PLANTS UNIT 2
- Created by: niall_q
- Created on: 21-04-14 20:26
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- PLANTS
- Structural defence mechanisms
- Spines - form a barrier that prevents grazing animals from reaching delicate tissues
- Ability to tolerate grazing
- leaves must regrow rapidly so photosynthesis can start again!
- Low meristems - eg. grasses have many buds or meristems near thebase of the plant; allowing large numbers of shoots to grow out rapidly
- Deep roots - remain undamaged by grazing and allow rapid regeneration of the plant
- Rhizomes - {eg. knotweed & some grasses} grow horizontal underground stems that act as storage organs. Rhizomes can produce the shoot and root systems of new plants and allow rapid spread
- High grazing levels - some plants are unable to tolerate damage inflicted by large numbers of grazing animals = species diversity decreases.
- Grazing on species diversity
- Medium grazing levels - dominant plants eaten, allowing less well-adapted plants more access to light such as light and space. Species diversity increases
- Low grazing levels - few plants damaged by grazing, interspecific comp allows best-adapted plants to dominate & displace other species. Overall no species is low
- Within a pasture, grasses can tolerate grazing bc shoots regrow quickly from ground level. Diversity of species is maintained mainly due to - unpalatibility of some species and some species growing so close to ground they avoid being eaten.
- Grazing on species diversity
- Shade plants reach comp point at different time fro light, bc they reach comp point at lower light intensities - therefore earlier in the day than sun plants. This gives them more time to produce & store food before light intensity decreases again.
- Some woodland sgade plants produce flowers in early spring bc their rate of photosynthesis is higher when the tress above have not yet produced leaves. Si more light reaches woodland floor.
- Shade plants more efficient at using green light for photsynthesis. important bc red & blue light are absorbed by sun plants in canopy layer - leaving transmitted green light to reach lower plant layers
- Maintaining a Water Balance
- The movement of water from the soil to the leaves is called the transpiration stream.
- Rate of transpiration can be affected by: temp, wind, relative humidity and atmospheric pressure
- Because the water concentration in the soil water surrounding the root hair is greater than that inside the epidermal cell, water passes by osmosis across the selectively permeable cell membrane into the cell. This then increases the water concentration of this cell compared to the adjacent cells in the cortex. Thus a concentration gradient exists in the cells from the root cells across the cortex to the xylem vessels. Water moves by osmosis from cell to cell down this concentration gradient.
- The movement of water up the xylem is briught about transpiration pull and is reliant on two forces; adhesion - water molecules stick to xylem and cohesion - water molecules bind to each other. Transpiration pull is caused by a low water conc in leaf cells pulling water up plant stem via xylem vessels
- Water exits via stomata by evaporation
- Stomatal Mechanism
- the way a plant demonstrates a degree of osmoregulation
- Transpiration through open stomata involves a huge water loss, so it helps survival for the stomata to remain closed when photosynthesis is not taking place
- The guard cells which surround have thickened inner walls which makes them curved when they are turgid & straighter when flaccid
- turgid guard cells open stomata and flaccid cells close stomata
- Hydrophytes
- plants adapted to live in fresh water
- air spaces in stomata to give buoyancy
- reduced central xylem gives flexible leaf stalks that can adjust to changes in water levels & prevent damage by currents
- Floating leaves with stomata on top surface on top surface for gas exchange, but none on lower surfaces to prevent filling with water
- Xerophytes
- plants which are adapted to live in very dry habitats
- reduced leaf surface area of cacti spines or pine needles gives fewer stomata for water loss
- small, thick leaves reduces surface area relative to leaf volume
- rolled leaves, hairy leaves and stomata in pits all allow a build-up of humid air outside the stomata and so decrease the water vapour concentration gradient
- Deep roots and superficial roots
- thick waxy cuticles prevent water evaporating through cuticle
- reversed stomatal rhythm where stomata are open during cool night and closed during hot daytime when transpiration would be greater
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