(b) : In monocot root, secondary growth is absent and vascular cylinder is in the form of several alternate and radial xylem and phloem bundles. The vascular bundles are arranged in the form of ring around central pith. Their number in maize ranges between $20 -30$ whereas in Pandanus and palms, theymay be upto $100$. Because of the presence of numerous xylem bundles and exarch condition, xylem ofmonocot root is polyarch. On the other hand, in dicot root, xylem and phloem are equal in number $(2 -6)$ and alternately arranged $i.e.$, they lie on different radii hence called radial bundles. According to number of ray (equivalent to number of xylem or phloem bundles) the roots may be diarch, triarch, tetrarch, pentarch or hexarch.