● The `color{violet}("transpiration")` driven ascent of `color{violet}("xylem sap")` depends mainly on the following `color{violet}("physical properties of water:")`

`color{green}(star)` `color{brown}("Cohesion")` – mutual attraction between water molecules.


`color{green}(star)` `color{brown}("Adhesion")` – attraction of `color{violet}("water molecules")` to polar surfaces (such as the surface of tracheary elements).


`color{green}(star)` `color{brown}("Surface Tension")` – `color{violet}("water molecules")` are attracted to each other in the liquid phase more than to water in the `color{violet}("gas phase.")`

● These properties give water `color{brown}("high tensile strength")`, i.e., an ability to resist a pulling force, and `color{brown}("high capillarity,")` i.e., the ability to rise in thin tubes.

● In `color{violet}("plants capillarity")` is aided by the small diameter of the `color{violet}("tracheary elements")` – the `color{brown}("tracheids")` and `color{brown}("vessel elements.")`

● The process of photosynthesis requires `color{brown}("water.")`

● The system of `color{brown}("xylem vessels")` from the root to the leaf vein can supply the needed water.

● As water evaporates through the `color{violet}("stomata")`, since the thin film of water over the cells is continuous, it
results in `color{brown}("pulling of water")`, molecule by molecule, into the leaf from the xylem.

● Also, because of `color{brown}("lower concentration")` of `color{violet}("water vapour")` in the atmosphere as compared to the `color{brown}("substomatal cavity")` and `color{violet}("intercellular spaces")`, water diffuses into the surrounding air. This creates a `color{brown}("‘pull’.")`

● Measurements reveal that the forces generated by `color{violet}("transpiration")` can create pressures sufficient to lift a xylem sized column of water over `color{brown}("130 metres high.")`

● Transpiration has more than one purpose; it:

`color{green}(star)` `color{brown}("Creates transpiration pull")` for absorption and transport of plants


`color{green}(star)` `color{brown}("Supplies water")` for photosynthesis


`color{green}(star)` `color{brown}("Transports minerals")` from the soil to all parts of the plant


`color{green}(star)` `color{brown}("Cools leaf surfaces")`, sometimes 10 to 15 degrees, by `color{violet}("evaporative cooling")`

`color{green}(star)` `color{brown}("Maintains the shape")` and `color{brown}("structure")` of the plants by keeping cells turgid.

● An actively `color{violet}("photosynthesising plant")` has an insatiable need for water.

● Photosynthesis is `color{brown}("limited")` by available water which can be swiftly depleted by transpiration.

● The `color{brown}("humidity of rainforests")` is largely due to this vast cycling of `color{violet}("water from root")` to leaf to atmosphere and back to the soil.

● The evolution of the `color{brown}(C_4 "photosynthetic system")` is probably one of the strategies for maximising the availability of `color{violet}(CO_2)` while minimising water loss.

● `color{brown}(C_4 " plants")` are `color{brown}("twice")` as efficient as `color{brown}(C_3 "plants")` in terms of fixing carbon (making sugar).

● However, a `C_4` plant `color{brown}("loses only half")` as much water as a `color{brown}(C_3 "plant")` for the same amount of `color{violet}(CO_2)` fixed.