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Summary BOT 2601 LEARNING UNIT 6
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 6ECOLOGICAL ANATOMY:
Textbook reference: chapter 8 pg. 295-337
HABITAT AND PLANT STRUCTURE:
o An adaptation is any aspect of a plant that promotes its welfare in its environment
o Adaptations are heritable and thus the result of evolutionary change
o Plants adapt by natural selection
o Plants that require abundant soil, water and a humid atmosphere and that live in regions of average water
availability are termed mesophytes
o Hydrophytes grow on the surface of water or are submerged. They have no need to conserve water but have
adaptions that enhance light absorption and provide more efficient gas exchange.
o Xerophytes inhabit dry regions
o The study of ecological plant anatomy has 3 approaches:
- The floristic approach, where the anatomical adaptations of a group of unrelated species in a single habitat
are investigated
- The systematic approach, where a group of related species that show adaptative radiation along a
geographical line are investigated.
Adaptative radiation is the diversification of a single species as it adapts to different ecological niches
- The experimental approach, where controlled experiments are performed on a single species
ECOLOGICAL LEAF ANATOMY:
o The variation in plant structure that are commonly affected by environmental factor are strongly expressed in
the anatomy of leaves
- Smooth leaves with entire margins and apical drip tips are more common in lowland tropical rain forests
- Leaf areoles of tropical and xeric climate plants tend to be smaller and with fewer freely ending veinlets
than those of plants from cooler areas with adequate rainfall
Leaf characteristics of plants adapted to sun and shade:
The level of illumination that a leaf receives during development is an influential environmental factor affecting
mature leaf structure
Sun:
o Within the same plant, leaves produced in bright light conditions (sun leaves) are smaller and thicker and have
increased mesophyll tissue, higher density of stomata, veins and chlorophyll than shade leaves
o Sun leaves fix carbon faster than shade leaves
o Low light can cause a reduction in vessel density
o In sun leaves, the mesophyll zone contains an increased number of compact palisade layers. The foliar cuticle is
thicker, and hairs are more abundant. The well-developed cuticle may reflect light rays and protect the
underlying tissues. Mechanical tissues are more strongly developed
o Thickened cell walls of sun leaves resist the mechanical action of wind and prevents these leaves from bending
o Sun leaves possess more stomata to produce a cooling effect
Shade:
o Leaves that develop in shade are thinner. They have a mesophyll
region composed of poorly defined palisade cells and abundant
intercellular spaces. They possess special epidermal cells on the lower
surface that are larger than neighbouring cells and contain many
chloroplasts – these cells function in the capture of light. Reduced
stomata and more chloroplasts
Plants with xeromorphic leaves and the 3 different survival strategies of
xerophytes:
o In xerophytes
o Plants that live in habitats where the water supply is deficient have been classified into broadly defined
categories on the basis of the adaptive strategies they have developed:
, - Drought-escaping species: plants with a compressed growth cycle that completes vegetative growth and
reproduction within the short period of time when conditions are favourable
- Drought-evading species: plants that are able to reduce water loss or compensate for water loss by
possessing specialised features
- Drought-enduring species: plants that are able to survive even when water uptake cannot take place or is
reduced. They possess specialisations such as temporary leaf loss, changes in leaf angle, rolling of leaf
blades, leaves reduced to spines, Bulliform cells.
Plants with ericoid leaves:
o A reduction in leaf surface area has resulted in the production of small leaves
o In some plants the reduction in leaf evolution formed needle-like projections or in the loss of leaves
o The formation of a short, linear leaf form is associated with a reduction in leaf surface.
o These leaves have stomatal grooves that extend the entire length of the lower surface. Trichomes may
completely fill the grooves or only be located on the rims.
Sclerophyllous leaves:
o In dry environments the leaves of some plants have retained a broad lamina and become leathery
o Xerophytes that have increased cutinisation and lignification of the leaves are called Sclerophyllous
o Leaves are thick, composed of highly lignified cells with thickened and strongly cutinized outer epidermal walls.
o Venation is dense and individual vascular bundles are increased in size.
o Trichome density is accentuated and sometimes ae lignified
o Layers of heavily lignified cells below the epidermis form a hypodermis
o The increase in sclerified cells is correlated with a well-developed and lignified bundle sheath around the veins
o Extensive sclerenchyma development is required for support
o Increased amounts of palisade parenchyma and a decrease in spongy mesophyll and intercellular spaces
o The mesophyll can be isobilateral – palisade mesophyll occurs on both sides of the blade
o Stomata are sunken in pits on the lower surface of the blade, with cuticular rims overarching the depression.
This protects the opening between the guard cells and the outside air
Succulent leaves:
o Fleshy succulent leaves
o Epidermal cells are thickened and cutinized
o Succulents are adapted to store lots of water
o Colourless storage cells are large and thin walled. Walls reinforced to prevent collapse when turgor is reduced
o Little or no sclerenchyma
o Another adaptation is a variant type of photosynthetic carbon dioxide fixation known as Crassulaceae acid
metabolism (CAM). Although this pathway is not efficient, it promotes photosynthesis in hot environments.
Succulents initially fix carbon dioxide into oxaloacetate. This occurs only at night. This results in the malic acid
content of photosynthesising cells increasing at night
o Stomata open at dawn and close at night
Poikilohydric plants:
o Their body structure and function vary dramatically with environmental water availability
o The cells, tissues and organs of these plants are able to remain viable after extreme dehydration and rehydration
o These plants are commonly referred to as resurrection plants
o When desiccated the leaves shrink and curl up, the epidermis wrinkles as the xylem elements contract
o Lose their leaf chlorophyll and thylakoid membranes degrade
Halophytic leaves:
o Plants that are able to tolerate saline environments
o This adaptation is a special type of xerophytism
o Succulent leaves with special colourless storage cells called window cells increase capacity for salt accumulation
o Some have secretory cells, enlarged tracheids terminating vein endings, salt glands, salt hairs and a hypodermis
o They concentrate salt in their tissues, moving it from the soil through the plant
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