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As at the root, the youngest part of a stem is located near the end, where the stem cells multiply. apical meristem, which allows growth in extension.
Just below the apical meristematic zone the cells begin the process of cell differentiation, which leads to the appearance of the various tissues that make up the stem.
Origin of axillary buds
Not all cells produced by the apical meristem undergo differentiation. As the stem grows, meristematic cell clusters remain under the epidermis, just above the insertion point of the leaves. These groups of cells form, in the leaf axils, lumps called axillary or lateral buds.
Stems that have not grown in thickness have a primary structure, characterized by the presence of libero-woody bundles located between the parenchyma cells that fill their interior.
Each libero-woody beam has outward-facing elements of the liber (phloem) and inward-facing elements of the wood (xylem).
In monocotyledonous plants, which usually show secondary growth, the conducting beams are diffusedly distributed within the stem. In dicotyledonous, the woody bundles are distributed regularly, forming a cylinder.
In the libero-woody bundles of the dicotyledons, the phloem faces the outside of the stem and the xylem inwards. Between the phloem and the xylem of a beam there is a meristematic tissue: the vascular exchange and the suberogen or felogen exchange.
The vascular cambium of the stem is formed from the fascicular cambium and the interfascicular cambium, the latter a secondary meristematic tissue, resulting from the de-differentiation of parenchymatic cells located between the libero-woody bundles. The fascicular exchange thus delimits an internal area of the stem where there is only xylem and parenchyma. As in the root, vascular cambium cells originate xylem vessels to the innermost region and phloem vessels to the outermost region. Gradually, the area delimited by the exchange rate becomes increasingly cylindrical.
Vascular exchange The stem, like the root, is also a meristem of mixed origin, primary and secondary. This is because it originates from both the fascicular cambium, a primary meristem, and the interfascicular cambium, a meristem that arose from the parenchymal cell de-differentiation.
Suberogen or felogen exchange
The activity of vascular exchange causes the stem to progressively increase in thickness. To keep up with this growth in diameter, the cortical parenchyma cell is undifferentiated and gives rise to a secondary meristem cylinder, the felogen. As we have seen, the activity of felogen produces feloderme inland and super to the outside, forming the periderm, which now covers the stem.