T.S. of phloem tissue: 

Structure of phloem: 

1. Phloem is a living tissue. It is also called as bast. 

2. It is responsible for conduction of organic food material from source (generally leaf) to a sink (other plant parts). 

3. On the basis of origin, it can be protophloem (first formed) and metaphloem (latterly formed).

4. It is composed of sieve elements (sieve cells and sieve tubes), companion cells, phloem parenchyma and phloem fibres. 

2. Sieve elements:

a. Sieve tubes are long tubular conducting channel of phloem. 

b. These are placed end to end with bulging at end walls. 

c. The sieve tube has sieve plate formed by septa with small pores. 

d. The sieve plates connect protoplast of adjacent sieve tube cells. 

e. The sieve tube cell is a living cell with a thin layer of cytoplasm, but loses its nucleus at maturity. 

f. The sieve tube cell is connected to companion cell through phloem parenchyma by plasmodesmata. 

g. Sieve cells are found in lower plants like pteridophytes and gymnosperms and sieve tubes are found in angiosperms.

h. The cells are narrow, elongated with tapering ends and sieve area located laterally. 

3. Companion cells: 

a. These are narrow elongated and living. 

b. Companion cells are laterally associated with sieve tube elements. 

c. Companion cells have dense cytoplasm and prominent nucleus. 

d. Nucleus of companion cell regulates functions of sieve tube cells through simple pits. 

e. From origin point of view, sieve tube cells and companion cell are derived from same cell. Death of the one result in death of the other type. 

4. Phloem parenchyma: 

a. Cells of phloem parenchyma are living, elongated found associated with sieve tube and companion cells. 

b. Their chief function is to store food, latex, resins, mucilage, etc.

c. The cells carry out lateral conduction of food material. 

d. These cells are absent in most of the monocots. 

5. Phloem fibres (Bast fibres): 

a. Phloem fibres are the only dead tissue among this unit. 

b. They are sclerenchymatous. 

c. They are generally absent in primary phloem, but present in secondary phloem. 

d. These cells have with lignified walls and provide mechanical support. 

e. They are used in making ropes and rough clothes.