Question

Phloem fibres are generally absent in
A) Protophloem
B) Metaphloem
C) Primary phloem
D) Secondary phloem

Answer 1

Hint:-The phloem is the vascular tissue that carries out the function of translocation i.e. the process of transporting photosynthesis products from the photosynthetic plant organs like leaves to various parts of the plant. The phloem is composed of the components as follows: sieve elements, companion cells, phloem sclerenchyma, and phloem parenchyma.

Complete Answer:-The phloem sclerenchyma consists of phloem fibres and the sclereids. Both the phloem fibres and the sclereids die at maturity and lose their protoplast and form secondary wall thickenings in between the primary cell wall and the plasma membrane. The secondary cell wall is then further thickened with lignin. Therefore these provide mechanical support. The phloem fibre is also called bast fibres and is made up of sclerenchyma are narrow and elongated cells. It is present in both in the primary phloem and the secondary phloem (but not present in the metaphloem).
In secondary phloem, the phloem fibres form the bast fibres which have commercial value as well. The phloem fibres are elongated, unbranched and have pointed apices. The first formed primary phloem consists of narrow sieve tubes and is called protophloem while the later formed phloem has larger sieve tubes and are called metaphloem. The primary phloem is formed in apical meristems of the roots and shoots and consists of protophloem or metaphloem. The sieve tubes of protophloem are destroyed and torn as the plant grows since it cannot stretch further.

Hence, the correct option is (C) 'Primary phloem'.

Note:- The elongated sclerenchymatous cells in the phloem, and is responsible for providing tensile strength without limiting flexibility Bast fibre is plant fibre collected from the phloem or bast surrounding the stem of certain dicotyledonous plants. It supports the conductive cells of the phloem and provides strength to the stem.