1. A mass of parenchymatous cells that forms at a wounded surface, for example where a branch has been cut off a tree. The callus tissue is produced by the cambium and initially forms a ring of thickening (wound wood) around the wound. Eventually it may completely cover the exposed wood. Callus tissue is also formed at the base of cuttings before roots are produced. Similarly callus tissue is important when propagating plants by leaf cuttings. For example, when a Begonia leaf is pegged down on the soil surface and the veins are cut, it is the callus tissue at the wound sites that gives rise to the new plantlets. Grafts also depend on the formation of callus for a successful graft union. The regenerative capacity of callus tissue is utilized considerably in *tissue culture
. Callus cells are grown in vitro on suitable nutrient media and under correct conditions new plantlets will develop. See also canker.
2. An accumulation of the carbohydrate *callose
on phloem sieve plates.