Secondary growth, the radial thickening of plant organs, is a major source of plant biomass and leads to a continual production of vascular tissues. The activity of the cambial meristem produces secondary xylem towards the inside and secondary phloem towards the outside of the plant body. A good understanding of the molecular mechanisms that drive cambial activity would aid attempts to increase biomass yield and to engineer plants that can better withstand extreme and unpredictable environments. The Arabidopsis hypocotyl is an ideal model organ for the study of secondary growth. The hypocotyl has two phases of secondary growth, separated at the onset of flowering. In the first phase, phloem and xylem are produced at equal rates, with the xylem tissue containing only parenchyma cells and water-conducting vessels. The second phase is known as âxylem expansionâ due to the increased rate of xylem production relative to phloem. It is also during this second phase that xylary fibres begin to differentiate, which provide structural support for the plant following flowering. The vascular tissue produced during the second phase is remarkably similar to the wood of trees. Here, xylem expansion expression datasets are used to identify novel regulators of hypocotyl vascular development. The phytohormone abscisic acid (ABA) is found to promote the onset of xylem fibre differentiation after flowering. Mutations in ABA-biosynthesis enzymes delays fibre production without affecting the relative rate of xylem production. Secondly, CLE17 is identified as a novel regulator of hypocotyl vascular development. Plants constitutively expressing CLE17 display various vascular defects in the hypocotyl and root but not the stem, including the loss of a clear cambial ring and delayed phloem differentiation. Experiments into other candidate regulators of xylem expansion are also described, including two Rapid Alkalinization Factors (RALFs). Although these two RALFs appear not to affect hypocotyl vascular development, the largest phylogenetic analysis of the RALF family undertaken to date is presented. It is found that a number of genes described in the literature as RALFs are actually highly diverged and could instead be referred to as RALF-related genes. Together, the work included in this thesis provides a novel insight into Arabidopsis vascular development.