ABSTRACT
As growth of the callus proceeds, centres of meristematic activity are formed. The variation depends on the parent tissue, the age of the callus and the growth conditions. Callus cultures are also predisposed to genetic instability and as a result, variation in phenotype within the same culture may occur. If the callus culture is to be used as part of a micropropagation programme, the parent plant must show the desired genetic characteristics. The formation of callus is usually achieved by placing the sterile explant onto an appropriate solid growth medium in a container such as a Petri dish or culture tube. Callus formation may be preceded by expansion growth of the plant tissue. Callus cultures are maintained at around 22-25 C under low-intensity fluorescent light with a dark/light cycle of 8 h:16 h. Callus cultures can be maintained under these conditions for several years with sub-culturing every 3-6 weeks depending on the species and the growth rate of the culture.
