Phloem is part of the vascular system in a plant. It carries food substances from the leaves, where they are manufactured, to all other parts of the plant.
Phloem cells are long, living cells with sieve plates at either end. These sieve plates allow substances in and out but will only allow sugars and other substances one way – away from the leaves. As the products of phtosynthesis travel, they are taken and stored in parts of the stem roots or anywhere where energy is required. these carbohydrats aso form the building blocks in important molecules which the plant uses for energy-using acts such as respiration, growing and making seeds and flowers.
Alongside the phloem cells are tiny cells called companion cells. It is not clearly understood what their function is but they appear to exert an influence over the phloem sieve cells and control those substances which are allowed in and out. This ensures that only substances required in a particular area of the plant at that time are allowed through and the rest can carry on in the phloem vessels to be used elsewhere. For example, if a plant is about to flower, an enornmous amount of food is going to be needed for energy to grow all the parts of the flower, for the flower to burst from the bud and other substances will be needed to create the waxy layer of the stigma, the pollen grains and all the other important parts of the flower. So the companion cell controls the substances allowed to that part of the plant through the phloem vessels and allows those products of photosyntheses which will combine with the minerals from the water dissolved in the xylem vessels to create the fatty materials and other products required to get enough energy and building materials to create that most important and precious item in a plant’s life – its flower,.
Phloem is often rich in sap – that sticky fluid full of dissolved sugars which it is carrying from the leaves to other parts of the cells. In monocotyledons, the vascular tissue is scattered but in dicotyledons it forms rings, central to which is a meristematic tissue called cambium, which divides to create xlylem on the inside and phloem on the outside. That phloem is towards the outer layer of a stem is used to their advantage by insects like aphids who stick their probing mouthparts (stylets) deep into the plant tissue and tap straight into this sugar-rich phloem.
Phloem above all is 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 with out true leaves).
Phloem is vital for a plant’s survival and together the sieve cells and companion cells work to make this component of the plant’s vascular system extremely efficient.
