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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.
By studying the phloem of the leaves in vivo through laser microscopy and the usage of fluorescent markers (placed in both companion cells and sieve elements), the network of companion cells with the compact sieve tubes was highlighted. The markers for sieve elements and companion cells was used to study the network and organization of phloem ...
One criticism of the pressure flow mechanism is that it does not explain the phenomenon of bidirectional movement i.e. simultaneous movement of different substances in opposite directions. The phenomenon of bidirectional movement has been demonstrated by applying two different substances at the same time to the phloem of a stem at two different ...
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.
Polar auxin transport (PAT) is directional and active flow of auxin molecules through the plant tissues. The flow of auxin molecules through the neighboring cells is driven by carriers (type of membrane transport protein) in the cell-to-cell fashion (from one cell to other cell and then to the next one) and the direction of the flow is determined by the localization of the carriers on the ...
Passive phloem loading transports solutes freely through plasmodesma in the symplast of the minor veins of leaves. Active transport occurs apoplastically and does not use plasmodesmata. 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 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.
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]