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Phloem (/ ˈ f l oʊ. əm /, FLOH-əm) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, [1] to the rest of the plant. This transport process is called translocation. [2]
Xylem and Phloem. A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, engages in photosynthesis, stores nutrients, and produces new living tissue. [1]
They provide structural support, particularly in growing shoots and leaves (as seen, for example, the resilient strands in stalks of celery). Collenchyma cells are usually living, and have only a thick primary cell wall [6] made up of cellulose and pectin. Cell wall thickness is strongly affected by mechanical stress upon the plant.
Sieve cells are long, conducting cells in the phloem that do not form sieve tubes. The major difference between sieve cells and sieve tube members is the lack of sieve plates in sieve cells. [1] They have a very narrow diameter and tend to be longer in length than sieve tube elements as they are generally associated with albuminous cells. [4]
The fascicular and interfascicular cambia thus join up to form a ring (in three dimensions, a tube) which separates the primary xylem and primary phloem, the cambium ring. The vascular cambium produces secondary xylem on the inside of the ring, and secondary phloem on the outside, pushing the primary xylem and phloem apart.
Stem parasitic plants, unlike most root parasites, germinate using the resources inside their endosperms and can survive for some time. For example, the dodders (Cuscuta spp.) drop their seeds to the ground. These may remain dormant for up to five years before they find a host plant. Using the resources in the seed endosperm, the
Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes—water and minerals. The phloem pressure can rise to several MPa, [12] far higher than atmospheric pressure. Selective inter ...
An intermediate type of loading exists that uses symplastic transport but utilizes a size-exclusion mechanism to ensure diffusion is a one-way process between the mesophyll and phloem cells. This process is referred to as polymer-trapping, in which simple solutes such as sucrose are synthesized into larger molecules such as stachyose or ...