In biology research a Callus of Cells is a mass of undifferentiated cells.
A callus is usually sustained on gel media, much in the same manner as bacteria are grown. Sufficient media consists of agar and the usual mix of macronutrients and micronutrients for the given cell type. For plant cells, enrichment with Nitrogen, Phosphorus, and Potassium is especially important. Water is provided as a constituent part of the gel media.
A plant cell callus consists of somatic undifferentiated cells from an adult subject plant.
A callus is not necessarily genetically homogeneous because a callus is often made from structural tissue, not individual cells. Nevertheless, callus cells are often considered similar enough for standard scientific analysis to be performed as if on a single subject. For example, an experiment may have half a callus undergo a treatment as the experimental group, while the other half undergoes a similar but non-active treatment as the control group.
Plant cell calluses may be made to differentiate into the specialized tissues of a whole plant, with the addition of a number of hormones or enzymes. This is an ability known as totipotency.
A callus is often the target of a gene gun for specific DNA insertion experiments.