Question

Which one is living and non-nucleated at maturity.?
A) Sieve tubes
B) Companion cells
C) Collenchyma
D) parenchyma

Answer 1

Hint: Non-nucleated cells are those that lack a nucleus. They lose their nucleus and many other organelles while maturing but continue to function normally because this is their distinguishing feature.

Complete Answer:
Sieve elements, companion cells, phloem parenchyma, and phloem fibres are the four types of cells found in phloem. Companion cells are only found in angiosperms. Pteridophytes and gymnosperms lack companion cells. Most angiosperms lack phloem fibres in their primary phloem. However, they are usually found in the secondary phloem.
Sieve elements:
Sieve elements are the phloem conducting elements. They have primary walls that are thick. Their end walls are either transverse or oblique in orientation. The end wall has a lot of pores and looks like a sieve. As a result, it is known as a sieve plate. The sieve elements are stacked on top of each other to form vertical sieve tubes. The nucleus is absent in mature sieve tubes.
It has a cytoplasmic lining layer. This is an important characteristic of sieve elements. It contains a unique protein known as slime body. Conduction of food material occurs via cytoplasmic strands. Sieve cells and sieve tubes are the two types. Pteridophytes and gymnosperms have sieve cells, while angiosperms have sieve tubes.
Companion cells:
Companion cells are thin-walled, elongated, specialised parenchyma cells that are associated with sieve elements. The companion cells, unlike sieve elements, have cytoplasm and a prominent nucleus. They are linked to the sieve tubes via pits in the lateral walls. The companion cells are only found in angiosperms and are absent in gymnosperms and pteridophytes.They aid the sieve tubes in the transport of food materials.
Phloem parenchyma:
Phloem parenchyma refers to the parenchyma cells that are associated with the phloem. These are actual living cells. They keep starch and fats in reserve. Some plants also contain resins and tannins. They can be found in all pteridophytes, gymnosperms, and dicots. Phloem parenchyma is usually absent in monocots.
Collenchyma:
In plants, collenchyma is the support tissue for living elongated cells with irregular cell walls. Collenchyma cells have thick cellulose deposits in their cell walls, making them appear polygonal in cross section. The thickened cell walls and longitudinal interlocking of the cells contribute to the tissue's strength.

Sieve tube elements, also known as sieve tube members in plant anatomy, are highly specialised types of elongated cells found in flowering plants' phloem tissue. Sieve elements are living cells, as opposed to water-conducting xylem vessel elements, which are dead when mature. They are unique in that they do not have a nucleus when they reach maturity.

Therefore the correct answer is option (A) Sieve tubes.

Note:
Sieve cells only have sieve areas on their lateral walls and are not arranged in linear rows one above the other. They are not linked to any companion cells. Sieve tubes are arranged in linear rows one above the other, with sieve plates on their end walls. They are linked to the companion cells. A substance called callose can sometimes block the pores in the sieve plate of mature sieve elements.