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The vascular cambium (also called main cambium, wood cambium, bifacial cambium; plural cambia) is a plant tissue located between the xylem and the phloem in the stems and roots of vascular plants.:125 It is a cylinder of unspecialized meristem cells that divide to form secondary vascular tissues. It is the source of both secondary xylem growth inwards towards the pith, and secondary phloem growth outwards to the bark. Unlike the xylem and phloem, it does not transport water, minerals or food through the plant.
Vascular cambia are found in dicots and gymnosperms but not monocots, which usually lack secondary growth. A few leaf types also have a vascular cambium. In wood, the vascular cambium is the obvious line separating the bark and wood. For successful grafting, the vascular cambia of the rootstock and scion must be aligned so they can grow together.
Structure and function
The cambium present between primary xylem and primary phloem is called intrafasicular cambium. During secondary growth, cells of meduallary rays, in a line with intrafasicular cambium, become meristematic and form interfascicular cambium. Therefore, the intrafascicular and interfascicular cambia form a ring which separates the primary xylem and primary phloem, and is known as cambium ring. The vascular cambium produces secondary xylem on the inside of the ring, and secondary phloem on the outside, pushing the primary xylem and phloem apart.
The vascular cambium usually consists of two types of cells:
- Fusiform initials (tall, axially oriented)
- Ray initials (smaller and round to angular in shape)
Maintenance of cambial meristem
The vascular cambium is maintained by a network of interacting signal feedback loops. Currently, both hormones and short peptides have been identified as information carriers in these systems. While similar regulation occurs in other meristems of plants, the cambial meristem receives signals from both the xylem and phloem sides for the meristem. Signals received from outside the meristem act to down regulate internal factors, which promotes cell proliferation, and promotes differentiation.
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