Question

Anatomy. Fully describe the entire path that water travels from the root hair to the hydathode as guttation occurs. Correctly use at least the following six anatomical terms in your description: hydathode, stele, tracheid, trichome, vascular bundle, and xylem. Note that you may need to use more terms than this to completely describe this anatomical pathway.

Answer 1

Water enters a plant through the hair on the root, and moves across the root cells into the xylem, which transports it up and around the plant. That, and solutes are moved around by the xylem and the phloem, using the root, stem and plant.

Root

The Apoplast and Symplast Pathways

Water enters the root through the root hair, and then takes one of three paths (apoplast, symplast and vacuolar) to the xylem vessel.

Soil to root

A root hair is a simple extension of the epidermis of a root cell. It reaches into the soil to absorb water by increasing the surface area and therefore the rate at which water can be absorbed. Some plants have fungi which act like fine roots, absorbing nutrients from the soil for the plant. Water moves into the root hair cells by osmosis because it is moving down a water potential gradient, since a root cell has a relatively low water potential due to its inorganic ions and organic substances. Water enters through the membrane and into the cytoplasm and vacuole.

Root hair to xylem

From the root hair cells, water again moves by osmosis down a concentration gradient toward the xylem, and can take one of three paths - apoplast, symplast, or vacuolar.

The apoplast pathway is where water takes a route going from cell wall to cell wall, not entering the cytoplasm at any point. The symplast pathway is where water moves between cytoplasm/vacuoles of adjacent cells. However, the apoplast pathway can only take water a certain way; near the xylem, the Casparian strip forms an impenetrable barrier to water in the cell walls, and water must move into the cytoplasm to continue. This gives the plant control over the ions that enter its xylem vessels, since water must cross a plasma membrane to get there. The vacoular pathway moves molecules through the vacuoles only of the plant.

Xylem

The xylem is constructed of three main elements:

Vessel elements, including tracheids - cells involved in water transport

Fibres - elongated cells with lignified walls that support the plant

Parenchyma cells - normal plant cells, except no chloroplasts.

Xylem vessels

These vessel elements make up the xylem - and are many elongated cells laid end to end, and normal plant cells have walls strengthened by lignin - a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody, a hard strong substance that is impermeable to water, and is designed to provide structure and strength to the plant. When these plant cells are strengthened by lignin, the cell inside dies, leaving a space inside. However, in some plasmodesmata, there was no lignin laid down and these appear as gaps in the xylem vessel, known as pits. These have permeable unthickened cellulose cell wall.

Thus, a continuous tube is formed, known as the xylem vessel. Xylem vessels are huge. They are used to transport the minerals and water and provide support to the plant.

Tracheids

Tracheids are dead cells with lignified walls, but they do not have open ends and thus do not form vessels - their ends are tapered. All plants have them, but primitive plants use them as main conducing tissue.

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells.

This decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.