THE PHYSIOLOGY AND PATHOLOGY OF PRUNING. 211 



position for a wound, especially if it were a large one. The 

 reason why this position is dangerous is perfectly clear. Every 

 wound while healing forms a cup shaped depression. If this 

 depression be left in an upright position it would be an excel- 

 lent device for catching various kinds of organic material to- 

 gether with bacteria, and fungus spores, which will produce 

 decay. In case it is necessary to cut a limb which would leave 

 a wound in this position', I think it would be better to cut it 

 somewhat diagonally in order to prevent the danger, even though 

 the wound does not heal so smoothly. 



In the healing of wounds the age of the injured organ is of 

 foremost importance, as the process of healing works more ra- 

 pidly the younger the injured tissues. The youngest tissues 

 of roots and shoots are made up of cells which are not differ- 

 entiated into permanent structures. These cells are still able 

 to undergo cell division and may be changed into a different 

 structure. If the tip be pinched from a young root it will be 

 found that the entire surface will begin to form new cells and 

 the tip will be entirely renewed in a short time. If, however, a 

 root or branch two or three years old be cut off it will be found 

 that only a small ring just outside the wood has the power of 

 forming healing tissues. At first the tissues formed by the 

 cambium and spreading over the cut surface, consist of delicate 

 soft cells (callus). Later these cells become greatly differ- 

 entiated and form permanent structures. Thus the delicate 

 callus cells gradually form the layers which cover the wound. 

 They may then be looked upon as a part of the cells which 

 formed the branch. 



The shape of the wound has a great deal to do with the rapid- 

 ity with which it heals. If we examine a wound we will find 

 that there is more healing tissue formed at the upper end than 

 at the lower. This is explained by the fact that the plastic sub- 

 stances which enter into the healing tissues descend from the 

 leafy portion of the branch. The upper surface of the wound 

 stops the downward current and causes a large amount of heal- 

 ing material to collect. Thus on a wound which is proportion- 

 ately broad a large amount of material will collect and the heal- 

 ing will go on more rapidly than if the greatest length of the 

 wound were parallel to the long axis of the main branch. 



