Question

Name two types of sieve elements found in phloem.

Answer 1

The sieve elements have two types namely sieve tubes and sieve cells. They are the major conducting cells in the phloem. The sieve element originates from the meristem or the meristematic tissue.

Complete answer: The sieve elements are the specialized cells that perform an important role in the functioning of the phloem. They are the major conducting cells in the phloem. The major function of these cells is to act as a channel to transport sugars over long distances through plants.
The sieve element originates from the meristem, or the meristematic tissue to be precise. The sieve cells are elongated cells that are parallel to the organ or tissue that they are located in. Usually, they lack a nucleus and contain none to a very small number of ribosomes. The two types of sieve elements are as follows –
1. Sieve tubes
2. Sieve cells
The sieve tube is a tube-like structure that is made up of living cells arranged in a row. Each sieve tube has two sieve plates on both sides. Just beside the sieve tube, a companion cell is present, that is permanently attached to it. The cells of the sieve tube are shorter and wider with a greater area for the transportation of nutrients. The sieve cells are the long, and narrow conducting cells in the phloem. Due to this, they have a smaller area for the transportation of the nutrients. The sieve cells differ from the sieve tubes in the respect that the former does not possess any sieve plates.
Additional Information: The phloem is a type of complex permanent tissue. This means that it is made up of different kinds of cells. It is formed by the differentiation and the redifferentiation of cells. The phloem is responsible for the conduction of food in plants. Food is transported from the source to sink through phloem tissue in the form of sugar or sucrose.
There are two kinds of phloem based on the stage of formation. These are –
1. Primary phloem
2. Secondary phloem
The components of the phloem tissue are. The phloem tissue is made up of four different kinds of cells actually. These cells are –
1. Sieve tubes
2. Companion cells
3. Phloem parenchyma
4. Phloem fibers of Bast fibers
The sieve tubes, companion cells, and the phloem parenchyma are the living parts of the phloem. Whereas the phloem fibers are dead as they are made of sclerenchyma tissue, and sclerenchyma is the dead tissue.

Note: The sieve elements were first discovered by the botanist Theodor Hartig. The sieve tubes are present in Angiosperms or flowering vascular plants. In non-flowering vascular plants like pteridophytes and gymnosperms, there are sieve cells in place of the sieve tubes. Also, sometimes in gymnosperms, in place of sieve cells, there is a special cell present called the albuminous cell.