Sexual Reproduction In Plants🌹🌾 Flashcards
What type of plant is an insect-pollinated one?
Dicotyledon
What type of plant is a wind-pollinated one?
Monocotyledon
What is the carpel?
- Stigma
- Style
- Ovary
What is the stigma?
- Sticky surface
* Pollen grains adhere
What is the style?
Joins the ovaries to the stigma
What is the ovary?
Forms fruit after fertilisation
What is the ovule?
Contains the egg cell which develops into a seed after fertilisation
What is a sepal?
- Enclose flower bud and protect it before it blossoms
* Green sepals can photosynthesise
What are calyx?
Collective name for sepals
What are the petals?
- Brightly coloured and scented
* Attract pollinating insects
What are corolla?
Collective name for petals
What is the stamen?
- Anther
* Filament
What is the anther?
Four sacs containing pollen grains in which the male gametes develop
What is the filament?
Supports the anther
What is the receptacle?
- Structure at the top of the flower stalk
* Supports the flower
Comparison of flowers IP/WP
- IP - conspicuous flowers with large brightly coloured petals
- WP - inconspicuous flowers with small drab petals/no petals
Comparison of scent IP/WP
- IP - scented
* WP - not scented
Comparison of nectaries IP/WP
- IP - nectaries present
* WP - nectaries absent
Comparison of pollen IP/WP
- IP - low pollen production, large and spiny (picked up by insects)
- WP - high pollen production, small, lightweight and smooth - able to be blown by wind
Comparison of stamens IP/WP
- IP - enclosed in flower - insect brushes past
* WP - pendulous and dangle outside the flower - increased chance of wind blown good
Comparison of stigmas IP/WP
- IP - relatively small and sticky
* WP - feathery and hang outside - large S.A for wind
Comparison of plant grouping IP/WP
- IP - often single or in small groups
* WP - often in dense groups covering a large area
Comparison of complexity IP/WP
- IP - very complex structural modifications to attract insects
- WP - relatively simple flower
What does self pollination lead to?
Self fertilisation - inbreeding
What does cross pollination lead to?
Cross fertilisation - out breeding
What is self pollination?
The transfer of pollen from the stamen to the stigma in the same flower
What is cross pollination?
The transfer of pollen from the stamens of one flower to the stigma of a different flower of the same species
What is the advantage of cross pollination?
•Provides more genetic variation
•Male gamete from on plant fertilises the egg cell of a flower on another plant
-zygote contains a new mixture of chromosomes from the two parents
Mechanisms to ensure cross pollination
- Being dioecious
- Being monoecious
- Anthers and stigmas maturing at different times
- Structural adaptations which make self pollination unlikely
- Self-incompatibility
Explain dioecious (mechanisms)
Separate male and female plants
Explain monoecious (mechanisms)
Separate male and female flowers on one plant
Explain anthers and stigmas maturing at different times (mechanisms)
- Protandry - anthers mature first
* Proyogyny - stigmas nature first
Explain self incompatibility (mechanisms)
- If self pollination occurs, it ensures that the pollen tube does not develop unless it has a different genetic composition from that of the stigma
- Prevents self fertilisation
Describe the structure of the carpel
- Each ovule is made up of a nucellus of parenchyma cells surrounded by integuments (protective layer)
- Micropyle - opening through integuments
- Ovule develops from tissues within the ovary called the placenta
- Ovule remains attached to the placenta by a stalk called the funicle
Describe the formation of female gametes
•Megaspore mother cell divides by meiosis to produce a row of 4 megaspores (n)
•3 megaspores degenerate, while 1 grows
•Enlarged megaspore develops into a mature embryo sac
•Nucleus divides by mitosis to form 8 haploid cells which have no cell walls
•2 cells are polar nuclei which may fuse to form a central cell (2n) in some plants
•1 nucleus close to the micropyle forms the egg cell
-it is flanked by 2 synergids
•Other 3 cells are antipodal cells and take up a position opposite the egg cell
Formation of an female gametes simple
•Megaspore mother cell (2n) -meiosis •4 megaspore cells (n) -3 disintegrate -mitosis x3 •3 antipodal nuclei (n) •2 polar nuclei (n) - fuse to form a polar nucleus (2n) •2 synergids (n) •Oosphere (n) - female gamete
What is the function of the nucellus?
Nourishment
What is the function of the 2 synergids?
- Nourishment
- Support
- Release chemicals to attract the pollen tube to the nucleus
Structure of the filament
- Stalk-like
* Contains vascular tissue which supplies food and water to the anther
Anther structure
4 pollen sacs in which pollen grains develop from pollen mother cells by mitosis
Describe the formation of male gametes
- Each pollen mother cell undergoes meiosis to form 4 pollen cells (n)
- The 4 pollen cells are grouped together to form a tetrad (n)
- After the pollen cells separate, each develops into a pollen grain with a thick outer wall (exine) and a thin inner wall (intine)
- The wall has thin areas called apertures out of which the pollen tube can grow
- In a mature pollen grain the haploid nucleus has divided by mitosis to form a generative nucleus and a tube nucleus
- After pollination, the generative nucleus again divides by mitosis to produce 2 male gametes (n)
Formation of male gametes simple
•Pollen mother cell (2n) -meiosis •4 cells in tetrad (n) -each does mitosis •Pollen tube nucleus (n) •Generative nucleus (n) -mitosis •Male gamete x2 (n)
What is the function of the tapetum?
It nourishes the pollen cells
How is pollen released from the anther?
•Once the pollen grains are matured and ready to be released from the anther, the anther dries out and splits open along lines of weakness
-this is called dehiscence
•Pollen grains are now exposed to the environment and can be picked up, e.g. by insects
Process of pollination and double fertilisation
•Pollen grain from a flower of one species lands on the stigma of a flower of the same species
•Pollen grains takes in water and germinates in sucrose solution secreted by the style
•Tube nucleus begins to form the pollen tube
•Hydrolase enzymes in the pollen tube digest their way through the style
-products are used by the pollen tube to grow
•Generative nucleus divides by mitosis to produce 2 gametes (n)
•Pollen tube enters embryo sac through micropyle
•Male nuclei enter the ovule and fertilisation occurs
•One haploid male nucleus fuses with the haploid female nucleus to form a zygote (2n)
•Other male nucleus fuses with both polar nuclei to form a triploid primary endosperm nucleus (3n)
•Seed and fruit develop
-zygote becomes the embryo (containing a plumule and a radicle and one or two cotyledons)
-integuments form testa
-ovule forms seed
-ovary forms fruit
•Triploid endosperm divides to form endosperm tissue
What is endosperm tissue?
An important food storage tissue in cereal grains
What are endospermic seeds?
Endosperm continues to grow and is kept as the seed’s food store
What are non-endospermic seeds
Endosperm becomes absorbed by developing cotyledons which provide the food store for the germinating seed
What is a monocotyledon?
- Have only one cotyledon
- E.g. maize
- Endosperm (3n)
- Testa
- Cotyledon
- Plumule
- Radicle
Function of testa
- Physical and desiccation protection of embryo
* Waterproof and hard (lignin)
What is a dicotyledon?
- Has two cotyledons
- E.g. broad bean
- Cotyledon
- Testa
- Plumule
- Radicle
- Micropyle
Describe seed dispersal
- Seeds have evolved as a strategy for reproducing successfully on land
- Plants disperse their seeds in different ways, reducing competition and increasing the chances of the seeds developing successfully into mature plants
- If a seed lands in an area where conditions are favourable, they will germinate
Seed adaptations
- Dormant - survive long periods
- Less that 10% water - low metabolic rate
- Seed dispersal - avoid competition and colonise new habitats
- Tough testa coat - chemical and physical protection
- Contain chemical inhibitors - controls germination until spring
- Endosperm/cotyledons - provides nutrients for germination until seeding can photosynthesise
What is the role of oxygen in germination?
Needed for the production of ATP by aerobic respiration
What is the role of water in germination?
Seeds require water to mobilise their enzymes, to form vacuoles in the cells and also for transport
Why is a suitable temperature required for germination?
- The temperature has to be close to the optimum temperature of the enzymes involved in germination
- This varies depending on the species of plant
Process of germination in non-endospermic seeds
- In right conditions, seed imbibes water through the micropyle
- Cotyledons swell and the testa splits, so O2 can enter the seed and aerobic respiration can occur
- Enzymes in the seed mobilise and break down energy storage molecules such as starch, proteins and lipids in the cotyledons to provide energy for plant development
- Testa breaks open and the plumule grows upwards (away from gravity) to form the shoot. The radicle grows downwards towards gravity and forms the roots
- Until the shoot can grow through the soil surface, all the energy for the growth must come from energy stores within the seed
Describe broad bean germination
- The plumule is folded into a hook shape so that, as it grows through the soil, it protects the developing leaves
- Once the plumule has grown clear of the soil, it straightens, the leaves open out and photosynthesis begins
What is the role of gibberellins?
- Stimulate the production of hydrolytic enzymes such as amylase, which break down stored nutrients such as starch
- Switches on/expressed genes for alpha amylase to be produced in the aleurone layer
Describe endospermic seed germination, e.g. maize
- After imbition of water, gibberellins are released by the embryo and diffuse to the aleurone layer
- The gibberellins stimulate the production of hydrolytic enzymes to break down stored nutrients such as starch
- Glucose and other nutrients that are products of this digestion diffuse to the embryo where they are used for aerobic respiration and growth
What is the advantage of preventing self-pollination?
•Prevent inbreeding •Increase genetic variation/diversity •Prevents homzyogosity •Larger gene pool Therefore: •Increases chances of survival •Adapt to changing environment
What is an advantage of self pollination?
The lack of variation does help preserve successful genotypes
What is dehiscence?
The anther dries out and splits open along line of weakness once the pollen grains are mature and ready to be released
