Sexual Reproduction In Plants Flashcards
What are the characteristics of an insect pollinated flower?
- Smell and coloured brightly to attract insects
- anther and stigma generally enclosed within petals
- pollen large and sticky, but only small quantities produced
What are the characteristics of A wind pollinated flower?
- Petals generally green and small as there is no need to attract insects
- anther exposed so that pollen can easily be blown away
- stigmas are feathery to catch pollen carried on wind
- pollen grains are small and light, occurring in very large numbers.
What are the anthers made up of ?
4 pollen sacs arranged in 2 pairs side by side
Pollen grains contain the male gamete and are formed inside the pollen sacs
How is pollen formed ?
1) Diploid mother cell (2n)
2) Meiosis occurs
3) Each haploid cell from the tetrad can undergo mitosis to form a pollen grain
4) A pollen grain with 2 haploid nuclei is formed (n)
5) The cell wall thickens, which then creates a mature pollen grain. Containing a generative nucleus, tube nucleus and cell wall
Why does the pollen grain contain a cell wall?
Prevents the pollen grain from dessication (drying out)
Describe the development of an ovule
1) The ovule containing a mass of cells, the nucleus surrounded by two protective integuments is carried on a short stalk called the funicle. One cell in nucleus enlarges into a megaspores mother cell
2) Meiosis of the mother cell produces 4 haploid megaspores
3) 3 of the haploid megaspores degenerate
4) 1 haploid megaspores develops into the embryo sac
5) 3 mitotic divisions occur and 8 haploid nucleus are formed
6) 2 nuclei move to the centre of embryo sac to form polar nuclei
7) The remaining nuclei develops cytoplasm around and become separated by cell walls
8) Three antipodal cells opposite the micropyle play no further role. 1 cell develops into female gamete
Define pollination
Transfer of pollen grains from the anther to the mature stigma of the same species
What is self-pollination?
Transfer of pollen from the anther of a flower to the mature stigma of the same flower or another flower on the same plant, leading to self fertilisation which leads to inbreeding
What does genetic variation depend on in self-pollination?
- Crossing over during prophase I of meiosis
- independent assortment during metaphase l of meiosis
- mutation
Less genetic variation
What are the advantages and disadvantages of self-pollination?
Greater chance of 2 potentially harmful recessive alleles combining
Successful, genomes are preserved which is an advantage in a stable environment but disadvantage if environment changes suddenly
What is cross-pollination?
Transfer of pollen from the anther of a flower to the mature stigma of another flower on another plant of the same species
Leads to cross-fertilisation which leads to outbreeding
What is genetic variation dependent on in cross-pollination?
- Crossing over during prophase l of meiosis
- independent assortment during metaphase I of meiosis
-Mutation - combining genotypes of gametes from 2 different individuals
- more genetic variation
Advantages and disadvantages of cross-pollination
- Reduced chance of producing harmful combinations of alleles
- advantageous for evolution, if environment changes suddenly it is likely that there are individuals within the population with a combination of alleles that allow a species to survive
What are some adaptations of flower to promote cross-pollination?
Chemical self - incompatibility
Irregular flower structure
Dichogamy
Monoecious Plants
Dioecious Plants
Explain these adaptations
1) gametes from the same parent plants are unable to fuse and form a zygote or, if the zygote forms, then it fails to develops
2) stigma and anther on different levels
3) anther and stigma mature at different times
4) separate female and male flowers on the same plant
5) have separate male and female plants
Explain the process of double fertilisation
1) pollen grain lands on stigma and absorbs water. If pollen grain & stigma are compatible pollen tube will germinate
2) Pollen tube grows through style under control of the pollen tube nucleus which codes for production of hydrolyse enzymes. Digesting a way through the style
3) During germination & growth of the pollen tube, the generative nucleus divides by mitosis to form 2 nucleus; the gametes
4) Pollen tube grows towards chemical secreted by embryo sac, growing through a gap in the integuments & passes into the embryo sac
5) Pollen tube nucleus disintegrates
6) double fertilisation occurs
Define double fertilisation
- one male gamete fuses with the female gamete to produce a diploid zygote (2n)
- second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n)
State the structure of the seed and fruit before and after fertilisation
Ovule - seed
Integument’s - testa (seed coat)
Micropyle - pore
Ovary Wall - Fruit Wall
Content of Ovary - Fruit
Attachment point of funicle to ovule- Hilum
What are some differences in structure between the broad bean and maize ?
Broad bean
- two cotyledons present (dicotyledons)
Broad been has micropyle
Maize
- one cotyledon present (monocotyledon)
- gas endosperm which stores starch which provides a food source for the growing embryo
What conditions need to be present for germination?
Water to mobilise enzymes for transport
Oxygen for aerobic respiration
Suitable temperature for enzymes
What is a dicotyledons?
A plant that produces seeds that contain two cotyledons - they have 2 primary leaves
What is the stamen ?
The male part of the plant consisting of an anther and a filament involved in the production of male gametes
Describe the role of the tapetum in pollen grain development
A specialised layer of cells in the anther
Provides nutrients to developing pollen grains
Describe the structure of the embryo sac ?
Two polar nuclei
Ovum
Two synergids
Three antipodal cells
Outer protective coat
Define micropyle
Pore in the integuments of an ovule through which the pollen tube enters the embryo sac
Remains as a pore in the testa
What happens to the ovule following double fertilisation ?
Develops into the seed
Describe the process of germination in a non endospermic seed
- Water is imbibed through the micropyle
- The cotyledons swell and the testa splits, the splitting allows entry of more water and oxygen
- Starch and protein reserves in the cotyledons are mobilised through hydrolysis
- Products of hydrolysis are used as a source of energy for respiration and the growth of plumules and radicle
Describe germination in an endospermic seed
- Following the entry of water into the seed, gibberellin is released via the embryo
- The gibberellin diffuses to the protein rich layer, the aleurone.
- Gibberellin induces the production of hydrolytic enzymes e.g amylase
- The hydrologic enzymes diffuse into the inner layer of the endosperm and catalyse the breakdown of stored nutrients e.g. starch
- Glucose and other breakdown products of the stored nutrients diffuse into the embryo where the are used for aerobic respiration and growth.
