Question

What is the correct sequence?
A. Xylem-Cambium-medulla
B. Cortex-Endodermis-pericycle-xylem
C. Cambium-xylem-cortex
D. None of the above

Answer 1

Hint: The outermost layer of organs is generally called the cortex unambiguously. It provides protection to the underlying structures.

Complete answer:
The correct sequence of internal tissues in plants from outwards to inwards can be written as Root hairs- the epidermis- Cortex- Endodermis- Pericycle- Protoxylem- Metaxylem- Pith.
The epidermis is the outermost layer of the plant tissues. It gives rise to thin, unicellular root hairs in the roots that protrude out into the soil to facilitate the absorption of water and minerals. Then lies the cortex. It consists of various thin-walled parenchymatous cells with intercellular spaces. The innermost layer of the cortex is known as the Endodermis. It is made up of uni-layered barrel-shaped cells with no intercellular spaces. In roots, the tangential and radial walls of endodermis are laden with an impermeable, waxy material called Suberin. Then lies the pericycle. It is a thickened parenchyma layer required for forming the lateral roots and vascular cambium. Pith consists of large rounded parenchymatous cells in the center. The vascular bundle i.e., Xylem and Phloem lie centrally.

So, the correct answer is Cortex-Endodermis-pericycle-xylem.

Note:
- Casperian strip: The hydrophobic, impermeable, suberin laden cells of endodermis of the root is known as the Casperian strip. It blocks the flow of extra soil solution through the xylem.
- Vascular cambium: They give rise to secondary xylem and secondary phloem during secondary growth i.e. the increase in girth of plant parts.
- In the dicotyledonous stem, the endodermal cells are very rich in starch grains and are therefore called the starch sheath.
- Protoxylem grows in a centripetal manner to form metaxylem.