Biology · Anatomy of Flowering Plants · NEET
A monocot root usually has MORE THAN SIX xylem bundles. Because the number is high, we call this condition POLYARCH. NCERT says: "there are usually more than six (polyarch) xylem bundles in the monocot root." A dicot root has fewer (2 to 4). NEET often asks you to pick the plant part from the word polyarch, and polyarch = monocot root.
Polyarch means many (poly) xylem strips (arch) arranged around the centre. A root with 2 xylem patches is diarch, 3 is triarch, 4 is tetrarch, and more than six is POLYARCH. Only the monocot root is polyarch. Dicot roots are di- to tetrarch. So if a question uses the word polyarch, the answer is monocot root.
Yes. A monocot root has a LARGE and well-developed pith in the centre. NCERT states "Pith is large and well developed." This is one of the two biggest differences from the dicot root, where the pith is very small or absent (xylem fills the centre). So remember: monocot root = big pith, dicot root = little or no pith.
It is EXARCH. Exarch means the protoxylem (first-formed xylem) is on the OUTER side, near the pericycle, and metaxylem is on the inner side. All roots (both monocot and dicot) are exarch. This is opposite to stems, which are endarch. NEET loves this: root = exarch, stem = endarch.
RADIAL. In a monocot root, xylem and phloem lie on separate radii, alternating with each other around the centre. This radial arrangement is the sign of a ROOT (both monocot and dicot). Stems have conjoint bundles (xylem and phloem on the same radius, side by side). So radial + exarch = root.
Because a monocot root has NO vascular cambium. Secondary growth needs a cambium ring to make new (secondary) xylem and phloem, and monocots lack it. NCERT says "Monocotyledonous roots do not undergo any secondary growth." So monocot roots stay thin their whole life. The dicot root DOES form a cambium later and shows secondary growth.
MONOCOT ROOT. Radial + exarch tells you it is a ROOT (not a stem). Polyarch (more than six xylem groups) tells you it is a MONOCOT root, not a dicot root (dicot roots are di- to tetrarch). Endodermis and pericycle are present in all roots. This exact combination was asked in NEET 2023, and the answer was monocot root.
The transverse section of a plant part showed polyarch, radial and exarch xylem, with endodermis and pericycle. The plant part is identified as :
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Polyarch. It has more than six xylem bundles. Diarch (two patches) is a dicot root feature. Only the monocot root is polyarch.
The monocot root pith is bigger. NCERT calls it "large and well developed." A dicot root has a small pith or almost none, because the xylem reaches the centre.
No. They have no vascular cambium, so no secondary xylem or phloem is made. NCERT clearly states monocot roots do not undergo secondary growth.
From outside in: epidermis (epiblema), cortex, endodermis, pericycle, radial vascular bundles (polyarch, exarch xylem), and a large pith. The layers are the same as a dicot root; only the xylem number and pith size differ.
Exarch. Protoxylem faces outward (toward the pericycle) and metaxylem faces inward. All roots are exarch; stems are endarch. This is a common NEET one-liner.
