Sexual Reproduction in Plants

2 Contents Definitions Flower Structure Flower parts & functions Two important facts Pollen Grain Development Embryo Sac Development Pollination Agents of Pollination Insect pollinated flowers – adaptations Wind pollinated flowers – adaptations Hay fever Fertilisation Plant Breeding Techniques Tissue Culture Seed Types & Structure Classification of seeds Fruits Fruit & Seed Dispersal Agents of dispersal Dormancy Germination Enzymes found in seeds Germination of Broad Bean Germination of a sunflower

3 Definitions sexual reproduction: reproduction involving the union of sex cells or gametes, which fuse at fertilisation to form a zygote. zygote: a diploid cell resulting from the union of two haploid gametes - a fertilised egg – will grow by cell division into a new individual. Two parents needed – male and female. New individual will possess traits of both parents.

4 Flower Structure flower: reproductive shoot of plant, containing four whorls – i.e. sepals, petals, stamens and carpels. Are formed from buds.

5 The structure of a typical flower

6 Another typical flower

7 Flower parts and functions (1/3) PartDescriptionFunction(s) ReceptacleTip of stem Forms the base of the flower Sepals Leaf-like, ususlly green Protects flower before it blooms PetalsUsually coloured Attract pollinators e.g. insects Stamens Male part made up of anther and filament Produces the male gametes

8 Flower parts and functions (2/3) AntherSac-like structure Produces pollen grains that produce the male gamete FilamentStalk-like structure Supports the anther and supplies food and water Carpel Female part made up of stigma, style and ovary Produces the female gamete: the egg StigmaTop of the carpel Place where pollen lands

9 Flower parts and functions (3/3) StyleNeck of the carpel Connects stigma to ovary Ovary Swollen base of carpel Contains ovules Ovule Bears an embryo sac Produces the egg cell and polar nuclei NectarySugar sac Provides energy-rich food for pollinators

10 Lilium, anther. T.S. L.P.

11 Two important facts Pollen grain produces male gametes Embryo sac produces an egg cell and polar nuclei

12 Pollen Grain Development (1/2) Pollen grains form in the anther – in pollen sacs Pollen sacs contain microspore mother cells (2n) – lined by tapetum – nutrition Divide by meiosis to produce four haploid microspores (n) These become the pollen grains – enclosed by a thick wall

13 Pollen Grain Development (2/2) Each microspore nucleus divides by mitosis to produce two nuclei – both haploid A tube nucleus (n) and a generative nucleus (n) After pollination the generative nucleus divides again by mitosis to produce two male gametes – both haploid (n)

14 Pollen grain development

15 A mature pollen grain Consists of a tube nucleus and two haploid male gamete nuclei.

16 Lilium, mature pollen grains

17 Embryo Sac Development (1/2) The carpel contains one or more ovules Each ovule contains nutritive tissue – nucellus Nucellus contains embryo sac mother cell = megaspore mother cell (2n) Divides by meiosis to produce four haploid cells Three of these degenerate

18 Embryo Sac Development (2/2) Remaining nucleus divides three times by mitosis to produce Eight haploid nuclei Arrange themselves as in diagram on next slide Note position of egg cell and polar nuclei Remaining cells play no part in reproduction

19 Embryo sac development 1

20 Embryo sac development 2

21 A mature embryo sac Consists of eight nuclei one of which is the haploid egg and two others are the polar nuclei.

22 Pollination pollination: is the transfer of pollen from the anther of the stamen of one flower to the stigma of the carpel of (a) the same flower or another flower on the same plant (self-pollination) or (b) another flower on a different plant of the same species (cross- pollination).

23 Agents of Pollination Insect Wind Self

24 A bee pollinating a flower

25 Insect pollinated flowers – adaptations (1/2) Flowers large, conspicuous Brightly coloured Small quantity of pollen produced Hooked, sticky pollen Nectaries produce nectar Scented Anthers and stamens inside flower Large pollen

26 Insect pollinated flowers – adaptations (2/2) Heavy pollen Small anthers Small stigma Stigma rounded Stigma inside flower

27 A wind-pollinated flower – rye grass

28 Wind pollinated flowers – adaptations (1/2) Small, inconspicuous Green Large quantity of pollen produced Smooth pollen No nectaries Not scented Anthers and stamens outside flower

29 Wind pollinated flowers – adaptations (2/2) Smaller pollen Lighter pollen Larger anthers Larger stigma Stigma feathery Stigma outside flower

30 Pollen grains, mixed wind and insect distributed

31 Clouds of pollen from hazel catkins

32 Hay fever A seasonal allergy The symptoms include sneezing fits, a blocked or runny nose, a tickle in the roof of the mouth and itchy watery eyes. It is caused by the pollen of certain wind- pollinated plants e.g. grass

33 Fertilisation fertilisation: the union of a haploid (n) male gamete with a haploid (n) female gamete resulting in the formation of a diploid (2n) zygote. How does this occur?

34 After pollination (1/3) Pollen grain has landed on stigma. It germinates and a pollen tube grows down through the stigma and style to the embryo sac. The two male gamete nuclei are inside the pollen tube. Pollen tube enters micropyle – tip of tube bursts open –

35 After pollination (2/3) tube nucleus disintegrates – allows free passage for male gamete nuclei into the embryo sac. FERTILISATION - is the union of one of the male gamete nuclei (n) with the nucleus of the egg cell (n) to produce a diploid zygote (2n).

36 After pollination (3/3) The second male gamete nucleus (n) fuses with the two polar nuclei to produce the triploid endosperm nucleus (3n). This is a double fertilisation.

37 A mature ovule

38 What becomes of the various parts The ZYGOTE develops into the embryo plant. The ENDOSPERM forms a food supply for the developing embryo. The OVULE becomes the seed. The OVARY becomes the fruit.

39 Plant Breeding Techniques (1/3) How can we produce wheat that will grow quickly, be resistant to pests and diseases, and produce a large quantity of good quality grain? Two methods. The first method Choose two plants that have all the desired traits between them – grow them.

40 Plant Breeding Techniques (2/3) Artificially pollinate – remove anthers from one plant – this prevents self-pollination When carpels are ripe dust them with pollen taken from the other plant Cover flower with bag – this prevents cross- pollination from other plants. Seeds produced will hopefully have the required traits.

41 Plant Breeding Techniques (3/3) The second method Tissue culturing Cells removed from required plant Cells grown on sterile nutritive medium One of these cells taken and grows into an embryo on another medium Embryo plant grows into new plant

42 Tissue culture

43 Seed Types & Structure (1/2) Seed embryo Plumule (immature shoot) Radicle (immature root) Cotyledon (food supply or seed leaf) endosperm Food store All seeds In some seeds

44 Seed Types & Structure (2/2) When seeds are mature they lose water and become hard and dry. They can survive adverse conditions – e.g. cold and drought. They can be dispersed away from the parent plant to reduce competition.

45 L.S. of cereal grain

46 Classification of seeds Based on two features 1. The number of cotyledons – if one then is a monocotyledon e.g. maize, grass, oats; if two then is a dicotyledon e.g. broad bean, snapdragon, buttercup. 2. The presence or absence of endosperm (food store) – if present seed is endospermic (e.g. usually monocotyledons); if absent non- endospermic (e.g. most dicotyledons).

47 Endospermic seed

48 A maize seed - endospermic

49 Broad bean seed – non- endospermic

50 Fruits A fruit is a ripened ovary Or sometimes a modified part of the flower (e.g. receptacle) that may contain seeds e.g. strawberry – red part = receptacle & pips on surface = fruits. Fruit- protects the seeds - enables seeds to be dispersed

51 Dry fruits

52 How do seedless fruits form? 1. May be a genetic variety and occur naturally e.g. some bananas & cucumbers. 2. Spray flowers with auxin or gibberellin (growth regulators) and fruits form without fertilisation taking place. 3. Gibberellin may also cause bigger fruit to form. 4. Ethene is used to ripen fruit and de-green skins of oranges, lemons and grapefruit.

53 Fleshy fruits

54 Fruit & Seed Dispersal Necessary to: Prevent overcrowding Prevent competition for water and light Allows colinisation of new areas

55 Agents of dispersal Wind – fruit / seeds light and winged or hairy e.g. dandelion, sycamore. Water – buoyant e.g. water lilies. Animal – (a) eat fruits – seeds indigestible – pass through gut and deposited elsewhere (b) seeds have hooks – attach to animal’s coat e.g. cleavers, burdock. Self-dispersal – seeds in pods – pods dry out – burst open – eject seeds from pod.

56 Dandelion fruits

57 A pea pod and seed

58 Dormancy (1/2) dormancy: a period of rest before growth, during which the rate of metabolism is reduced. Seed will not germinate during this time even if given ideal conditions, because other requirements may be necessary before germination can occur. These include: -

59 Dormancy (2/2) Seed coat too hard - must wait for it to be softened by nature. Cold conditions necessary - ensures springtime germination. Desert plants - chemical inhibitors in seed must be washed out by heavy rain - ensures water for further growth. Embryo not mature - seed not 'ripe' - must wait.

60 Advantages of Dormancy Allows time for dispersal before germination. Allows seed to avoid unfavourable growing conditions of winter. Maximises growing conditions in springtime. Provides a long growing season the first year.

61 Germination germination: is the beginning of growth of a seed into a new plant.

62 Factors necessary for Germination Water – enters through the micropyle - hydrates cell cytoplasm, activates enzymes, dissolves food reserves, and bursts testa. Oxygen - for respiration - ATP production. Temperature - allows enzymes to work.

63 Enzymes found in seeds EnzymeSubstrateProductUse of Products AmylaseStarchMaltoseRespired for energy MaltaseMaltoseGlucoseCellulose cell walls ProteinaseProteins Amino Acids For making enzymes and cell membranes LipaseLipids Fatty Acids and glycerol For energy and making cell membranes

64 Germination of Broad Bean Seeds (1/2) Seeds absorb water and swell Radicle bursts through testa and grows downwards into soil Hooked plumule emerges and grows upwards out of soil Above the soil the plumule straightens out and leaves emerge

65 A germinating seed

66 Germination of Broad Bean Seeds (2/2) Radicle develops primary and lateral roots Cotyledons remain below ground – act as food supply for embryo plant – eventually shrivels up and withers when true leaves begin to photosynthesise.

67 Germination of a broad bean

68 Germination of a sunflower

69 END