Q.47 In Angiosperms, normally ‘Exarch Xylem’ occurs in
(A) dicot stem
(B) monocot stem
(C) dicot root
(D) dicot leaf
Exarch xylem in angiosperms roots, specifically dicot roots, features protoxylem positioned towards the periphery and metaxylem towards the center. This arrangement supports efficient water absorption from soil. The correct answer to the question is (C) dicot root.
Option Analysis
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(A) Dicot stem: Vascular bundles are conjoint, collateral, and open with endarch xylem, where protoxylem faces the pith center and metaxylem the periphery.
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(B) Monocot stem: Bundles scatter with endarch xylem; protoxylem towards center, metaxylem outward.
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(C) Dicot root: Radial vascular bundles show exarch xylem; protoxylem peripheral (near pericycle), metaxylem central, aiding root water uptake.
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(D) Dicot leaf: Veins have collateral bundles with xylem adaxial (upper) and phloem abaxial; endarch condition dominates.
Exarch vs Endarch Xylem
Exarch xylem develops centripetally (from outside inward), typical in roots of both monocots and dicots. Endarch develops centrifugally (inside outward), standard in stems and leaves.
| Feature | Exarch (Roots) | Endarch (Stems/Leaves) |
|---|---|---|
| Protoxylem | Peripheral (towards cortex) | Central (towards pith) |
| Metaxylem | Central | Peripheral |
| Development | Centripetal | Centrifugal |
| Function | Water entry from soil | Sap ascent to shoots |
In angiosperms, exarch xylem in angiosperms defines root anatomy where protoxylem faces outward for optimal soil water absorption. This contrasts with endarch patterns in stems, crucial for CSIR NET preparation.
Vascular Bundle Types
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Radial in roots (xylem/phloem alternate radii).
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Conjoint collateral in stems/leaves (xylem-phloem side-by-side).
Dicot roots feature 2-6 (di-to-hexarch) exarch bundles; monocot roots polyarch.
Why Roots Have Exarch Xylem
Protoxylem’s peripheral position prevents collapse during soil penetration, metaxylem enlarges centrally. Stems use endarch for upward transport efficiency.
Exam Insights
For competitive exams, recall: roots = exarch (both dicot/monocot), stems/leaves = endarch. Practice TS diagrams to visualize.
