FIGURE 2.

Green plant life cycle as represented by the PO. The developmental stages of plants are often arranged into a life “cycle” (thick green lines) in which the gametophyte and sporophyte phases alternate, although no individual plant undergoes the entire cycle. Different plant structure development stages in the PO occur sequentially along the cycle, as mapped by the thinner, external lines, although the senescent stages can begin at any point. The beginnings and endings of different development stages are defined by different developmental processes, shown as radial lines intersecting the thick green lines.