Abstract: Boron is one of the essential micro elements for plants. The growth of plant root is inhibited lacking boron. There are four different types of nucleolus in the nuclei of the meristematic cells in root tips of Cucumis sativus L. In the nucleolus, which is provided with boron, the fibriller components, fibrillar centres and nucleolar vacuoles are seen clearly, while in those of the boron deficiency ones, the fibrillar components and the'NOR'are contracted and thickened, the fibrillar centres and nucleolar vacuole can not be seen, the size of the central big vacuole becomes smaller in annular nucleolus.