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Xylem is the vascular tissue responsible for the transport of water and nutrients from the roots in the soil to the above-ground parts of the plant. Hormones and other small molecules also travel through the xylem as they are moved throughout the plant.
Xylem consists of three different types of cells: Xylem parenchyma cells and xylem fiber cells, which simply provide structural support for the most important xylem cells, the tracheary elements (TE). The TE are completely dead at maturity, and act like pipes to allow water and dissolved minterals to flow through them. There are two types of tracheary elements: vessel elements and tracheids. Though they have some structural differences, these two types function in the same manner. Given the TE are dead at maturity, they have a completely passive role in the transport of water through the plant. Water is moved from the roots to the rest of the plant due to two factors: Root pressure, in which osmosis moves water from the soil into the roots, and transpirational pull, where water is lost by transpiration in the leaves and the resulting surface tension pulls water up the xylem.
To serve their function without collapsing, xylem tracheary elements must be stronger than ordinary cells. Therefore, a key feature of their structure are secondary cell wall thickenings. These thickenings form in distinctive patterns, frequently rings, to provide maximum structural support.
The development of a xylem tracheary element consists of a thickening reinforcement of the cells walls, followed by a controlled death of the cell. The reinforced, empty cell wall then acts like a pipe for water to flow through.
Cytokinin acts against the other hormones described above by preventing procambial cells from differentiating into xylem. Therefore, cytokinin is key to maintaining the procambium, and allowing some cells to differentiate into phloem. Cytokinin has a mutually repressive relationship with the gene APH6: Cytokinin represses the expression of APH6, and APH6 suppress cytokinin signaling. Therefore, this gene and hormone fight it out, and which ever one there is more of, suppresses the other.