What is Phloem?

Image Credit: 
Main Image: 
Image of Phloem-sieve plates

In plants, Phloem is the living tissue that carries organic nutrients, known as Photosynthate, in particular, sucrose, a sugar to all parts of the plants where needed. In trees, the phloem is the innermost layer of the bark, hence the name derived from the Greek word, ‘Phloos’ means ‘bark’. The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation. Phloem is a type of tissue found in plants. Phloem and xylem make up the big transportation system of the vascular plants. Both these tissues are found throughout the plants each carries different substances to and from different parts of the plant. Their function is very similar to the arteries and veins in an animal body, which transmit blood to all parts of the body.  Phloem is used to transport dissolved substances, sap around the plant, while xylem transports water. Phloem cells are long, living cells with sieve plates at either end. 


What is the origin of Phloem?

The Phloem originates, and grows outwards from, meristematic cells in the vascular cambium. A meristem is the tissue in most plants consisting of undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place. Vascular cambium is the source of both the secondary xylem (inwards, towards the pith) and the secondary phloem (outwards), and is located between these tissues in the stem and root. Phloem is produced in phases. Primary phloem is laid down by the apical meristem and develops from the procambium, which produces the vascular cambium. Secondary phloem is laid down by the vascular cambium to the inside of the established layer(s) of the phloem.


What is the Structure of Phloem?

Phloem tissue structure consists of sieve-tube cells and companion You do not have access to view this node.

  • Sieve-tube cells: The sieve tube cells lacks a nucleus, have a very few vacuoles, but contain other ribosome’s. The sieve tube is an elongated rank of individual cells, called sieve-tube members, arranged end to end. Theses members are joined end to end to form a tube that conducts food materials throughout the plant. The end walls of these cells have many tiny pores or holes and are called sieve plates which allow the dissolved nutrients to move easily through the sieve-tube.
  • Companion cells: The survival of sieve-tube members depends on a close association with the companion cells. All of the cellular functions of a sieve-tube element are carried out by the companion cells, a typical plant cell. Companion cell is more metabolically active than a typical plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.


What is the Function of Phloem?

Phloem often rich in sap- the sticky fluid full of dissolved sugars which it is carrying from leaves to the other parts of the cells. In monocotyledons (a plant that has only one seed leaf), the vascular tissue is scattered but in dicotyledons  (a plant with 2 seed leaves)  it forms rings, central to which is a meristematic tissue called cambium, which divides to create xylem on the inside and phloem on the outside. The phloem, which is towards the outer layer of a stem, is used to their advantage, by insects like aphids which stick their probing mouth parts (stylets) deep into the plant tissue and tap straight into this sugar rich phloem. Phloem is all the feeding channel of the plant. It brings food in the form of carbohydrates which are the building blocks for all the substances used in a plant’s life, to all parts of the plant from where they were made-the leaves (or stem in some plants without true leaves). Phloem is vital for the plant’s survival and together with the sieve cells and companion cells work to make this component of the plant’s vascular system exceptionally efficient.


External References
Related Videos: 
See video