Development of the vascular system in the inflorescence stem of Arabidopsis

The development of the vasculature in the inflorescence stem and pedicel of two common Arabidopsis thaliana ecotypes is reported, describing the tissu es and events in primary and secondary growth of the stem vasculature. Light and transmission electron microscopy was used to investigate the vasc ular system during plant growth; cellulose, lignin and callose were detecte d by cytochemistry. The innermost cortical layer differentiated as a starch sheath during prima ry growth. The outermost parenchymatous layers of the interfascicular regio n lignified the walls at the onset of fascicular cambium activity. The seco ndary structure formed during silique production. In the internodes, the in terfascicular cambium was mainly produced by starch sheath cells. In the in terfascicular arcs of the internode, the secondary xylem mainly consists of xylem parenchyma whereas in the bundle region, it comprises vessels. Secon dary phloem and xylary fibres were produced in limited quantities. Medullar y sheath and phloem cap cells also became lignified. The results show that the secondary vasculature develops in the stem and pe dicel, and that, simultaneously, some primary tissues lignify, enhancing st em rigidity. Secondary vasculature is compared with the vascular alteration s reported for some mutants.