Plant vascular system is the principal means to carry water and food throughout the plant body. It is composed of two types of conducting vascular tissues, xylem and phloem. Xylem carries water and minerals from the root to the shoot, whereas phloem transports photosynthates from leaves to other parts of the body. The evolution of vascular system which solved the problem of water and food transport was considered to be one of the key events for the successful emergence of vascular plants on the land from aquatic environments. Although vascular tissues in almost all vascular plants consist of xylem and phloem, diverse arrangements of vascular tissues within the bundles and of vascular bundles in the stele were evolved (1). The occurrence of diverse vascular patterns in vascular plants offers an excellent opportunity to study the evolutionary mechanisms controlling pattern formation. In this report, we present our studies on the vascular differentiation and pattern formation in the inflorescence stems of Arabidopsis thaliana.