Question

What is the position of the Xylem in the secondary xylem?
A) Exarch
B) Endarch
C) Mesearch
D) None of the above

Answer 1

Xylem is a complex permanent tissue because it is made up of more than one type of cell. These help to move the materials in one direction only. They also help to transport water and mineral salts from roots to other tissues of the plant.

Complete answer:
The xylem is classified as – primary xylem and secondary xylem. The primary xylem that is first formed is called the protoxylem and the later formed are called the metaxylem. These are associated with vertical growth.

Different conditions that are linked with the primary xylem are –
i) Exarch - This condition occurs in monocots and dicots roots, in this, the protoxylem lies towards the periphery and the metaxylem towards the centre.
ii) Endarch – This condition occurs in dicots and monocots shoots, in this, the protoxylem lies towards the centre and the metaxylem lies towards the periphery of the organ.
iii) Mesarch – This condition occurs in some plants like in the shoots of ferns, in this, the protoxylem is located in the centre that is surrounded by a metaxylem. Protoxylem develops first and develops centrifugally and centripetally in a ring.

The secondary xylem is those that are formed by secondary growth. These are associated with lateral growth. In this, the protoxylem and metaxylem do not show any variation.

So, the correct answer is option (D) None of the above.

Note:
- Procambium is made of primary meristem and gives the primary xylem while vascular cambium gives the secondary xylem.
- Secondary xylem is present in trees and shrubs and absent in non-woody plants. The cell wall is made up of lignin and thickened. They also provide mechanical support to the plants.