The phloem is a vascular tissue that is responsible for translocation. Translocation pertains to the process that transports the photosynthate materials to different parts of a vascular plant. The phloem may be primary or secondary depending on which type of growth it came from or on which type of cambia (i.e. whether procambium or vascular cambium) it came from.
The primary phloem is a type of phloem that forms from the procambium during the primary growth. The primary growth is responsible for the growth in length in plants. The procambium is a meristematic tissue that enables the primary growth of a plant. Some of the cells produced by the procambium may become a component of the primary phloem (others as component of primary xylem, cork cambium, or vascular cambium). The primary phloem is made up of protophloem or metaphloem.
The primary phloem is located in the primary plant body parts. It occurs towards the periphery as opposed to the secondary phloem that forms inner to the primary phloem. The primary phloem lacks a radial system, which occurs in the secondary phloem. Comparing the primary phloem with the secondary phloem in terms of cellular components, the primary phloem has fewer phloem fibers, sieve tubes, and phloem parenchyma. Sclereids are also typically absent in the primary phloem.