Shoot is a term used to refer collectively to the stem and associated leaves of a vascular plant. In most plants, however, the primary function of the stem is to support leaves in a position where they are well exposed to light.
At the tip of every shoot is a terminal bud. If the leaves of this bud are carefully peeled away, a tiny, rounded dome of tissue is exposed—the shoot apical meristem (SAM). A meristem is a region of the plant where cell division is concentrated. Cell division and differentiation in the shoot apical meristem produce the tissues that form the stem and leaves of that shoot.
The structure of the shoot apical meristem varies, depending on the plant being examined. Seedless vascular plants, such as ferns, have a large, pyramidally shaped apical cell that ultimately gives rise to all the cells and tissues of the shoot.
The seed plants have a multicellular apical meristem differentiated into several zones. Each zone is associated with one of three primary meristems that form the tissues of the shoot system. A surface layer of cells covers the dome and differentiates into protoderm, the primary meristem that forms epidermal cells. The epidermis is a continuous protective layer that covers the stem and leaves of the shoot system.
Within the apical meristem, certain cells begin to elongate and form the procambium, the primary meristem that forms vascular tissues. The remainder of the internal cells of the apical meristem become ground meristem, the primary meristem that forms ground tissues, such as cortex and pith.
As the apical meristem grows and new cells are formed, the dome of tissue enlarges. This growth is not symmetric; rather, surface or subsurface cells of a localized region will proliferate more rapidly than the rest to form a small bulge of tissue. This bulge is the first sign of initiation of a new leaf, a leaf primordium. As the leaf primordium enlarges, the protoderm also proliferates to maintain a continuous covering over the developing leaf that is contiguous with the developing stem. This continuity is maintained as cells differentiate into mature epidermal cells.
After initiation of a leaf primordium, the shoot apical meristem continues to grow, and procambium begins to differentiate within the developing node. These procambial cells connect to already formed procambium in older portions of the stem to form the template for the vascular system of the stem. Simultaneously, as the leaf primordium enlarges, procambial strands differentiate in the leaf base region. Further development is bidirectional.
Cells from the basal end of each strand differentiate into the node and connect with the already-formed procambium there. Cells from the upper end of each strand continue to develop into the enlarging leaf primordium. In this way the vascular tissues of the leaf and stem are integrated into a continuous system. Similarly, the ground tissues of leaf and stem are continuous as the leaf primordia are initiated and develop.