Dicot Stems

In dicot stems, vascular bundles are arranged in a circle near the outside.  The phloem are found on the outside of the vascular bundle and the xylem are on the inside.  There is a pith located in the center made up of mostly parenchymal cells.  A cortex is external to the ring.  Both pith and cortex are part of the ground tissue system.  They are connected by pith rays, thin layers of ground tissues between the vascular bundles.  Many stems are strengthened by collenchyma cells, located just beneath the epidermis.

In stems and roots, most vascular plants grow in girth as well as length.  The increase in diameter is called secondary growth.  After the apical meristem produces the primary stem, two lateral meristems, vascular cambium, and cork cambium are formed.

Stems are one of the two organs that make up the shoot system.   The other is the root.  Along the stem, nodes, the points at which leaves are attached, alternate with internodes, the stem segments between nodes.

Stolons are horizontal stems that grow along the surface of the ground.  An example of this is a strawberry plant.  Rhizomes, such as aspens, are horizontal stems that grow underground.  Some rhizomes end in large tubers where food is stored as in white potatoes.  Bulb, such as those in a tulip or onion, are vertical underground shoots with leaves modified for food storage.

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