Have you ever seen white bumps on cuttings? Some plants tend to produce these things before rooting. It is not young roots and it is not a disease. It is called callus. In the past, this word refers to cells that cover a plant wound.

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Example of callus is also doughnut-shaped formation on the tree trunk after pruning around the wound. This formation protects the wound from infection. In the best case, eventually it will cover the whole wound.

Now term callus is used for more broad category – a mass of unorganized plant cells. Usually you can hear this term in the context of plant tissue culture. Very simplified idea of plant tissue culture is the following:

1. take plant tissue,
2. provide proper conditions,
3. get a clone of the initial plant.

Sounds easy and no callus 😀 Ok, let me add some more details.

1. Everything should be sterile 🙂
2. Take specific plant tissue – explant. Almost any part of almost any plant works, but some parts of some plants work much better.
3. Prepare media. It essentially consists of a physical base (for example, agar), nutrients and plant growth regulators – phytohormones. It should be different for explants of a different nature. 
4. Put explant into the media in the controlled physical conditions to get callus. Let me go into cell biology a little bit here. There are different cell types. Some cells could change their type, usually to more specialised. This process calls cellular differentiation. Cell’s ability to differentiate into other cell types is called cell potency. In descending order of cell potency: totipotent, pluripotent, multipotent, oligopotent, and finally unipotent. Totipotency is the ability to differentiate into any cell type. Pluripotency is the ability to differentiate into any cell type of an adult organism. Totipotency/pluripotency + ability to divide = ability to produce an entire organism. So, if we want to produce a new organism in vitro, we need cells with high potency. We, as mammals, have only a few of such magic cells – a result of fertilization process and close-in-time processes. But plants are more lucky from this perspective. They have quite a number of such super cells in shoots, roots, cambium and other parts. This fact makes vegetative reproduction of plants, natural and artificial, possible. So, let’s take one or just a few of these plant stem cells and get new organism in vitro! Hm… In theory it is possible but in practice it is very hard to isolate these cells from the plant without damaging them. But clever men already solved this problem using another great ability of plant cells – de-differentiation. With phytohormones and nutrients in the controlled physical conditions it is possible to force explant to de-differentiate into totipotent or pluripotent cells and start dividing. Here comes the callus! This obtained a mass of de-differentiated cells with high potency.
5. Plantlet forms from callus in the media as the result of condition’s manipulation (mainly, phytohormones). Then it is put in an artificial environment (growing light, microclimate, properly maintained soil, etc). And finally, the plant is introduced into a natural environment. Voilà – you have a fully-functional clone of the initial plant!