noun, plural: protophloems
The phloem is a vascular tissue that is responsible for transporting photosynthates to different parts of a vascular plant through a process called translocation. The phloem may be primary or secondary. A primary phloem is the type of phloem that is produced by the procambium during the primary growth (vertical growth) whereas a secondary phloem is one that is produced by the vascular cambium during the secondary growth (lateral growth).
The primary phloem is comprised of the protophloem and the metaphloem. The protophloem is the first phloem that develops from the procambium. The protophloem develops first but is more sensitive to stretch or mechanical stress thus often does not develop fully compared to the metaphloem that attains full development when the plant reaches maturity, or when the internodes stopped from elongating.
Although they look similar, the protophloem can be seen near the exterior whereas the metaphloem is located adjacent to the metaxylem. The protophloem can be further differentiated from the metaphloem in terms of cellular components and morphology. In protophloem, the sieve elements are relatively thin-walled, narrow, elongated, and short-lived compared with those of the metaphloem. The companion cells may or may not be present in protophloem.
Word origin: Greek prôtos (“first”) + phloem