Abstract: Using light microscope to observe the change of the dissection structure of Rorippa islandica (Oed.)Borb in the development course of its root and finding that the root of Rorippa islandica (Oed.)Borb. has anomalous secondary structure. In the secondary structure in early period of the first year, formed supernumerary cambium in the surrounding of the vessel and from supernumerary cambium to inside and outside splitted and formed a lot of parenchyma, thus making the root increase thick fast, on the transverse section of the root, the vessel dispersed in the parenchyma. In the secondary structure in later period of the first year, using the dedifferentiation of the sells of parenchyma in the xylem to form supernumerary cambium, many dispersive supernumerary cambium snippets connected and formed cambium cylinder, cambium cylinder splitted into inside and outside and formed tertiary xylem and tertiary phloem, thus composing tertiary fibrovascular cylinder , the formative direction of the tertiary fibrovascular cylinder was from outside to inside and might form multicylinder. In the second year the parenchyma of the phloem and cortex of Rorippa islandica (Oed.)Borb. dedifferentiated and formed supernumerary cambium, and using supernumerary cambium snippets to connect and form supernumerary cambium cylinder, thus splitted and formed tertiary fibrovascular cylinder, and also formed multicylinder in the cortex, and its formative direction was form outside to inside. The anomalous secondary structure of Rorippa islandica (Oed.)Borb. in the different development period of its root corresponded to its physiological activities.

Key words: Rorippa islandica (Oed.)Borb., anomalous secondary structure, Rorippa islandica (Oed.)Borb., tertiary structure, anomalous secondary structure, tertiary structure