Question

Nucleus is absent in
(a) Companion cells
(b) Sieve tubes
(c) Phloem parenchyma
(d) Cambium

Answer 1

Phloem is a living complex tissue that mainly conducts food materials mostly in the form of sugars. The components of phloem are sieve elements, companion cells, and phloem fibres. The region of the cell wall with sieve pores is called the sieve area. The function of sieve tubes is controlled by the nucleus of the companion cells.

Complete answer:
-Phloem transports food materials usually from leaves to other parts of the plants. It is composed of sieve tubes. The sieve elements are of two types: sieve cells and sieve tubes.
-Sieve tubes are made up of a long tube-like structure called sieve tubes elements which are arranged longitudinally and are associated with the companion cells. The end walls of sieve tubes elements are perforated in a sieve-like manner to form a sieve plate. Mature sieve elements possess a peripheral cytoplasm and a large vacuole but lack a nucleus.
-The companion cells are specialized parenchymatous cells and are closely associated with sieve tube elements. The sieve tube elements and companion cells are connected by pit fields present in their common longitudinal walls. The companion cells help in maintaining the pressure gradient in the sieve tubes by driving the water into the sieve tubes.

So, the correct answer is ‘Sieve tubes’.

Additional information:
Phloem parenchyma is made up of elongated tapering cylindrical cells that have dense cytoplasm and a nucleus. Phloem parenchyma cell wall is composed of cellulose and has pits through which the plasmodesmata connection exists between the cells. The phloem parenchyma stores food materials and other substances like resins latex and mucilage. Phloem parenchyma is absent in most of the monocotyledons. Phloem fibres are made up of sclerenchymatous cells with a thick cell wall. Phloem fibres are generally absent in the primary phloem but are found in the secondary phloem. Companion cells are associated with the sieve tubes of angiosperms both ontogenetically and physiologically. A mature sieve tube differs from a vessel in the absence of lignified walls mature sieve tubes have the only cytoplasm.

Note:
Sieve elements are the main conducting elements of phloem with only the primary wall having minute pores called sieve pores. Sieve elements are enucleated at maturity. Sieve elements are of two types called sieve cells and sieve-tube members. Sieve cells are elongated sieve elements with tapering ends and overlap each other at the ends and cells have indistinct sieve areas. The phloem of gymnosperms has albuminous cells and sieve cells. Gymnosperms and pteridophytes lack sieve tubes and companion cells.