9.3 – Reproduction in Angiospermophytes
9.3.1 – Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower
- Cover the flower structure while the flower is developing
- Sometimes modified into petals
- surround the male and female flower parts
- function is to attract animal pollinators
- Where meiosis occurs to produce haploid pollen
- a stalk that supports the anther
- male reproductive structure
- consists of the anther and filament
Pistil, or carpel
- female reproductive structure
- consists of the stigma, style and ovary
- the surface on which pollen lands
- receives pollen from the anther
- the pollen grows down to the ovary
- where meiosis occurs to produce haploid ovules
- pollen grows in a tube down through the style
- connects the stigma to the ovary
9.3.2 – Distinguish between pollination, fertilisation and seed dispersal
The transfer of pollen grains from the mature anther to the receptive stigma
The fusion of the male gamete with the female gamete to form a zygote
Seeds are moved away moved away from the vicinity of the parental plant before germination to reduce competition for limited resources. Mechanisms for this include fruits, winds, water and animals.
9.3.3 – Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed
Broad Bean – Vicia faba
- Testa – seed coat to protect the plant embryo and the cotyledon food stores. Formed from the ovule wall.
- Radicle – the embryonic root. Attached to and sandwiched between the cotyledons
- Plumule – the embryonic stem.
- Cotyledons – contain food store for the seed. Two in the seed
- Micropyle – a hole in the test from pollen tube fertilisation, through which water can enter the seed prior to germination. A hand lens is needed to see it.
- Scar – where the ovule was attached to the carpel wall, or ovary/ fruit.
9.3.4 – Explain the conditions needed for the germination of a typical seed
Many seeds have a dormant period, in which they do not germinate as soon as they are dispersed. This happens for a number of reasons, including:
- incomplete seed development – the embryo is immature, which is overcome in time
- presence of a plant growth regulator – such as abscisic acid, which inhibits development, and disappears with time
- impervious seed coat – eventually made permeable by abrasion with coarse soil or the action of microorganism
- requirement for pre-chilling – under moist condition, before the seed can germinate, sometimes below 5°C for 50 days (the equivalent of winter in temperate climates
Once dormancy is overcome, germination will occur some certain essential external conditions are met
- water – uptake occurs so that the seed is fully hydrated and the embryo is able to be physiologically active
- oxygen – must be present in high enough partial pressure to sustain aerobic respiration. Growth requires continuous supply of metabolic energy in the form of ATP that is best generated by aerobic cell respiration in all the cells
- suitable temperature – close to optimum temperature for the enzymes involved in the mobilisation of stored food reserves, translocation of organic solutes in the phloem, and the synthesis of intermediates for cell growth and development.
Seeds also may require fire, freezing, passing through the digestive system of a seed dispersing animal, washing to remove inhibitors, light or the erosion of the seed coat.
9.3.5 – Outline the metabolic processes during germination of a starchy seed
- Water absorption, causing the testa to split, combined with adequate oxygen and favourable temperature. Cotyledon cells are activated
- Gibberellic acid (GA) is formed by the embryo in the embryo’s cotyledon
- GA passes to the food stored in the cotyledons, and stimulates the production of amylase
- Starch is broken down into maltose, which is then broken down into glucose
- Glucose is released for energy to sustain respiration and growth. It may also be polymerised into cellulose for cell wall formation
9.3.6 – Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome
The blue-green pigment called phytochrome is present in very low concentrations, which is a highly reactive protein. It is a photoreceptor pigment in the leaves, able to absorb light of a particular wavelength, and changes its structure as a consequence.
Forms of Phytochrome
PR is a blue pigment that mainly absorbs red light, which has a wavelength of 660nm. PFR is a blue-green pigment that mainly absorbs far-red light, with a wavelength of 730nm. This inhibits flowering in short day plants, and promotes flowering in long day plants. PR is converted into PFR when exposed to light (or just red light). It is converted back into PR in the dark, or just far-red light
These bloom when days are longest and nights are shortest, such as midsummer. They include radishes, spinach, lettuce, barley, wheat and clover. They need sufficient exposure to light. In daylight, PR is converted into PFR, but during the short night, the PFR does not have a long time to convert back into PR. At the end of the night period, the concentration of PFR is high. The high PFR concentration is the trigger for flowering. The short night is the critical element.
These only flower if the period of darkness is longer than a certain critical length. They typically flower in the spring or autumn when day length is short. In daylight or red light, PR is converted into PFR, which only requires brief exposure. At the end of the night period the concentration of PFR is low, which is the trigger for flowering. The long night is the critical element.