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In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Ancient Greek word φλοιός (phloiós), meaning "bark". [3] [4] The term was introduced by Carl Nägeli in 1858. [5] [6] Different types of phloem can be distinguished. The early phloem formed in the growth apices is called protophloem.
In the stems of some Asterales dicots, there may be phloem located inwardly from the xylem as well. Between the xylem and phloem is a meristem called the vascular cambium. This tissue divides off cells that will become additional xylem and phloem. This growth increases the girth of the plant, rather than its length.
Botanists define vascular plants by three primary characteristics: Vascular plants have vascular tissues which distribute resources through the plant. Two kinds of vascular tissue occur in plants: xylem and phloem. Phloem and xylem are closely associated with one another and are typically located immediately adjacent to each other in the plant.
The pressure flow hypothesis, also known as the mass flow hypothesis, is the best-supported theory to explain the movement of sap through the phloem of plants. [1] [2] It was proposed in 1930 by Ernst Münch, a German plant physiologist. [3]
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem , which exists in two forms: xylem and phloem . Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues.
It serves several critical functions, including structural support, as the primary vein helps the leaf maintain its shape and structure; food and water transportation, as it contains xylem and phloem tissues that transport water, minerals, and nutrients to and from the leaf; [2] and connection to the stem, as it links the leaf to its vascular ...
The phloem is the living portion of the vascular system of a plant, and serves to move sugars and photosynthate from source cells to sink cells. Phloem tissue is made of sieve elements and companion cells, and is surrounded by parenchyma cells. The sieve element cells work as the main player in transport of phloem sap.
It is a derivative of the most rich fatty acid in the lipids of leaf membranes, alpha-linolenic acid. When plants experience mechanical wounding or herbivory, JA is synthesized de novo and induces genome-wide changes in gene expression. [5] JA travels through plants via the phloem, and accumulates in vascular tissue. [6]