The transport of sap

Differentiate between raw sap (xylem sap) and elaborated sap (phloem sap): what each transports, which vessel it travels through, and in which direction it moves.

Indicate which cellular elements form the xylem (tracheids and vessel elements) and the phloem (sieve tubes and companion cells), and state a structural difference between them related to their function.

Besides sugars, the phloem transports amino acids, phytohormones and alarm signals. Explain what it means that elaborated sap travels "from source to sink" and name two different sinks depending on the time of year (roots, fruits, new buds).

The diagram shows the two vascular tissues in the stem of a vascular plant. a) Indicate which number corresponds to xylem and which to phloem, justifying your answer by the direction of the arrows. b) Name the sap transported by each vessel and its composition. c) Explain which force drives the ascent through vessel 1.

Explain the cohesion-tension theory of xylem sap ascent: what role leaf transpiration, cohesion between water molecules, and adhesion to xylem walls play, and why water can rise tens of metres without a pump.

The cohesion-tension theory explains the ascent of water without a pump. Explain which three properties of water and of the xylem make it possible (evaporation at the stoma, cohesion between molecules, adhesion to the vessel walls) and discuss why the water column does not break in such narrow vessels.

During droughts, air bubbles can form inside the xylem (cavitation and embolism). Explain why an embolism breaks the water column and puts that vessel out of service, and name two plant strategies to avoid or limit it (narrower vessels, closing stomata, spreading risk among many vessels).

Distinguish between guttation and transpiration: when each occurs (humid nights versus dry days), through which structure the water exits in each case, and what guttation demonstrates about the pressure pushing the sap from the root.

What is root pressure? Explain in which conditions it shows itself (nights of high humidity, sap oozing when a stem is cut in spring) and why it is not enough to lift water to the crown of a tall tree.

Where does soil water enter the plant, and through which structures is most of it lost to the atmosphere? Name the process of water loss and explain its importance.

Explain what plant transpiration is and discuss what percentage of the water absorbed by the root is actually used in photosynthesis (what happens to the remaining $98\ \%$ and what is the point of losing it?).

List the journey of water from the soil to the atmosphere (absorbent hair → root cortex → xylem → leaf mesophyll → stoma), indicating which tissue it crosses at each step.

A dry soil "grips" water more strongly than a humid one. Explain what the tension of water in the soil is (water potential, qualitatively), what the permanent wilting point is, and why watering often with little water is a worse strategy than watering deeply.

Describe the pressure flow mechanism (Münch hypothesis) of phloem sap transport, explaining what happens at the 'source' (leaf) and at the 'sink' (root, fruit) and why this transport requires energy.

Fruit growers sometimes ring a branch (they remove a ring of bark) so that its fruits grow fatter. Explain which sap accumulates above the ring, why it improves the fruit, and what risk applying the technique to the whole trunk has (what dies first?).

Aphids drive their stylet into the phloem and the sap flows in by itself under pressure. Explain what this demonstrates about the pressure inside the phloem, what the honeydew aphids excrete is, and how ants exploit it (mutualism).

In horticulture, girdling (removal of a ring of bark) is sometimes practiced on a specific branch to obtain larger fruits, but if it is performed at the base of the main trunk, the tree eventually dies.

1. a) Explain, according to the pressure-flow model (source-sink), why the fruits on the girdled branch increase in size and sugar content.
2. b) A worker states: “The tree girdled at the trunk should not die, since the xylem remains intact and the leaves can still receive water and continue photosynthesizing normally.” Refute this argument by predicting the temporal sequence of physiological failures in roots and canopy that lead to the tree's death.