The development of the aboveground portion of seed plants, also called the shoot, is characterized by four phases: embryonic, juvenile vegetative, adult vegetative, and reproductive. The embryonic stage includes the period between the fertilization of an egg and the formation of a mature embryo within a seed. In contrast to animal embryos, plant embryos do not contain precursors of all of the organs found in an adult plant. Instead, the embryonic phase establishes the basic axes of growth, and new organs are continually formed throughout the life of the plant from a pool of stem cells at each end of the shoot-root axis. These stores of stem cells are called the shoot apical meristem (SAM) and the root apical meristem (RAM), respectively. Thus, while the apical end of an angiosperm embryo includes only one or two cotyledons, one or more immature true leaves, and the SAM, upon germination the plant repeatedly initiates new organs from the SAM—first vegetative organs (leaves, stems, and branches) and then reproductive ones (flowers).