In an intact plant there are many kinds of cells all cells, and soft thin-walled parenchymatous tissue, are having different forms and functions. Meristematic said to be undifferentiated, while specialised cells are said to be differentiated. The cells of callus and suspension cultures are mainly undifferentiated, and it is not yet possible to induce them to become of just one differentiated type. This is partly because culture systems are usually designed to promote cell growth: differentiation frequently occurs as cells cease to divide actively and become quiescent. Furthermore, the formation of differentiated cells appears to be correlated with organ development, therefore the prior expression of genes governing organogenesis may often be required. The in vitro environment can also be very different to that in the whole plant where each cell is governed by the restraint and influence of other surrounding cells. In suspension cultures, for example, cells are largely deprived of directional signals, influences from neighbouring differentiated tissues, and correlative messages that may normally pass between adjacent cells by way of interconnecting strands of protoplasm (plasmo-desmata).
The differentiated state is also difficult to preserve when cells are isolated from a plant. Askani and Beiderbeck (1988) tried to keep mesophyll cells in a differentiated state. The character of palisade parenchyma cells with regard to size, cell form, colour and size, and distribution of chloroplasts could be preserved for 168h, but after this the chloroplasts became light green, their distribution was no longer homogeneous and some of the cells began to divide. Differentiated cells are most effectively produced in vitro within organs such as shoots and roots; even here there may not be the full range of cell types found in intact plants in vivo.
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