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Having described the major external and internal signals that modify plant growth and development, we now turn to processes and control at the genetic level. To generate even a single plant cell requires expression of many genes; to achieve an efficient multicellular organism that is capable of responding to changing environmental factors requires differentiation into many different cell and tissue types, each with specific patterns and sequences of gene expression. First, we look at how cells are produced, and then at how intracellular and intercellular coordination are achieved (Section 10.1). In Section 10.2, we consider briefly the many possible pathways for plant cell differentiation, and how cells can switch between some of these options. Next, expression of the genome during devel-opment and in response to external factors is covered, focusing on tissue-specific and sequential patterns (Section 10.3). Finally, in Section 10.4 we cover genome manipulation, or genetic engineering, discussing how foreign genes can be inserted into plant genomes, and then expressed to give commercially valuable or novel plants, and how such techniques have become a new repertoire of tools for plant physiologists.