4.2 - Sexual Reproduction in Plants Flashcards
What are flowering plants called?
Angiosperms
Are most angiosperms diploid or haploid?
Where are the male and female spores in the plant?
Most flowering plants are diploid, meiosis takes place within the reproductive tissues and produces haploid spores which contain the gametes
- male spores are the pollen grains, produced in the anther
- the female spore is the embyo sac produced in the ovule, in the ovary
Describe the structure of a angiosperm flower?
- the outermost ring of structures is the calyx, which comprises the sepals, they are usually green and protect the flower and the bud, although some flowers, eg lillies, the sepals are coloured
- inside the sepals is the corolla, a ring of petals, these range from absent to small and pale green to large and brightly coloured. There may be a nectary at the base, releasing nectar which is scented and attracts pollinators such as insects
- inside the petals are the male parts of the flower, the stamen. each stamen consists of a filament supporting an anther which produces pollen grains. The filament contains a vascular tissue, which transport sucrose, mineral ions and water to the developing pollen grains. The anther usually contains 4 pollen sacs arranged in 2 pairs, side by side. When mature, the pollen sacs dehisce, which means they open and release pollen
- in the centre of the flower are one or more carpels, which are the female parts of the flower. Each carpel is a closed structure in which one or more ovules develop, the lower part of the carpel, surrounding the ovules, is the ovary and its tip bears the style, which ends in a receptive surface, the stigma
Compare insect pollinated flower and wind pollinated flowers.
Insect pollinated;
- colourful petals, nectar guides
- scent and nectar (sucrose)
- anthers within the flower
- stigma within the flower
- small quantities of sticky, sculptured pollen
- produces larger pollen grains
Wind pollinated flowers;
- petals absent or small, green
- no scent or nectar
- anthers hang outside the flower
- large, feathery stigmas outside
- large quantities of smooth pollen
- produces smaller pollen grains
How are the male gametes produced? (long process)
- in the pollen sacs of the anther, diploid mother cells undergo meiosis - each forms a tetrad, containing 4 haploid cells, which become 4 pollen grains
- the tapetum, a layer of cells around the pollen sac, provides nutrients and regulatory molecules to the developing pollen grains, it has a significant role in the formation of the pollen cell wall, which is tough and resistant to chemicals
- the cell wall resists desication, so the pollen grains can be transferred from one flower to another without being dried out. UV light cannot penetrate the cell wall, so the DNA in pollen that is carried at high altitude is protected from mutation
- inside the pollen grain, the haploid nucleus undergoes mitosis to produce 2 nuclei, a generative nucleus and a tube nucleus, the generative nucleus produces 2 male nuclei by mitosis
- when the pollen is mature, the outer layers of the anthers dry out, causing tension in the lateral grooves, eventually dehisence occurs in which the tension pulls the walls of the anther apart and the edges of the pollen sacs curl away, an opening called the stormium exposes the pollen grains and they are carried away by insects or the wind
How is the female gamete developed?
- the ovary contains one or more ovules
- in each ovule, a megaspore mother cell undergoes meiosis making 4 haploid cells
- 3 disintergrate
- the remaining cell undergoes 3 rounds of mitosis, producing 8 haploid nuclei, one of which is the female gamete, sometimes called the egg or the oosphere
- 2 haploid nuclei fuse to make a diploid nucleus called the polar nucleus
What is pollination?
The transfer of pollen grains from the anther to the mature stigma of a plant of the same species. Pollination brings pollen grains, containing the male gametes, into contact with the female part of the flower, or another flower, which can result in fertilisation
What is self-pollination?
The pollen from the anthers of a flower is transferred to the mature stigma of the same flower or another flower on the same plant
Self pollination leads to self fertilisation, causing inbreeding
What is cross pollination?
Most angiosperms use cross pollination, in which pollen is transferred from the anthers of the flower to mature stigma of another flower on another plant of the same species
Cross-pollination leads to cross-fertilisation, causing outbreeding
How can cross pollination be ensured?
- dichogomy, ie the stamen and the stigma opening at different times, in potrandry, the stamen opens first, in protogyny the stigmas open first
- the anther is below the stigma so pollen cannot fall on it
- genetic incompatibility, pollen cannot germinate on the stigma of the flower that produced it
- separate male and female flowers on the same plant
- separate male and female plants
Describe the (long) process of double fertilisation in plants.
- when a compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma, and produces a pollen tube
- the pollen tube nucleus is at the top of the tube, with the 2 male nuclei behind
- the pollen tube grows out of the pollen grain through the gap in the cell wall, called a pit, and down the style, up a gradient of chemoattractants eg GABA from the ovule
- the pollen tube nucleus codes for the production of hydrolases, including celluloses and proteases, and it digests its way through the tissues of the style, the products of digestion are used by the growing pollen tube
- the pollen tube grows through a gap in the integuments, the micropyle and passes into the embryo sac
- the pollen tube nucleus distintergrates, presumably having completed its function of controlling the growth of the pollen tube
- the tip of the pollen tube opens, releasing the 2 male gametes into the embryo sac
- the male and female gametes are haploid. One of the male gametes fuses with the female gamete, the oosphere, to form a zygote which is diploid
- the other gamete fuses with the diploid polar nucleus to form a triploid nucleus. This triploid nucleus is the endosperm nucleus, when it subsequently divides repeatedly by mitosis, it generates the endosperm tissue, which takes over from the nucellus in providing nutrients for the developing embryo
After fertilisation, how do fruits and seeds develop?
- the diploid zygote divides by mitosis, becoming an embryo which consists of a plumule (the developing shoot), a radicle (the developing root), and one or two cotyledons (seed leaves)
- the triploid endosperm nucleus develops into a food store, providing food for the developing embryo
- the outer integuments dry out, harden and become water proof, with deposits of lignin, they become the seed coat or testa, the micropyle remains as a pore in the seed
- the ovule, comprising the embryo, endosperm and testa, becomes the seed
- the funicle or stalk of the ovule becomes the funicle of the seed, it attaches to the seed at the hilium
- the ovary becomes the fruit, in some species, such as cherry, the ovary wall becomes sweet, juicy and pigmented. In others, such as almond, the ovary wall becomes dry and hard
Compare the differences in monocots and dicots.
Monocots;
- one cotyledon in seed
- leaf veins are parallel
- sepals, petals and stamens are in multiples of 3
- vascular bundles scattered in stems and roots
Dicots;
- two cotyledons in seed
- leaf veins form a network
- sepals, petals and stamens are in multiples of 5
- vascular bundles in a ring of stems
- vascular bundles in the centre of roots
What are the different ways seeds can be dispersed?
- wind
- transport
- rolling
- bursting
- water
- carrying
How are seeds adapted for survival?
- dormant seeds have a low metabolic rate and so they survive very cold weather
- the testa is chemically resistant and so seeds survive adverse chemical conditions
- the water content of a dormant seed is reduced below 10% and so seeds can survive very dry conditions
- the testa can physically protect the embryo
- the endosperm or cotyledons provide a supply of nutrients, which lasts until the emerging seedling can photosythesise adequately
- seeds can be dispersed great distances form the parent plant and so do not compete with it
- dispersal allows colonisation of new habitats
- inhibitors may only allow germination at a suitable time of the year, they are broken down in very cold weather, in a process called vernalisation, so that the seed can germinate in the spring
