Slide 19: Cross-section of a Coleus stem below the apical meristem. The more darkly-stained tissue in the stem is the procambial cells which appear as a more or less continuous hollow cylinder within the ground tissue, separating one part of the ground tissue from another. The outer ring of ground tissue is the cortex, and the inner region is the pith. The outer procambial cells will differentiate into primary phloem and the inner ones into primary xylem.