|This micrograph shows the
beginning of secondary growth within the Bougainvillea stem. Note the cambial
zone (CZ) immediately beneath the starch sheath (SS), and the central part of the stem
which is occupied by primary bundles, within which secondary growth is occurring.
The internal vascular bundles are associated with conjunctive tissue. The primary bundle (encircled) has been formed by the vascular cambium.
Cells to the outside (exarch of) the phloem in this primary bundle are termed conjunctive tissue -- they remain meristematic and the cambial cells so formed, produce new secondary growth.