Plant Vascular System Development
Xylem and phloem are the conducting elements of vascular plants. They function in the transport of water, nutrients, sugars, proteins and RNA throughout the plant. The xylem and phloem are generally found together in vascular bundles and can lie in various positions relative to each other. A collateral arrangment has xylem and phloem next to each other with phloem to the outside and xylem to the inside. In a bicollateral arrangement, xylem in found pressed between two areas of phloem. In an amphivasal arrangement, the phloem is surrounded by a ring of xylem. In an amphicribral arrangement, xylem is surrounded by a ring of phloem.
Collateral Bicollateral Amphivasal Amphicribral
Leaf section showing the phloem below xylem in the leaf vein
A cross-section through a stem showing collateral arrangment with
phloem lying to the outside of the vascular cambium with xylem to the inside
As a plant grows from the apical meristem at the tip of the shoot (see animation below), new, undifferentiated cells first differentiate into procambium(yellow). The procambium then divides and further differentiates into xylem and phloem.
It is important that the vascular tissue develops in the right location, and in neat columns so that the xylem and phloem cells line up and act like pipes to move water and nutrients. One theory of how the cells get properly aligned is the auxin canalization theory, illustrated below.
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