| Abstract: | Seedlings of Brassica napus L. 2–11 days after germination were used. However, the most investigation was concentrated on the 6-day old seedlings. The primary root has a diarch protostele, the two groups of primary phloem alternate with the primary xylem. At higher level, the metaxylem is gradually differentiated in a lateral direction. Being coincident with this changes of the metaxylem, the groups of phloem cell are extended. The stele of the lower hypocotyl is root-like and has no pith. In the middle hypocotyl, there is a further lateral differentiation of the metaxylem. At the higher level, four metaxylem arms appear and the groups of phloem are extended circumferentially to form two crescent shaped sectors. In the upper hypocotyl below 0.2 cm of the cotyledonary node, a central pith has been formed which separates the differentiating primary xylem into two distinct units. At a slightly higher level, each primary phloem divides into two small groups, at this time, each xylem unit and the two adjacent groups of phloem constitute a cotyledonary trace. The foliar traces of the first two foliage leaves appear in the inter-cotyledonary plane between the vascular elements of the cotyledonary traces. At this level, the vascular tissue of the hypocotyl forms a siphonostele made up of two cotyledonary traces and the two foliage leaves, where the root-stem transition has nearly been completed, while the endarch condition is not attained in the hypocotyl. At incresing distances from the cotyledonary node upwards, in the cotyledonary petiole, the protoxylem occupies a more and more adaxial position and the metaxylem a more and more abaxial direction and, thus, the endarch condition is attained. The primary system of the root, hypocotyl, and cotyledons forms a complete circular system, the plumular vascular elements are directly connected by secondary elements formed by the cambium in the region of the hypocotyl. As for the results mentioned above, the authers have not detected that the primary xylem has a rotation of 180˚, as described by Van Tieghem. |
