Question

What are the constituents of phloem?

Answer 1

Vascular plants have vascular bundles which help in the transportation of various substances. There are two types of vascular bundles found in plants - Xylem and Phloem.

Complete answer:
Phloem is basically a living tissues which helps in conduction of soluble organic compounds which are produced during the process of photosynthesis downwards from leaves,
The components of phloem are: -

Sieve element :They are long tubular structures and are placed one above the other forming a sieve tube. They lack a nucleus. Sometimes they have a special type of thickening called nacreous thickening. Sieve tubes are syncytes i.e. formed by fusion of cells.
Companion cell:They are living, and are associated with sieve tubes, the common wall between companion cell and sieve tube shows the presence of five pits which are transversed by plasmodesmata. The companion cells are absent in pteridophytes and gymnosperms.
Phloem parenchyma :These cells are living and store food in the form of starch and fats. They are absent in monocots. In secondary phloem, they are oriented in vertical as well as radial direction. The cells of the radial system are called phloem ray cells. Main function of phloem parenchyma is translocation and storage of food reserves.
Phloem fibers : They have cellulose and lignified thickenings. They are non – living cells that provide mechanical support to plants. They are also known as bast fibres. They have a great commercial use as they have great tensile strength and flexibility.

Note: On contrary, the other vascular bundle, i.e. xylem helps in transport of water and minerals from the soil to the stem and leaves and also helps in providing mechanical strength. There are four types of xylem tissues.
->Tracheids
->Xylem parenchyma
->Xylem fibre
->Trachea