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The differentiation of cambial cells into vessels or tracheids with lignified walls is triggered by auxins. Auxins will induce xylogenesis in a number of plant tissues and, as in all other hormone mediated processes, there may also be a requirement for gibberellins and/or cytokinins (Sugiyama and Konamine, Cell Differentiation Dev. 31, 77, 1990).

Explants of Jerusalem artichoke (Helianthus tuberosus L.) tuber have been shown to develop wound vessel members (WVMs) on a nutrient agar containing an auxin. The addition of cytokinins and gibberellins alters the arrangement of the tracheids (Dalessandro and Roberts, Am J Bot. 58, 378, 1971; Dalessandro, P Cell Physiol. 14, 1167, 1973).

In this practical, you will produce sterile explants and grow these on different hormone-containing media. A week later, you will stain differentiated xylem elements, record patterns within the explants, and quantify the extent of differentiation under the different treatments.
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Review: The control of vascular development
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Review: Xylogenesis - initiation, progression and cell death
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The contribution of auxin and cytokinin to symmetry breaking in plant morphogenesis.