The stem is a complex structure to which  roots, leaves, flowers and fruits are attached.  All stems require some mechanical support, which varies from plant to plant. Structural modifications may also vary depending on the environment that the plant is exposed to. Many  stems are divided into cortex and stele, with the division occurring just outside the vascular bundles. Staining a fresh section in iodine for example, may reveal a high proportion of starch in this boundary layer, which is appropriately, called the starch sheath.

However, the young Zea mays stem shown to the left  contains very little mechanical tissue, suggesting other means -- possibly hydrostatic pressure -- as a functional support system during the early stages of stem development. The stem contains a number of  vascular bundles -- an outer bundle arranged in one ring, and within this, a large number of bundles which spiral inwards. There is no stele. A  narrow ring of differentiating fibers occur below the thin-walled epidermal cells. Some of these cortical cells are green, indicating that they are capable of supporting photosynthesis. The stem is not divided into cortex and stele as would be the case in dicotyledons or gymnosperms, instead, the vascular bundles are supported by thin-walled parenchymatous cells.  This arrangement of vascular bundles is typical of an atactostele.