Detail, Bidens pilosa stem - development associated with the vascular cambium.  

This cross-section of a  Bidens stem, shows a detail of the vascular tissues in the stem. The cambium is active and is producing secondary vascular tissues.  The  phloem contains sieve tubes (ST) and companion cells. These are primary (1P) and secondary (2P) beneath a thin layer of perivascular primary phloem fibres (PV). The cortex is separated from the stele, by a starch sheath (SS)

The cambial zone contains a few differentiating  secondary xylem e(2P) elements.

The image was taken using a microscope with  polararized light.