XYLEM

• Xylem cells are larger than phloem cells.
• Within one vascular bundle, phloem cells tend to be closer to the outside of the plants in stems and roots.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.
• The apoplast pathway is a route through plant cells that allows water to flow freely.
• The symplast pathway is a route through plant cells that is packed with organelles which slow the flow of water.
• Transport of water from roots to leaves during transpiration occurs through both the apoplast and symplast pathways.
• All the vascular tissue is grouped in the centre of the roots, with xylem in a star shaped area and the phloem between the points of the star.
• The xylem vessels can be identified by their large size, thick walls and rounded shape in transverse section.
• Xylem walls may be stained red in microscope images because they are lignified.
• Other cells in the root are unlignified and are usually stained blue.
• Phloem cells are smaller than xylem with thinner wall.
• The outer layer of cells in the root is epidermis with small cells that may have root hairs protruding.
• Between the vascular tissue and the epidermis there is cortex with relatively large and thin walled cells.
• Xylem vessels are responsible for the transport of water and soluble mineral nutrients from the roots to the different parts of the plants (stems then leaves) that use water.
• A strong tension force (pulling force) is created within the xylem vessel as long as there is a continuous column of water.
• Xylem vessels are composed of thickened walls impregnated with a polymer called lignin, which strengthens the walls and prevents them from collapsing under low pressures.
• The pressure inside xylem vessels is usually much lower than atmospheric pressure but the rigid structure prevents the xylem vessels from collapsing.
• Primary wall areas in xylem include pits or pores that allow water to move laterally.
• Secondary walls in xylem are often interrupted by areas of primary wall.
• Water molecules cling together by hydrogen bonds between the molecules, a property known as cohesive forces, which helps water to be pulled through the plant.
• Transpiration pull is a continuous column of water in a xylem vessel, these tensions are transmitted from the leaf to the roots, strong enough to move water upwards against the force of gravity to the top of the tallest tree.
• Xylem vessels also allow minerals absorbed from the soil to be transported through the xylem to the leaves.
• The cohesive property of the water provides an unbroken column of water in the xylem throughout the plant, failure to have the cohesive property would stop all flow of water through the xylem vessels.
• When mature, xylem cells are non-living, so the flow of water along them must be passive.
• Water vapour is lost from air spaces through the stomata due to different concentration of water vapour between air spaces and air surrounding the leaf.
• Diffusion occurs through the cytoplasm of cells and via the cytoplasmic connections between cells - the plasmodesmata.
• Water molecules leave xylem and move through the cell wall of the spongy mesophyll cell.
• Root hairs are extensions of individual epidermal cells and are relatively short lived.
• Adhesion also helps prevent the column of water-filled xylem vessels from breaking.
• Adhesion is important when sap starts to rise in plants that were leafless through the winter.
• The root system provides a huge surface area in contact with soil, because plants have a system of branching roots that continually grow at each root tip, pushing through the soil.
• The cell walls of xylem vessels are charged, attracting water molecules.
• Contact with the soil is enhanced by the region of root hairs that occurs just behind the growing tip of each root.
• Mass flow of water occurs through the interconnecting free spaces between the cellulose fibres of the plant cell walls.
• The adhesive property of water and evaporation generate tension forces in leaf cell walls.
• The adhesive attraction of water to xylem vessel walls moves them up the stem against gravity.
• Evaporation from mesophyll cells and adhesion to xylem walls generates tension and creates an upward pulling force on the transpiration stream.
• The apoplast pathway includes the water-filled spaces of dead cells and the hollow xylem vessels.
• Water molecules adhere to hydrophilic xylem wall through hydrogen bonding which causes an upward force on transpiration stream.
• This route of water movement through the plant is a highly significant one.