Tracheids of the primary xylem, the part that which matures first, are stretched during their development and their secondary walls are usually in the form of rings or spirals. The secondary walls of secondary xylem develop after all length-wise growth has ceased and they are not stretched during their development. Their secondary walls are, therefore, more continuous.
Phloem is a complex tissue, which contains conducting, cells as well as supportive fibres and parenchyma. The principal vertical conductive elements in phloem are the sieve elements, which are arranged, in a vertical series to form structure called a sieve tube. Unlike the tracheids and vessels of the xylem, the sieve tubes retain their cytoplasm at maturity but their nuclei disintegrate. The sieve elements are elongated cells with specialized porous areas in their end walls called sieve plates. Strands of protoplasm connect the contents of one cell with those of the next through these pores.
Food material located in the cytoplasm can move from one cell to the next by means of cytoplasmic streaming.
Companion cells. Usually one or more specialized, elongated parenchymatous cells, called companion cells are closely associated with the sieve elements in most flowering plants. The companion cells are derived from the same original cell as the associated sieve element. Mature companion cells retain both their cytoplasm and their nuclei. It has been suggested that the nucleus of the companion cell controls both its own cytoplasm and the cytoplasm of the adjoining sieve element, after its nucleus has disintegrated.
Relate the structure of the xylem and phloem to their function.
Xylem: The function of the xylem is to transport water, and because water is important to the plant it needs to be in continual supply. This is helped by the way the xylem is formed. It is made from vessel elements, which in turn are a column of parenchyma cells that have lost their ends, forming a long drainpipe tube. This long continual drainpipe means that the water can flow in an uninterrupted manner.
The xylem needs to be well protected. In the roots it is concentrated to the central core, not only being protected, and the economy of space but giving the root flexibility. In the other parts of the plant the xylem has reason for its change in distribution. In the stem it is spoked around the outside. This will give the stem more support than if the xylem was just confined to the centre. In the leaf it is in the centre, which gives the leaf better support.
In the tracheids there are large boarded pits, which reduce the resistance of flow caused by the presence of these walls. The pits also allow the cross movement of water between the xylem. The walls of the xylem are impregnated with lignin. Lignin is a strong, hard substance and is impermeable to water. This was laid down when the plant was growing, and gives the xylem many added properties. The first being that it has added mechanical strength, and helps stop the structure collapsing under the large forces in transpiration, and stops it blowing apart under the large hydrostatic pressures.
Impermeability will stop any unwanted materials from entering the xylem. It also adds to the adhesive effect of the xylem. This means that the water molecules will stick to the inside of the xylem helping the water molecules to move up the inside. The lumen size of the xylem is also important, and it is by no mistake that the xylem has no living contents. The fact that there are no living organelles means a number of things. It increases the size of the lumen, allowing more water to be moved in vast quantities. As there are no organelles the flow remains uninterrupted.
Phloem: In the phloem the pores allow continuous flow. Being in the middle of the vascular bundle means that it is well protected. Being alive means that when loading and unloading the sucrose it can be done by means of active transport. If it were dead this would not be possible. There is also an increased amount of mitochondria, which produce ATP. This means that active transport can happen, and happen quicker because the ATP is situated near by.
The sieve plates are design so that they clot when the phloem is broken. This is helpful because it means that the flow of assimilates can be stopped if necessary and none will be lost. As there is no lignin the phloem it does not have a main structural role. Although the plates do provide horizontal support, forming a ladder like effect.
The companion cells are an important part of the phloem. They have all the organelles, unlike the sieve elements, which are deficient in their nuclei. This means that the companion cells act as the ‘brain’ for the sieve element. It is also the companion cell that starts the process of loading the sucrose at the source.
