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The movement in phloem is multidirectional, whereas, in xylem cells, it is unidirectional (upward). [citation needed] [15] After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks. Because of this multi-directional flow, coupled ...
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts of the plants such as stems and leaves, but it also transports nutrients .
The cells in vascular tissue are typically long and slender. Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, it is not surprising that their form should be similar to pipes. The individual cells of phloem are connected end-to-end, just as the sections of a pipe might be.
The cells of the vascular cambium (F) divide to form phloem on the outside, located beneath the bundle cap (E), and xylem (D) on the inside. Most of the vascular cambium is here in vascular bundles (ovals of phloem and xylem together) but it is starting to join these up as at point F between the bundles.
There is also a tissue between xylem and phloem, which is the cambium. The xylem typically lies towards the axis ( adaxial ) with phloem positioned away from the axis ( abaxial ). In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior.
Xylem cells are dead, hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall usually contains the polymer lignin. The phloem, on the other hand, consists of living cells called sieve-tube members. Between the sieve-tube members are sieve plates, which have pores to allow molecules to pass ...
Stained Xylem Vessel in Red/Orange (x400 magnification) In secondary xylem – the xylem that is produced as a stem thickens rather than when it first appears – a vessel element originates from the vascular cambium. A long cell, oriented along the axis of the stem, called a "fusiform initial", divides along its length forming new vessel elements.
The fibers of the xylem are always lignified, while those of the phloem are cellulosic. Reliable evidence for the fibre cells' evolutionary origin from tracheids exists. [10] During evolution the strength of the tracheid cell walls was enhanced, the ability to conduct water was lost and the size of the pits was reduced.