A typical unorganised plant callus, initiated from a new explant or a piece of a previously-established culture, has three stages of development, namely:
• the induction of cell division;
• a period of active cell division during which differentiated cells lose any specialised features they may have acquired and become dedifferentiated;
• a period when cell division slows down or ceases and when, within the callus, there is increasing cellular differentiation.
These phases are similarly reproduced by cell suspensions grown in a finite volume of medium (a batch culture), where according to a variety of different parameters that can be used to measure growth (e.g. cell number, cell dry weight, total DNA
content) an S-shaped growth curve is generally obtained (Fig. 1.4). The phases are:
• a period of exponential and then linear growth;
• a period when the rate of growth declines;
• a stationary phase when growth comes to a halt. Some differentiation of cells may occur in cell cultures during the period of slowed and stationary growth, but generally it is less marked and less complete than that which occurs in callus cultures. Cultures cannot be maintained in stationary phase for long periods. Cells begin to die and, as their contents enter the nutrient medium, death of the whole culture accelerates. Somewhat similar patterns of growth also occur in cultures of organised structures. These also cease growth and become moribund as the components of the medium become exhausted.
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