Internal Structure of Flowering Plants
Leaf Structure Textbook Diagram: transverse section of leaf.
Dermal Tissue: outer single cell layer protective tissue.
Cuticle: layer of waterproof wax on the outer surface of the dermal tissue.
Ground Tissue: usually two layers, closely packed upper layer and loose lower layer – photosynthetic tissue.
Air Spaces: rapid diffusion of carbon dioxide to the cells for photosynthesis.
Guard Cells: control the closing and opening of the stomatal pore.
Stomata: rapid entry of carbon dioxide into the leaf from the air.
Stem Structure Textbook Diagram: transverse and longitudinal sections of young dicot stem showing tissue distribution.
Dermal Tissue: outer single layer of protective tissue.
Vascular Tissue: in vascular bundles.
Vascular bundles in a ring
xylem tissue – inner layer of vascular bundle
phloem tissue – outer layer of vascular bundle
special meristematic separates the xylem and phloem
Ground Tissue: in centre of stem, between the vascular bundles and between dermal tissue and vascular bundles.
Xylem Structure and Function Textbook Diagrams: tracheid and vessel member.
Tracheids and vessel members specialise in efficient water transport.
Long, narrow, dead cells with walls thickened and strengthened with lignin.
Tracheids have intact end walls and are tapered at their ends.
Vessel members do not have end walls.
A series of vessel members forms a long continuous open tube called a xylem vessel.
Pits in the thickened walls allow easy water transfer to neighbouring cells.
Tracheids and vessel members also give great mechanical support to the plant.
Phloem Structure and Function Textbook Diagrams: sieve element and companion cell.
Sieve Elements
Specialises in efficient transport of food.
Living cells but do not have a nucleus.
Long, narrow, thin walled living cells.
End walls are heavily perforated – called a sieve plate.
A series of sieve elements is called a sieve tube.
Companion Cells
Assist the sieve element in food transport.
Live narrow cells with a prominent nucleus.
Its nucleus also controls the sieve element.
Dense cytoplasm particularly rich in mitochondria.
Dicot Root Textbook Diagram: transverse section of young dicot root to show tissue distribution.
Textbook Diagram: longitudinal section of young dicot root to show tissue distribution.
Root Cap: protects the apical meristem.
Apical Meristem: formation of new cells by mitosis for root growth.
Elongation Zone: expansion of cells by osmotic intake of water.
Differentiation Zone: formation of specialised cells for particular functions – dermal, ground, xylem and phloem tissue.
Root Hairs: increase absorption of water and mineral nutrients.
Meristems
A meristem is a group of plant cells with the ability of dividing indefinitely by mitosis.
The major function of a meristem is to provide new cells for plant growth.
The new cells will expand to mature size.
Then will differentiate into dermal, ground or vascular tissue.
Apical meristems are at the growing tips of stems and roots.
Their function is produce new cells increasing the length of the stems and roots.
Gas Exchange in Plants
Leaf Adaptation
Stomata: free unhindered diffusion of CO2 into leaf and O2 out of leaf.
High Stomata Density: the more stomata the greater the amount of gas exchanged.
Thin: the shorter the distance the faster the rate of diffusion of CO2 and O2.
Great Surface Area: the greater the surface area the greater the gas exchanged.
Flat: maintains the highest possible concentration difference for fastest diffusion.
Internal Air Spaces: diffusion of CO2 and O2 is much faster in air than in water.
Moist Internal Surface: required for the absorption and release of gas from the leaf cells.
Textbook Diagram: external view of a leaf.
Textbook Diagram: transverse section of a leaf to show internal anatomy.
A high rate of photosynthesis can only be maintained if atmospheric carbon dioxide can pass swiftly to the photosynthetic cells and oxygen gas escape from the leaf.
Control of Gas Exchange
Textbook Diagram: closed and open stomata.
The stomata are by far the most influential structures in gas exchange.
Gas exchange is essential in light when photosynthesis is taking place.
The guard cells respond to carbon dioxide concentration in the leaf.
At low CO2, during photosynthesis, the guard cells are turgid and the stomatal pore is open.
If photosynthesis stops the CO2 level rises in the leaf and the guard cells become flaccid.
Flaccid guard cells cause the stomatal pore to close and so gas exchange stops.
Generally: stomata open in light and closed in the dark.
Closed in the dark to reduce transpiration and so conserve water.
Lenticels
Textbook Diagram: structure of lenticel.
Lenticels are raised loose cork tissue in woody stems, roots and some fruits.
Function in gas exchange for aerobic respiration.
Live non-photosynthetic cells tissue cells below the dead cork layer need oxygen gas.
They also need to get rid of carbon dioxide waste gas.
Lenticels allow oxygen in and carbon dioxide out.
Flower Structure
What is a Flower?
A flower is a leafy shoot containing the sexual organs of a flowering plant.
It is adapted for sexual reproduction.
It is a modified terminal bud typically composed of four sets of modified leaves.
Parts of a Flower
Textbook Diagram: structure of insect-pollinated flower.
Textbook Diagram: structure of wind-pollinated flower.
Carpel: female part of the flower.
Stigma: collects the pollen from the pollinating agent, insect or wind, and chemically stimulates pollen germination.
Style: positions the stigma for effective pollen collection.
Ovary: site of fertilisation, protects the developing seeds, aids in seed dispersal.
Stamen: male part of flower.
Anther: pollen formation and release.
Filament: positions the anther for effective pickup of pollen by the pollinating agent.
Petal: Attracts insect pollinators by colour, food reward, and fragrance.
Sepal: Protects the flower bud and support the petals of the open flower.
The receptacle is the swollen end of the stem from which the modified leaves of the flower arise.
Note:
Monocots: flower parts in units of 3.
Dicots: flower parts in units of 4 or 5.
Adaptations to Insect and Wind Pollination
Adaptations of Insect Pollinated Flowers
Attract Pollinators: brightly coloured petals, petal shape, food reward – pollen and/or nectar, fragrance – volatile chemicals released into the air.
Pollen Collection by Insect: sticky pollen – stays in contact with anther until insect arrives.
Pollen Capture By Flower: sticky stigmas so pollen from insect will transfer to them.
Adaptations of Wind Pollinated Flowers
Pollen Collection by Wind: smooth – easily removed from anther, small pollen easily transported by the wind.
Pollen Capture by Flower: stigmas large and feathery – greater surface area to intercept pollen, stigmas outside the flower
Massive Pollen Production: compensate for the excessive losses.
Note:
An adaptation is a feature (behaviour or modified structure) which fits the organism better to carry out a particular function.
Ideally name or describe the feature and then give its advantage.
The absence of a feature is not an adaptation.
Sexual Reproduction in Flowering Plants
Textbook Diagrams:
Insect Pollinated Flower
Wind Pollinated Flower
Transverse Section of ‘Mature’ Anther
Longitudinal Section of Carpel to show ovule location
Longitudinal Section of ‘Mature’ Ovule with Embryo Sac
Pollen
Pollen is a specialised ‘spaceship’ carrying the male gametes to the carpel of a flower.
A pollen grain is not a gamete
it does not fuse with the egg cell
it produces the two male gametes
it is binucleate — a gamete has only one nucleus
it is formed by meiosis of a diploid cell — gametes in flowering plants are made by mitosis
Pollen Grain Development
Microspore mother cells in the anther undergo meiosis each producing four haploid microspores.
The microspore nucleus undergoes mitosis forming a binucleate haploid cell.
This binucleate cell develops into a pollen grain.
The mature anther opens allowing transfer by a dispersal agent.
Pollen Structure
haploid
single cell
double wall — exine and intine
binucleate — tube nucleus (n) and generative nucleus (n)
Pollination
Pollination is the transfer of ‘male gametes’ in pollen from the anther to the stigma.
Cross-pollination: pollen is transferred to the stigma of another plant. Increases genetic variation, population more resistant to environmental change.
Self-pollination: pollen transferred to the stigma of the same flower or a flower of the same plant. Guarantees reproduction if pollinating agent is absent or not efficient.
Embryo Sac
The embryo sac is the haploid female structure that contains an egg cell in the ovule of the ovary.
It is a haploid multicellular structure
two haploid polar nuclei in the centre
one haploid egg cell (female gamete) at the micropyle end
three haploid cells at the opposite end to the micropyle
Embryo Sac Development
In the ovule a diploid megaspore mother cell undergoes meiosis.
Four haploid megaspores are produced.
Three of the megaspores disintegrate.
Only one megaspore survives in each ovule.
The haploid nucleus of the surviving megaspore undergoes three mitotic divisions.
Eight haploid nuclei are now present.
Within the swollen ‘megaspore cell’ six haploid cells and a two ‘polar nuclei’ are formed.
The entire structure is called the embryo sac.
One of the cells near to the micropyle end of the ovule is the haploid female gamete (egg cell).
Fertilisation
The pollen grain ‘germinates’ forming a pollen tube.
The pollen tube grows through the stigma into the style.
The two nuclei of the pollen grain enter the pollen tube, the tube nucleus at the tip.
The growth of the pollen tube to the ovule is controlled by the haploid tube nucleus.
The haploid generative nucleus divides by mitosis forming two haploid male gamete nuclei.
The pollen tube enters the ovule by way of the micropyle.
The two male gamete nuclei are released into the embryo sac.
The tube nucleus disintegrates.
‘Double Fertilisation’
Fusion of the haploid egg cell and haploid male gamete nucleus forming a diploid zygote.
Fusion of the two haploid polar nuclei and a haploid male gamete nucleus forming a triploid endosperm nucleus.
Seed Formation
The micropyle closes.
The endosperm nucleus leads to the formation of triploid endosperm, a food tissue.
The diploid zygote, by mitosis, develops into a plant embryo.
The developing embryo draws nourishment from the endosperm.
The embryo ceases development and goes dormant.
The ovule becomes a seed: dormant plant embryo, food reserve and the protective coat called the testa.
Endospermous seed: the food reserve is endosperm outside the plant embryo, e.g., maize, wheat.
Non-endospermous seed: the food reserve is within the cotyledon(s) of the plant embryo, e.g., broad bean.
Fruit
The ovary becomes a fruit. The fruit protects the developing seeds and plays an important role in seed dispersal.
Important Textbook Diagrams:
Growth of pollen tube from stigma to embryo sac of the ovule.
Ovule with developing embryo.
Endospermous seed structure.
Non-endospermous seed structure.
Seedless Fruits
Genetically Determined: fruit is formed but no seeds are made e.g. banana.
Spray flowers with growth regulators: fruit formation without seeds e.g. seedless tomatoes.
Seeds - Dispersal and Germination
Seed Dispersal
Seed dispersal is the scattering of offspring away from each other and from the parent plant.
Advantages of Dispersal
Improved chance of success by reducing competition and overcrowding.
Enables colonisation of new suitable habitats — increased chance of species survival.
Methods of Seed Dispersal
Wind
light weight seeds, e.g., orchid
high air resistance, e.g., ‘parachute’ of dandelion, ‘wings’ of sycamore
Water
buoyant fruit, e.g., sedge
buoyant seed, e.g., water lily
Animal
passive, e.g., burdock
active — the animal seeks the fruit as a food source, e.g., tomato.
Mechanical
pea — the drying pod ‘flicks’ out the seeds.
Adaptations of Seeds as Dispersal Agents
Can survive a long period.
Large food reserve — improved chance of successful establishment on germination.
Early growth accomplished in parent plant before dispersal — improved the chance of successful seedling establishment on germination.
Seed Dormancy
Seed dormancy means that the living plant embryo will not begin active growth even if the external environmental conditions are favourable.
Advantages of Seed Dormancy
Survival of plant embryo during long adverse growing conditions of winter.
Germination is timed at the beginning of a long growing period improving seedling success.
Allows more time for more widespread dispersal.
Dormancy varies - one year’s seeds do not germinate the same year – improves species survival.
Dormancy in Agriculture and Horticulture Seed dormancy results in many seeds germinating later or not at all in the present growing season. It is advantageous to have as all the seeds of crop plants germinating at the same suitable time.
Techniques to overcome the problem of dormancy
Use growth promoter to stimulate seed germination, e.g., gibberellins.
Soften the testa of the seeds by special treatment, e.g., soak seeds in alcohol.
Store seeds in dry air.
Storage in moist cold air can break the dormancy of some seeds.
Seed Germination
Seed germination is the restart of growth by the plant embryo using the food stored in the seed.
Water, oxygen and a suitable temperature are the major factors for successful germination.
Water
Plant cells are 90% water.
Water is essential for increase in cell number during growth.
Water is needed for food reserve digestion and transport of nutrients to the growing points.
Oxygen
Oxygen is needed for efficient ATP production from the reserve food.
Fermentation is nineteen times less efficient than aerobic respiration.
Suitable Temperature
Required for optimum enzyme activity and so for optimum growth.
Some of the enzymes are involved in the digestion of the complex food reserve.
Other enzymes are involved in growth and ‘housekeeping’ activities.
Stages of Seedling Growth
Textbook Diagram: seed germination
Conditions for germination are suitable and dormancy is over.
Water and oxygen enter the seed.
Metabolism speeds up.
Enzymes digest the complex food reserve (starch, protein, lipid).
The digested food is transported to the growing areas of the embryo.
The food is used for growth.
Most of the food is used in aerobic respiration resulting in loss of dry weight.
The radicle grows out forming a rooting system.
The plumule emerges forming a shoot system.
Photosynthesis begins.
Dry weight increases and seedling is independent when photosynthesis is greater than respiration.
Mandatory Activities
To Investigate the Effect of Water, Oxygen and Temperature on Seed Germination
Textbook Diagram: set up of the apparatus
Four small clear glass jars kept in darkness.
Jar A: water-soaked seeds on soaked cotton wool, open to air at 20ºC.
Jar B: dry seeds on dry cotton wool, open to the air at 20ºC.
Jar C: 4°C – water-soaked seeds on soaked cotton wool (in a fridge) open to the air.
Jar D: water-soaked seeds at 20°C in a sealed jar that does not have oxygen in the air – oxygen removed by pyrogallol or wet iron filings on filter paper.
One week later check the results.
Results
A: water, oxygen and suitable temperature together present / germination.
B: no water / no germination.
C: low temperature / no germination.
D: no oxygen / no germination.
Conclusion: water, suitable temperature and oxygen are together required for seed germination.
To Show Digestive Activity During Seed Germination
Textbook Diagram: set up of apparatus
Soak maize seeds in water for two at 20°C – stimulates germination.
Place half of the seeds in boiling water to kill them.
Cut the seeds in half so the entire plant embryo is only in one half.
Sterilise the seed halves in disinfectant solution – no digestive activity of micro-organisms.
Rinse seed haves in sterilised water.
Sterilise forceps by flaming.
Use forceps to place seed halves with live embryos into the sterile starch agar in a petri dish.
Repeat for seed halves with dead embryo in a second sterile starch agar plate – control plate.
A third unopened starch agar plate is used as a second control.
Minimum opening of plates when placing the seed halves.
Sterilise the forceps after transferring the seed halves.
Incubate all plates upside down for 3 days at 20°C.
Open plates and remove the seed halves.
‘Flood’ the plates with iodine solution.
Let soak for 2 minutes and pour off the iodine solution.
Results:
Unopened plate: uniformly blue-black – no starch breakdown.
Dead Embryos: uniformly blue-black – no starch breakdown.
Live Embryos: yellow-brown areas at seed sites – starch breakdown.
Conclusion: starch digestion occurs during germination.
Repeat to verify the results.
The Syllabus Also Requires You To Know: Definition and advantages of “dormancy”.
Mention of dormancy in agricultural and horticultural practices.
Germination: definition, factors necessary, role of digestion and respiration.
Stages of seedling growth.
Vegetative reproduction.
Practical Activities:
Investigate the effect of water, oxygen and temperature on germination.
Use of starch agar or skimmed milk plates to show digestive activity during germination.