Stems, like roots, grow in length by division and elongation of cells at their tips. The youngest cells of stems (but not roots) are organized into two zones: the tunica and the corpus. In the tunica, cell divisions are perpendicular to the stem axis and give rise to a sheet of tissue several layers thick that covers the outside of the tip. Cell divisions in the corpus are in all directions and produce an interior mass of cells. Derivatives of cells in both tunica and corpus continue to divide and produce three recognizable primary (transitional) meristems—protoderm, ground meristem, and procambium—which, as they elongate and differentiate, create the three primary tissue systems— dermal, ground (fundamental), and vascular.
Cell divisions of the apical meristem give rise to leaf primordia close to the tip and so consistently, one after the other, that nodes and internodes can't be distinguished until elongation and differentiation start. At the base of the leaf primordia in the internodal region in monocot shoots, a zone of meristematic cells (an intercalary meristem) remains undifferentiated and retains the ability to divide throughout the life of the plant, causing elongation of the monocot leaves from the base upward.
The central cylinder of a primary plant body is called the stele. It consists of the primary xylem and phloem tissues together with any pith that may be present. Three types occur: 1.) protostele, the simplest, is a solid vascular core and is found in primitive vascular plants and the roots of eudicots (but not monocots); 2.) siphonostele, a hollow cylinder of vascular tissue surrounding a central core of pith is common in ferns; 3.) eustele, is a system of separate vascular bundles surrounding a pith and is the type in almost all seed plants.
Types of primary bodies
Herbaceous plants—in contrast to trees and shrubs—are composed essentially of primary tissues. If secondary growth does occur, the tissues are used for rigidity and not conduction. Two basic variations in the primary body of eudicots include a hollow cylinder of xylem, cambium, and phloem surrounding a central pith and, in others, a system of discrete vascular bundles, also with xylem, cambium, and phloem, arranged in a regular pattern between the pith and the cortex (see Figure ). The primary body of monocots consists of vascular bundles, with no cambium, scattered in an undifferentiated parenchyma called ground tissue.