Question

A mature sieve tube is different from a vessel in
A) Lacking a functional nucleus
B) Absence of lignin in walls
C) Being dead
D) Lacking cytoplasm

Answer 1

Hint: Xylem and phloem elements are the system by which plants transport water and solutes as well as nutrients like carbohydrates, proteins and so on throughout the plant body. Sieve tubes are part of the phloem.

Complete Answer:
The elements of xylem consist of dead tissues. Phloem on the other hand is made up of living cells, which arise from actively dividing meristem once these elements attain maturity, they lose their nucleus. Option A is not the correct answer.
Sieve tube cells form along with companion cells from the phloem parenchyma tissue. However, while transport elements of the xylem tissue are lignified, and lignification helps in maintaining cytoplasmic connections, Sieve tubes themselves are not. The correct answer is option B.
Phloem tissues are living tissues that transport carbohydrates, sugar, proteins and so on. They are not dead, producing and using ATP for transport. This is both upwards and downwards through the entire plant. Xylem tissue on the other hand is dead. Option C is incorrect.
As living cells, responsible for the transport of solids, sieve tubes contain cytoplasm through which all the substances are transported. Option D is incorrect.

Hence the correct answer is option B.

Note: Interestingly, though the sieve tubes are living cells, they are dependent on the companion cells to provide them with all the proteins necessary for their function. This is because at maturity these cells lose their ribosomes and nucleus. However, immature sieve cells do have their nucleus. Because of this and their close association with companion cells.