Compared with the root apical meristem, the shoot meristem is difficult to study, because it is encased in vegetative tissue, and lack morphological evidence of cell lineage.
In Arabidopsis, the meristem is very tiny, down to about 11 cells across! However, in some plants such as the barrel cacti the meristem can be quite large, even a millimeter or more.
The SAM is formed very early in development, during embryogenesis. In embryos as early as the globular stage, a gene called SHOOTMERISTEMLESS (STM) which participates in organizing the embryo, is turned on in a specific set of a few cells, and this is the first sign of a meristem. During embryogenesis, rudimentary leaf primordia are produced, so in the mature embryo the meristem is mostly formed and ready to go.
The domed SAM of both dicots and monocots exhibits two, SUPERIMPOSED organizations:
Tunica-corpus organization (left and above):
Outer layers form what's known as the TUNICA; specifically the epidermis (aka L1 layer) and the underlying subepidermal or (L2) layer(s). The so-called CORPUS is found beneath the tunica and is essentially the center of the meristem. Cells in corpus divide in apparently random orientation, whereas cells in tunica divide almost exclusively sideways (anticlinally and/or transversely) to perpetuate the layers, almost never periclinally. Therefore, tunica layers are clonally distinct. In this picture we can clearly see layers formed by these sideways divisions.
Zonal organization (rapidly growing vegetative SAM) (left, lower):
At the very top of the meristem is the so-called CENTRAL ZONE (CZ) - in the L1 and L2 there are a dozen or so nondifferentiated cells with large vacuoles and these cells divide relatively slowly., These cells sometimes referred to as plant 'stem cells'. Cell division in the CZ pushes cells outward and downward into the :
PERIPHERAL ZONE (PZ)- which surrounds the CZ on all sides. This region is about 4 cells across in Arabidopsis. Here cells divide rapidly and become designated as founder cells, and start to become RECRUITED into PRIMORDIA. The rapid cell division in the PZ are eventually displaced outwards and downwards into the region where organ differentiation takes place.
In the L3 at the base of the SAM is the so-called RIB ZONE (RZ, RMZ)- the rib zone contributes cells to the stem, including the vasculature and pith of the stem.