Transpiration

• Transpiration is when plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves. Carbon dioxide enters, while water and oxygen exit through a leaf’s stomata.
• The transpiration stream, which is the movement of water up the stem, enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is necessary for all these processes.
• The transpiration stream also supplies the plant with the required minerals, whilst enabling it to control its temperature via evaporation of water.
• Transpiration involves osmosis, where water moves from the xylem to the mesophyll cells.
• Transpiration also involves evaporation from the surface of mesophyll cells into intercellular spaces and diffusion of water vapour down a water vapour potential gradient out of the stomata
• The rate of transpiration can be investigated with the help of a potometer where water vapour lost by the leaf is replaced by water in the capillary tube
• Therefore, measuring the movement of the meniscus can be used to determine the rate of transpiration.
• Factors affecting the rate of transpiration include: number of leaves, number/size or position of stomata, presence of waxy cuticle, the amount of light present, the temperature, humidity, air movement and water availability.
• Xerophytes are plants adapted to living in dry conditions
• They are able to survive in such conditions because of various adaptations which serve to minimise the water loss
• Both densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation
• Moreover, xerophytes respond to low water availability by closing the stomata to prevent water loss
• Apart from this, they contain hairs and pits which serve as a means of trapping moist air, thus reducing the water vapour potential
• Xerophytes also roll the leaves to reduce the exposure of lower epidermis to the atmosphere, thus trapping air
• Hydrophytes are plants that actually live in water such as water lillies
• As such they need their own adaptations for this environment
• The hydrophotes have a very thin or absent waxy cuticle as they don’t need to conserve water
• Many constantly open stomata are found on the upper surfaces of leaves to maximise gas exchange
• Wide, flat leaves give a large surface area for light absorption
• Air sacs are found in some hydrophytes to enable leaves to stay afloat.
• Many large air spaces to make leaves and stems more buoyant