HOW PLANTS HEAL THEIE WOUNDS 



103 



rapidly give rise to cork. Under the bark that forms the edges of 

 the wound, the tissues remain that can make fresh cork and wood, 

 and the mass of new material 

 which they at once begin to pro- 

 duce, and which constitutes what 

 is called the callus, creeps gradu- 

 ally over the exposed surface. 

 Figure 217 shows a section of a 

 stem that has been wounded on 

 the side, and figure 218 represents 

 the wound itself, in process of heal- 

 ing. When a wound is very large 

 or in a particularly exposed 

 position, the uncovered wood may 

 rot before it is again protected, 



FIGURE 217. A section across a side wound in 

 an Ash branch. The shaded part indicates 

 where the exposed wood has begun to rot. 



and the wound in consequence 

 may never heal satisfactorily. 



If two wounded stems are held together sufficiently closely, and 

 precautions taken to prevent drying or rotting, as well as to make 

 the breadth and length of the wounds equal, so that the live parts 

 coincide, the callus at the two edges of one stem will in certain cases 



unite with that of the other 

 (see figure 219) which grows 

 against it, and in the following 

 year (see figure 220) the two 

 stems will jointly form a new 

 annual ring which will per- 

 manently hold them together. 

 photograph by willed Mark Webb. This is what is done in inarch- 



. Side wound on an Ash branch. ing and grafting, though in the 



