A key question in the development of multicellular organisms is what controls the identity, or “fate,” of individual cells. In plants, each cell is bound to its neighbor by the cell wall and thus fixed in place. This means that cell identity must be determined by where a cell is located within the plant. For cells at the surface, the absence of neighboring cells on one side can be the signal to become an epidermal cell. For cells in the interior, chemical signals determine what sort of cell is formed. These signals provide information on both the identity of neighboring cells and the cell’s position in the plant.

Some of the chemical signals that guide plant development affect only a small area. For example, in section 31.1 we mentioned that the signals that cause cells to express meristem identity genes are restricted to the region close to the shoot tip. Other signaling molecules, or their precursors, enter the vascular system and are transported the entire length of the plant. Because plants grow and develop in many places at once, signaling molecules that can move from one part of the plant to another play an important role in coordinating growth and development.