787 



surface, the more the elements of this wood are found to resemble the 

 normally elongated, thick-walled elements (g, h'). In the drawing, the 

 transition from the short vascular elements to the long ones is interrupted 

 by the continuation of an old medullary ray (w) into the medullary ray 

 (m') of the new wood. 



Besides this formation of new wood and independent of it still another 

 cell increase manifests itself in the bark near the hard bast bundle. The 

 parenchyma cells divide and increase, thereby, the thickening of the original 

 bark, which is forced out by these new growths and causes the externally 

 visible swelling (Figs. 179 a, 180 a, 181 a). Under certain circumstances, 

 the new growth within the bark is so intensive that a meristematic zone is 

 produced, which remains active for some time, producing in turn wood and 

 vascular elements, and gives rise to the formation of wood fibres in the 

 bark, said to have been found in the production of gnarl tubers. 



The drawing of a cut branch, reproduced in Fig. 181, does not agree 

 entirely with the structure found in the overgrowing cross-wound of the 

 stump of a branch. The reason for ^this is that we usually think of such 

 cuts as having been made late in the spring or summer on older branches. 

 In these cases the drying back of the tissue from the surface of the wound is 

 not extensive until the time when the wound begins to heal, i. e. until the 

 formation of the overgrowth edge {nr, nh). This overgrowth edge soon 

 appears above the cut surface and lies in a curve over the old wood, which 

 had been formed before the time of pruning and is indicated by ah. The 

 arrangement of the elements then corresponds to the formation of the callus 

 roll in the cuttings illustrated in a later figure ; the nature of the cell elements 

 remains that shown in Fig. 181. 



As the branch becomes older and the wood layers, formed from cambial 

 zones, become increasingly thicker, the overgrowth edge, projecting on all 

 sides above the cut surface of the branch, also becomes thicker and thicker 

 until the opposite sides touch one another and unite in a cap which entirely 

 encloses the cut surface. 



Each overgrowth edge begins in the way shown in cross section in Fig. 

 175. It can, therefore, be said, figuratively, that the new wood layers, 

 formed after injury, spread over the old wood body, laid bare by pruning, 

 and finally shut it in by a cap. 



Girdling Callus. 



By "girdling" is understood the removal of a small circular strip of 

 bark around the whole axis, usually at the time of the greatest cambial 

 activity, since only at this time can the bark body be loosened easily and 

 completely from the wood. 



In girdling, only the part of the branch lying above the wound receives 

 the plastic material prepared by its leaf apparatus. This cannot, as des- 

 tined, be used to strengthen the wood ring for the whole length of the 

 branch, but is held back above the place of girdling, thus conditioning a 



