Adhesion and cohesion and xerophytic adaptations

Question 1

describe the cohesion tension theory

Answer 1

• The mass flow is helped by the polar nature of water and the hydrogen bonds (H-bonds) that form between water molecules which results in cohesion between water molecules and adhesion
• So due to the evaporation of water from the mesophyll cells in the leaves a tension is created in the xylem tissue which is transmitted all the way down the plant because of the cohesiveness of water molecules. The cohesive force results in a continuous column of water with high tensile strength (it is unlikely to break) and the adhesive force stops the water column from pulling away from the walls of the xylem vessels so water is pulled up the xylem tissue from the roots to replace what was lost in the leaves. This mechanism is called the cohesion-tension theory

Question 2

state the xerophytic adaptations

Answer 2

fleshy succulent leaves, ‘hinge cells’ shrink when flaccid, spines or needles or curled leaves, stomata closed during daylight and open during night, reduced no. of stomata, suken stomata, stomata on upper epidermis, waxy cuticles

Question 3

explain xerophytic adaptations

Answer 3

1. fleshy succulent leaves: to store water for long periods of time
2. hinge cells: causes leaves to roll trapping humidity withing folded space
3. needle leaves; reduces surface area for transpiration
4. stomata open and closing times: during the day water loss is minimised
5. reduced number of stomata: less water loss due to less pores
6. sunken stomata: water loss is minimised by trapping water close to area of water loss reducing diffusion gradient
7. stomata on upper epidermis: open into the humid spae created by hinge cells and sunken stomata
8. waxy cutic;e: water loss reduced via cuticle