Chapter 6: Sexual Reproduction in Flowering Plants

6.1 Structure of a Flower

• Flowers are the reproductive organs of angiosperms; they contain male and female reproductive organs. (p.114)
• Flower parts include peduncle, sepals and petals. (p.114)
• Bisexual flowers (hermaphrodites), e.g. lily, have male and female organs in the same flower. (p.114)
• Stamen (male): filament and anther; produces pollen grains. (p.115)
• Carpel (female): stigma, style and ovary; produces the embryo sac. (p.115)

6.2 Development of Pollen Grains and Embryo Sac

• Pollen sacs in each anther lobe contain diploid microspore mother cells. (p.116)
• Microspore mother cells divide by meiosis to form four haploid microspores called a tetrad. (p.117)
• Each microspore develops into a pollen grain; the nucleus divides by mitosis to form a generative nucleus and a tube nucleus. (p.117)
• The pollen sac wall breaks when the pollen matures and pollen grains are released. (p.117)
• Wind-pollinated flowers (e.g. corn, grass, paddy) have small, smooth, light pollen; insect- and animal-pollinated flowers have rough, sticky pollen. (p.118)
• Ovules attach to the ovary wall by a funicle; the placenta supplies nutrients to the ovule. (p.119)
• The nucellus is inside the ovule; the integument forms its outer layers and the micropyle is the opening. (p.119)
• The megaspore mother cell (diploid) divides by meiosis to form four haploid megaspores; three degenerate and one develops. (p.119)
• The remaining megaspore nucleus divides mitotically three times to form eight nuclei. (p.119)
• The mature embryo sac has three antipodal cells, two synergid cells, one egg cell, and two polar nuclei in the centre. (p.119)

6.3 Pollination and Fertilisation

• Pollination is the transfer of pollen grains from the anther to the stigma, assisted by insects, mammals, birds, water or wind. (p.120)
• The pollen grain germinates on the stigma and forms a pollen tube that grows down through the style towards the ovule. (p.121)
• The generative nucleus moves down the pollen tube and divides by mitosis to form two male gametes (n). (p.121)
• The pollen tube enters the ovule through the micropyle; the tube nucleus degenerates and both male gametes enter the embryo sac. (p.121)
• Double fertilisation: one male gamete fuses with the egg cell to form a diploid zygote. (p.123)
• The second male gamete fuses with the two polar nuclei to form a triploid endosperm nucleus. (p.123)
• Double fertilisation restores the diploid number in the zygote and provides food for the embryo, enabling survival of plant species. (p.124)

6.4 Development of Seeds and Fruits

• The triploid endosperm nucleus divides by mitosis to form endosperm tissue, which stores food for the embryo. (p.125)
• The zygote divides by mitosis into a larger basal cell that forms the suspensor and a smaller terminal cell that forms the embryo. (p.125)
• The embryo consists of plumule, radicle and cotyledon. (p.125)
• After fertilisation, petals fall off, stigma and style wither, and the ovary develops into a fruit. (p.125)
• The ovule develops into the seed; the integument forms the seed coat. (p.125)
• The ovary wall forms the pericarp, which consists of exocarp, mesocarp and endocarp. (p.125)
• Simple fruits develop from a single carpel or fused carpels, e.g. peas. (p.126)
• Aggregate fruits develop from many carpels in one flower, e.g. raspberry. (p.126)
• Multiple fruits develop from carpels of a cluster of flowers, e.g. pineapple. (p.126)
• Accessory fruits develop from tissues near the carpel, e.g. apple. (p.126)

6.5 Importance of Seeds for Survival

• Seeds are the main means by which most angiosperms reproduce, maintaining species survival. (p.128)
• The testa is strong and water-impermeable, protecting the seed from spoiling. (p.128)
• Seeds can enter a dormant state, allowing them to be stored for a long time. (p.128)
• Endosperm or cotyledon provides nutrients for energy when germination occurs. (p.128)
• Seeds have features such as lightness and spongy tissue that help dispersal to other places and reduce competition. (p.128)
• Seeds from sexual reproduction give more genetic variation than asexual reproduction. (p.128)