Some ornamental plants, including orchids and most transgenic crops, are reproduced by tissue culture (Figure 3.4). This involves the infliction of a wound to the parent plant so that it forms a callus from parenchyma cells. Parenchyma cells are most similar to meristem cells and represent up to 80% of all the cells in the plant. Meristem cells have a large nucleus and can repeatedly divide to produce daughter cells. Meristem cells are the plant equivalent of animal stem cells and are found in the meristem region at the tips of roots and shoots and in seeds. It is only when meristem cells migrate out of the meristem region that they differentiate into specialized cells that make up the rest of the plant. Once a meristem cell differentiates, it generally will not divide again, but parenchyma cells can divide in response to a wound and produce a callus, which is a mass of undifferentiated cells. These undifferentiated cells from the callus can be induced to differentiate into the specialized cells that will produce a fullgrown plant.
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