Bidens stem, secondary growth

You can see the early stages of the development of the secondary plant body in the micrograph to the left. Note in particular -

The cambial zone (CZ) which forms a ring of thin-walled cells, which appear across the the middle of each vascular bundle and across the interfascicular parenchyma. The cambial zone consists of the true vascular cambium and on either side of it, the cambial derivatives, which are almost indistinguishable from it. The derivatives formed centrifugally, are destined to differentiate into secondary phloem and the derivatives formed centripetally are destined to differentiate into secondary xylem. The xylem forms a wedge-shaped mass of cells which contains larger diameter xylem vessels  and smaller diameter  xylem tracheids. The first-formed xylem, called the protoxylem occurs on the inner (centripetal) face of the mass of cells.

Note that the cells within the cambium are arranged in regular radial rows, that they are   tangentially flattened, that the tangential walls are thinner than the radial walls and that the cells contain cytoplasm and nuclei. If seen in longitudinal section, you would notice that the cambial cells are elongated, with chisel-shaped ends.

The primary phloem contains recognizable darkly-stained companion cells and the larger diameter sieve elements.

The starch sheath performs an important physiological function. It is a barrier through which materials moving inward from the cortex, to the stele and outwards from the stele to the cortex, must pass. In some instances a casparian strip will force symplasmic movement.

Click here to see what this stem looked like at the end of primary growth.