Question

Two cross-sections of a stem and root appear simple, when viewed by naked eye. But under microscope, they can be differentiated by
A. Exarch condition of root and stem
B. Endarch condition of stem and root
C. Endarch condition of root and exarch condition of stem
D. Endarch conditions of stem and exarch condition of root

Answer 1

Hint: Endarch and exarch are two forms of primary xylem growth in vascular land plants. The cross-section can be distinguished by their arrangement of the primary xylem.

Complete Answer:
- Complex tissues are composed of more than one type of cells which function together as a unit. Xylem and phloem are complex tissues of plants. Xylem acts as a conductive tissue for water and minerals from roots to stems and leaves.
- There are two types of primary xylem. The first formed primary xylem elements are known as protoxylem, while the primary xylem that forms later is called metaxylem. In stems, the protoxylem is present towards the middle (pit) and the metaxylem is towards the edges. This form of primary xylem is called endarch.
- Usually, the protoxylem matures first and the metaxylem grows later. The cells of the protoxylem are smaller than the cells of the metaxylem. As the protoxylem matures first, endarch refers to the development from the middle. Thus, protoxylem occurs in the middle, while metaxylem, which matures later, occurs in the edge.
- In roots the protoxylem is found towards the outside and the metaxylem lies towards the middle. This structure of the primary xylem is called exarch. Thus the cross-sections of the stem and the root look identical as seen by the naked eye, but under a microscope, they can be identified by the endarch condition of the stem and the exarch condition of the root.

Thus the correct answer is option D i.e., Endarch conditions of stem and exarch condition root.

Note: Endarch arrangement results in the centrifugal pattern of development while the exarch arrangement results in the centripetal pattern of development.