Image: primary stem of a dicot (mallow), stained with safranin / Alcian blue.
Epidermis: it is the outer layer of the stem. The epidermis is a one cell-thick layer with an irregular free surface, but not showing a thick cuticle.
Collenchyma: it is the tissue immediately below the epidermis. Mechanical support is the main function of collenchyma. In this case, it is the annular collenchyma.
Cortical parenchyma: it is the tissue found below the collenchyma. Parenchyma is made up of large cells with thin cell walls stained in blue.
Sclerechyma: it is a layer of sclerenchyma fibers found between the cortical parenchyma and vascular bundles. The sclerenchyma fibers usually form a vascular cap or sheath around each vascular bundle, but in this case form a continuous layer.
Primary phloem: it is the vascular tissue specialized in transporting organic molecules. The primary phloem is organized in a small group of cells located between the outer surface of the procambium meristem and the sclerenchyma.
Procambium: it is the meristem found between the primary phloem and the primary xylem. By proliferation and differentiation, the procambium gives rise to the primary phloem outward and to the primary xylem inward.
Primary xylem: it is the vascular tissue specialized in transporting water and minerals. The xylem is made up of large cells with lignified cell walls (in pale pink), as well as parenchyma cells (small cells with blue cell walls).
Pith parenchyma: parenchyma cells forming the stem pith account for the majority of the stem volume. These cells are large cells with thin cell walls stained in blue.