Discussion
Which one of the following plant organs typically exhibits all the four features listed below?
(i) Starch-enriched endodermis
(ii) Conjoint, collateral and open vascular bundle
(iii) Endarch xylem
(iv) Presence of medullary rays
Answer: D
All four features occur together in the dicot stem.
In a young dicot stem the endodermis is rich in starch grains and is called the starch sheath (i). The vascular bundles are conjoint (xylem and phloem on the same radius), collateral (phloem outside xylem) and open, with cambium between them (ii). The protoxylem faces the centre, so the xylem is endarch (iii). Between the bundles run parenchymatous medullary rays (iv).
Roots, dicot or monocot, have radial bundles with exarch xylem. A monocot root has no cambium. A dicot leaf has collateral veins without cambial activity and no medullary rays, since it has no pith around which the bundles form a ring.
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