Definition - What does Callus mean?
A callus is the protective epidermal layer that forms over a wound on a woody plant. It is the plant equivalent of a scab on an animal's skin. Whenever a shrub or tree is pruned, the area of the incision will produce a callus to cover it.
MaximumYield explains Callus
In nature and in the garden there are many things that can injure a woody plant. It may be the chewing of insects or animals, or the tapping of a tree for its sap. If a callus did not form over the wound, it would continue to leach or bleed valuable nutrients or moisture.
Sap leaking from an injured area attracts insects, birds, and parasites that can cause further damage. Additional insect damage may occur before the callus has formed, and this may result in the formation of a gall at the wound site.
Although not considered a practical method of propagation, plant callus tissue culture has been successful in carrots, tobacco, and asparagus. The advantage of callus tissue in propagation is that the callus culture produces a genetic variability and often the cells can double their chromosomes.