tabular cork cells. When the 

 twig has grown to a branch, and has therefore formed several 

 annual rings of wood, these layers split the cork mantle open 

 so as to be able to expand themselves. Close to these natural 

 splits new cork layers are formed, which project like arches 

 into the outermost layers of the cortex, and by cutting off 

 such layers cause their ultimate death. These dead pieces 

 are pushed outwards by the development of new bast and 

 cortex, and project from the surface of the stem. The repeti- 

 tion of this mode of procedure causes the formation of the 

 so-called bark scales. These scales, consisting as they do of 

 dead tissues, absorb water during moist periods, and give it 

 off again in dry weather, the cells contracting at the same 

 time. By this contraction the dead scales become gradually 

 separated from the living tissue beneath them. In this way, 

 the stems protect themselves naturally against an excessive 

 pressure of the bark, and the expansion or growth in thick- 

 ness of the stem is able to take place. The scales drop off 

 most readily when the weather is alternately wet and dry. 

 If, in some cases, the tree retains its bark for too long a 

 period, it becomes necessary to scrape the stems. The scraping 

 of the stems should therefore be looked upon as one of the 

 regular operations in arboriculture. 



In those cases in which orchards have not been carefully 

 treated (especially in damp situations), we find trees from 

 which the bark scales have not been removed naturally or 

 artificially for several years, and by decomposing have formed 

 suitable places for an overgrowth of lichens or mosses. In 

 such cases the pressure of the bark is greater than where the 

 scales have been removed, and the growth in thickness of the 

 trees is retarded. We may here begin by scraping the stems 

 and then proceed to slit the bark along the whole length of 

 the stem. 



Sometimes we only desire to strengthen certain portions of 

 stems or branches, where the growth in thickness has been re- 

 tarded by the pressure of other branches or of stakes. What 

 considerable tension is exerted upon the bark by the thickening 



