TYPES OF TISSUE CULTURE

3.1. ORGAN CULTURES
Differentiated plant organs can usually be grown in culture without loss of integrity. They can be of two types:
• Determinate organs which are destined to have only a defined size and shape (e.g. leaves, flowers and fruits);
• Indeterminate organs, where growth is potentially unlimited (apical meristems of roots and nonflowering
shoots).

In the past, it has been thought that the meristematic cells within root or shoot apices were not committed to a particular kind of development. It is now accepted that, like the primordia of determinate organs such as leaves, apical meristems also become inherently programmed (or determined) into either root or shoot pathways (see Chapter 8). The eventual pattern of development of both indeterminate and determinate organs is often established at a very early stage. For example, the meristematic protrusions in a shoot apex become
programmed to develop as either lateral buds or leaves after only a few cell divisions have taken place (see Chapter 10).
3.1.1. Culture of determinate organs An organ arises from a group of meristematic cells. In an indeterminate organ, such cells are theoretically able to continue in the same pattern of growth indefinitely. The situation is different in the primordium of a determinate organ. Here, as meristematic cells receive instructions on how to differentiate, their capacity for further division becomes limited.
If the primordium of a determinate organ is excised and transferred to culture, it will sometimes continue to grow to maturity. The organ obtained in vitro may be smaller than that which would have
developed on the original plant in vivo, but otherwise is likely to be normal. The growth of determinate
organs cannot be extended by subculture as growth ceases when they have reached their maximum size.