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Vascular Tissue: Xylem and Phloem

Together, xylem and phloem tissues form the vascular system of plants. Xylem and phloem: Xylem and phloem tissue make up the transport cells of stems.  They transport water and other substances throughout the plant.

Xylem transports and stores water and water-soluble nutrients in vascular plants.

Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.

Vascular plants are able to grow higher than other plants due to the rigidity of xylem cells, which support the plant.

xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood

phloem: a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot

tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts

Xylem tissue 

Xylem has the dual function of supporting the plant and transporting water and dissolved mineral salts from the roots to the stems and leaves. It is made up of vessels, tracheids, fibres and parenchyma cells. The vessels and tracheids are non-living at maturity and are hollow to allow the transport of water. Both vessels and tracheids have lignin in their secondary walls, which provides additional strength and support.

Xylem vessels are composed of a long chain of straight, elongated, tough, dead cells known as vessel elements. The vessel elements are long and hollow (lack protoplasm) and they make a long tube because the cells are arranged end to end, and the point of contact between two cells is dissolved away. The role of xylem vessels is to transport water from roots to leaves. Xylem vessels often have patterns of thickening in their secondary walls. Secondary wall thickening can be in the form of spirals, rings or pits.

Tracheids have thick secondary cell walls and are tapered at the ends. The thick walls of the tracheids provide support and tracheids do not have end openings like the vessels. The tracheids’ ends overlap with one another, with pairs of pits present which allow water to

The Phloem tissue 

Phloem tissue is the living tissue responsible for transporting organic nutrients produced during photosynthesis (mainly as the carbohydrate sucrose) to all parts of the plant where these are required. The phloem tissue is made up of the following major types of cells:

• Sieve elements: these are conducting cells which transport sucrose.
• Parenchyma cells: which store food for transport in phloem.
• Companion cells: are associated with parenchyma cells and control the activities of sieve tube elements, since the latter have no nuclei. Companion cells are responsible for providing energy to the sieve elements to allow for the transport of sucrose. Companion cells play an important role in loading sieve tubes with sucrose produced during photosynthesis. Companion cells and sieve tube elements are connected via connecting strands of cytoplasm called plasmodesmata.
• Fibres: unspecialised cells and supportive cells.

In the table below, the key structural features of the phloem are related to their function

Figure 1:  Xylem and phloem are the main transport vessels in plants.

The figure above shows how vascular tissues are arranged in a vascular bundle.

Dicot vascular bundles are arranged in a circle, while monocot veins are scattered

Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs.

The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity.

Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights. Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range.

By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and precious nutrients in the soil. The tracheids do not have end openings like the vessels do, but their ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass horizontally from cell to cell.

Phloem tissue is responsible for translocation, which is the transport of soluble organic substances, for example, sugar. Plants transport sucrose rather than glucose because it is less reactive and has less of an effect on the water potential.

The substances travel along sieve elements, but other types of cells are also present: the companion cells, parenchyma cells, and fibers. The end walls, unlike vessel members in xylem, do not have large openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve tubes is controlled by companion cells through plasmadesmata.