somatic embryos. In maize, these two callus types are designated as type I (compact) and type II (friable). Callus can directly undergo direct organogenesis and/or embryogenesis where the cells will form an entirely new plant. Callus cell death Callus can brown and die during culture, mainly due to oxidation of phenolic compounds. In Jatropha curcas callus cells, small organized callus cells became disorganized and varied in size after browning occurred. Browning has also been associated with oxidation and phenolic compounds in both explant tissues and explant secretions. In rice, presumably, a condition which is favorable for scutellar callus induction induces necrosis too. Uses Callus cells are 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 calluses derived from many different cell types can differentiate into a whole plant, a process called regeneration, through addition of plant hormones to the culture medium. This ability is known as totipotency. A classical experiment by Folke Skoog and Carlos O. Miller on tobacco pith used as the starting explant shows that the supplementation of culture media by different ratios of auxin to cytokinin concentration induces the formation of roots – with higher auxin to cytokinin ratio, the rooting (rhizogenesis) is induced, applying equal amounts of both hormones stimulates further callus growth and increasing the auxin to cytokinin ratio in favor of the cytokinin leads to the development of shoots. Regeneration of a whole plant from a single cell allows transgenics researchers to obtain whole plants which have a copy of the transgene in every cell. Regeneration of a whole plant that has some genetically transformed cells and some untransformed cells yields a chimera. In general, chimeras are not useful for genetic research or agricultural applications. Genes can be inserted into callus cells using biolistic bombardment, also known as a gene gun, or Agrobacterium tumefaciens. Cells that receive the gene of interest can then be recovered into whole plants using a combination of plant hormones. The whole plants that are recovered can be used to experimentally determine gene function(s), or to enhance crop plant traits for