Although the plant emerges from embryogenesis with few outwardly visible structures, there is nonetheless already a significant amount of organization in place in the plant embryo. Chief among the organizing structures are two apical meristems, one at the shoot tip and another at the root tip. From these two regions will come all future branches, roots, leaves, and flowers.
Apical meristems are collections of dividing, undifferentiated cells. When a meristematic cell divides into two daughter cells, one of them is known as an initial, and the other is known as a derivative. Assuming the meristem is neither increasing or decreasing in size over time, only one of these two daughter cells – the initial – will remain in the meristem. The derivative cell will “leave” the meristem eventually, not by cell migration as seen in animals, but by being pushed farther and farther from the meristem proper by continued rounds of division and expansion. As you study the image of a root apex at right, notice how cells form files that almost seem to ‘flow’ from the tip toward the base.
Meristems are therefore a source of new, undifferentiated cells in the tips of growing stems and roots. As they are pushed out of the meristem through division and expansion, these ‘blank slate’ cells begin to take on a cell identity as one of the three major tissue types: dermal, ground, or vascular. In addition to adopting a cell identity, each cell takes on a shape appropriate for its type and location. The shape of individual cells provides the driving force for the formation of organs like leaves or petals.