Question

Which of the following is not an element of phloem tissue?
1. Tracheids
2. Sieve tubes
3. Phloem fibers
4. Companion cells

Answer 1

Hint: A complex tissue can be defined as a collection of dissimilar cells structurally, performing a common function. Xylem and phloem are the complex tissues found in all vascular plants. The main function of the phloem is the transport of food products from the place where it is available(source) to the place where it is required(sink). This is also called bast. Conducting part of the phloem is known as leptom.

Complete answer:
Phloem is a complex tissue made up of four kinds of cells:
1. Sieve elements
2. Companion cells
3. Phloem fibres (phloem sclerenchyma)
4. Phloem parenchyma
Tracheids: It is a part of xylem tissue, the cells of tracheids are long, tubular with tapering, chisel-like ends and are dead. The cell wall is lignified. The thickening in the wall of metaxylem is generally of the type scalariform, reticulated with bordered pits but tracheids in protoxylem possess annular and spiral thickening.
Sieve elements or Sieve tubes: This is part of phloem tissue, the Sieve elements in angiosperms are Sieve tubes arranged one above the other in distinct linear rows and have Sieve plates on their end walls and are associated with companion cells. In gymnosperms and pteridophytes the Sieve elements have Sieve plates on their lateral walls, the companion cells are absent and there is no distinct arrangement of Sieve cells in linear rows; such types of Sieve elements are called Sieve cells.
Phloem fibres(Phloem sclerenchyma): These are sclerenchymatous fibres but because of their presence in phloem they are called phloem fibres. They have thick walls with simple pits. They provide mechanical support. They are used for making rough cloth and ropes.
Companion cells: These long, narrow, thin walled cells are usually attached to the lateral side of the Sieve tube, this is called a companion cell. They contain cytoplasmic contents and a large nucleus. Companion cells are supposed to help in transport of food along with the Sieve tubes. They are absent in gymnosperms and pteridophytes. The companion cells and Sieve tubes maintain close cytoplasmic connections with each other through fine pits.

Hence, tracheids are not an element of phloem tissue, so the correct option is (1) Tracheids.

Note:
Phloem parenchyma is also a part of phloem tissue ,these cells are like ordinary parenchyma and are living. They are intermingled with Sieve tubes. They are absent in monocots and in some dicots( e.g. Ranunculus). They store food material and also help in transport of food. In most plants at the end of growing season or during winter the Sieve plate is surrounded by a highly refrangible substance called callus (formed by a carbohydrate called callose).