PLANT BIOLOGY 



turn become bark-bound, and the healing may stop. On 

 large wounds it progresses more rapidly the first few years 



than it does later. This roll or 

 ring of tissue is called a callus. 



The callus grows from the liv- 

 ing tissue of the stem just about 

 the wound. It cannot cover long 

 dead stubs or very rough broken 

 branches (Fig. 64). Therefore, 

 in pruning the branches should be 

 cut close to the tntnk and made 

 even and smooth ; all long stubs 

 must be avoided. The seat of 

 the wound should be close to the 

 living part of the trunk, for the 



FIG. 64. ERRONEOUS 

 PRUNING. 



stub of the limb that is severed 

 has no further power in itself 

 of making healing tissue. The 

 end of the remaining stub is 

 merely covered over by the 

 callus, and usually remains a 

 dead piece of wood sealed in- 

 side the trunk (Fig. 65). If 

 wounds do not heal over speed- 

 ily, germs and fungi obtain 

 foothold in the dying wood 

 and rot sets in. Hollow trees 

 are those in which the decay- 

 fungi have progressed into the 

 inner wood of the trunk ; they 

 have been infected (Fig. 66). 



Large wounds should be protected with a covering of 

 paint, melted wax, or other adhesive and lasting material, 



FIG. 65. KNOT IN A HEMLOCK 

 LOG. 



