Abstract: Studies on the ontogeny of the primary vascular system of the seedling of Thalictrum petaloideum L. showed that a cotyledon node zone (CNZ) and two transition regions with CNZ being the centre existed in Th. petaloideum. The cotyledon node-root transition region, namely hypocotyl, was from CNZ downward and the cotyledon node-stem transition region was the meristematic tissue in the middle and upper parts of CNZ. The results were the same with what were observed by Gu Angen et la. (Studies on Cotyledon Node Zone).^[7] A mesarch and diarch haplostele with metaxylem in double hook form was observed in the lower part of CNZ of Th. petalcidrum.