page 12 STEM ANATOMY Herbaceous or Young Woody Dicot or Gymnosperm Stem Primary Growth Vascular bundles are arranged as a ring between the cortex and pith. The pith and cortex are usually comprised of parenchyma cells.  Inside each vascular bundle, the phloem is orientated towards the outside and xylem towards the inside of the stem. The outer surface is covered by the epidermis. Woody Dicot or Gymnosperm Stem Secondary Growth Remnants of the pith occur in the center, surrounded by rings of xylem (one ring for every year), then the cambium. The phloem is ridged (dilated). Rays transverse the xylem and extend into the phloem (where they  dilate). The outer surface is covered by the periderm or bark, which occurs as irregular layers.