noun, plural: metaphloems
Vascular plants have vascular tissues that are responsible for the conduction of food materials, water, and nutrients to different plant parts. The complex permanent tissues include the xylem and the phloem. The xylem is the vascular tissue that conducts water and minerals from the roots upwards. The phloem is the vascular tissue that plays the role of transporting food materials (called photosynthate) from photosynthetic organs to different parts of the plant. This process is called translocation. The phloem tissues may be classified as either primary or secondary. A primary phloem is one that arises from the procambium and is involved in the primary growth (i.e. growth in length) of plants whereas a secondary phloem is one that comes from the vascular cambium during the secondary growth (i.e. growth in girth). The primary phloem is found in the primary plant body parts and occurs towards the periphery. The primary phloem has fewer phloem fibers, sieve tubes, and phloem parenchyma compared with the secondary phloem. Sclereids are also typically absent in the primary phloem.
The primary phloem is comprised of the protophloem and the metaphloem. Both the protophloem and the metaphloem arise from the procambium. However, the protophloem is the first to develop. The metaphloem may be differentiated from the protophloem in terms of cellular components and morphology. For instance, the sieve elements of the metaxylem are relatively longer, conspicuous, and with a wider lumen. The companion cells are always present in metaphloem as opposed to those in protophloem that are often absent.
Word origin: Greek prôtos (“first”) + phloem