Reproduction in Plants (part 3): Fertilization & Post-Fertilization Changes

1. Fertilization & Post- Fertilization Changes

3. ***Fertilization • After pollination, pollen grain germinates in response to sugary fluid secreted by the mature stigma • From each pollen grain, a pollen tube grows out

4. ***Fertilization (continued) • The cytoplasm and 2 nuclei (vegetative + generative nuclei) pass into the pollen tube • The growth of the pollen tube is controlled by the vegetative nucleus • Enzymes are secreted to digest the tissue of the stigma and style as the tube grows • The pollen tube penetrates right through the style as it grows and enters the ovule through the micropyle (opening in the ovule wall)

5. vegetative

6. ***Fertilization • Along the way, the generative nucleus divides to form 2 male gametes (vegetative nuclei degenerates) • Within the ovule the tip of the pollen tube absorbs sap and bursts releasing 2 male gametes • One male gamete + ovum → zygote (fertilization!) • One male gamete + definitive nucleus → endosperm nucleus Double fertilisation

8. Post-fertilization changes • The zygote develops into the embryo of the seed with cotyledons, developing shoot (plumule) + developing root (radicle) • The endosperm nucleus divides and gives rise to the endosperm • In some seeds the endosperm is completely absorbed by the embryo which stores the food materials in the cotyledons • A fruit is formed from the ovary and the ovules • The ovary walls ripens to form the pericarp (hard/dry OR fleshy and succulent) • The ovules becomes the seeds • Fruit chambers = loculi Refer to table 20.2 page 322

12. Endospermic vs Non- endospermic seeds

13. Terms to familiarise • Monocotyledonous vs dicotyledonous • Endospermic vs non-endospermic seeds

14. Endospermic vs non-endospermic seeds • Endospermic seeds ➢The endosperm is present in the mature seed and serves as food storage organ ➢Testa and endosperm are the two covering layers of the embryo • Non-endospermic seeds ➢The cotyledons serve as sole food storage organs as in the case of pea (Pisum sativum) ➢During embryo development the cotyledons absorb the food reserves from the endosperm ➢The endosperm is almost degraded in the mature seed and the embryo is enclosed by the testa

15. Endospermic vs non-endospermic seeds (ENDOSPERMIC SEED) (NON-ENDOSPERMIC SEED)

16. endosperm Testa ( )

17. Structures of a seed the scar on seeds marking its point of attachment to the funicle

18. Structure of seeds (dicotyledons) 1 2 4 5 6 3 7

19. Structure of seeds (dicotyledons)

20. Germination

21. Conditions for germination 1. Sufficient water 2. Suitable temperature 3. Adequate O2supply

22. Dormancy • Period in which seeds of plants will not germinate even though environmental conditions are favourable • May be days, weeks or even years (depending on species) • Seeds usually dry (lack of water) • Respire anaerobically as vital activities are much reduced (lack O2) • Allow seeds to survive unfavourable seasons e.g. winter/drought (unfavourable temperature)

23. Changes during germination • The absorption of water by the seed • Seed swells 1) testa becomes more permeable to O2 and CO2 2) and may rupture the testa

24. Roles of enzymes in germination • With water absorption, cotyledons produce enzymes to digest stored food so that growing embryo can use it • In endospermic seeds, the enzymes flow into the endosperm to digest the food stored there • In non-endospermic seeds, the digestion of the stored food occurs within the cotyledons • Digested food are transported to growing regions of the embryo e.g. plumule and radicle

25. Enzyme digestion

27. Utilization of food substances for growth during germination • Carbohydrates and fats used in tissue respiration • Some carbohydrates+ fats used for formation of cell walls + cell membranes • a.a. assimiliated in building of new protoplasm

28. Types of germination 1. Epigeal germination – cotyledons are carried above the ground e.g. sword bean seed 2. Hypogeal germination – cotyledons remain below the surface of the soil e.g. maize

29. Parts of a germinating seed 6 5 4 3 2 1

30. Parts of a germinating seed

31. Stages in the germination of sword bean seed (epigeal germination) 1. Radicle grows rapidly and pushes against testa at micropyle. The testa splits as the radicle emerges and grows downwards. Lateral roots develop. Root hairs formed behind the tips of the radicle and lateral roots help to absorb H2O and mineral salts 2. Meanwhile, the stem below the cotyledons elongates, pulling the cotyledons above the ground and leaving the testa behind in the soil. At first the cotyledons are hook- like with the cotyledons bent over and still closed together to protect the delicate plumule 3. Soon the stem straightens, the cotyledons turn green and spread out, exposing the first foliage leaves with the bud between them. The bud will grow into a future shoot. The foliage leaves expand, turn green and carry out photosynthesis. The seedling is now a self- supporting plant

32. Germination of a dicotyledon Epigeal germination

33. Hypogeal germination

34. Dispersal of fruits

35. Dispersal of fruits and seeds • By wind e.g. angsana • By animals e.g. tomatoes • By water e.g. coconut • Explosive mechanism e.g. rubber fruits

36. Wind dispersal What do you think will be the factors that will aid dispersal by wind? • small and light (able to float in air and blown about by wind) • large flattened wing-like structures OR parachute of fine-hairs → enlarges surface area → increases buoyancy in air