XVI International Botanical Congess

Xylem morphogenesis is induced and controlled by growth regulators. Two major hormonal trends shaped xylem evolution: (i) specialization of the signals for each vascular element, and (ii) increase in the sensitivity of the vascular meristems. Typical examples are: (i) during evolution, the original mechanism for tracheid differentiation, a combination of auxin and gibberellin, has become more specific for each of the xylem elements: auxin by itself induces vessels, whereas gibberellin, in the presence of auxin, induces fibers, and (ii) during evolution of temperate deciduous hardwood trees, an increase in cambium sensitivity occurred in limiting environments resulting in the development of ring-porous trees from diffuse-porous species. The sensitive cambium enables wide earlywood vessel differentiation by low auxin levels.