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Plant Biology - The Shoot System Flowers, Fruits, Seeds

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    Study Guide Plant Biology – The Shoot System: Flowers, Fruits, Seeds 1. Features of the Angiosperm Life Cycle Angiosperms, or flowering plants, have a life cycle with two alternating stages : • a sporophyte stage (2n) • a gametophyte stage (n) This pattern is called alternation of generations . The sporophyte is the plant we usually see — trees, grasses, shrubs, and crops. The gametophytes are very small and develop inside the flower.

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    Study Guide 1.1 Sporophyte Stage: The Flowering Plant The adult angiosperm plant is diploid (2n) and produces flowers , which are the reproductive organs. Inside the flower: • Anthers produce microspores • Ovules (inside the ovary) produce megaspores These spores are formed by meiosis , a type of cell division that reduces chromosome number. 1.2 Formation of the Male Gametophyte (Pollen) Inside the anthers : 1. Microsporocytes (2n) undergo meiosis. 2. This produces haploid microspores (n) . 3. Each microspore develops into a pollen grain . A pollen grain contains: • a tube cell • a generative cell Before or during pollination, the generative cell divides to form two sperm cells . At this stage, the pollen grain is the immature male gametophyte . 1.3 Formation of the Female Gametophyte (Embryo Sac) At the same time, the female gametophyte develops inside the ovule in the ovary. Structure of the Ovule Each ovule contains: • Nucellus (internal tissue) • Two integuments (protective layers) • A small opening called the micropyle • A stalk called the funiculus (attaches ovule to ovary wall)

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    Study Guide 1.4 Development of the Embryo Sac 1. A megasporocyte (2n) undergoes meiosis. 2. This forms four haploid megaspores (n) . 3. Three megaspores degenerate . 4. The remaining megaspore divides three times by mitosis , forming eight nuclei . These nuclei arrange into: • Egg apparatus (micropylar end) o 1 egg cell o 2 synergids (helper cells) • Three antipodal cells (chalazal end) • Two polar nuclei in the center When cell walls form, this structure becomes the mature embryo sac , which is the female gametophyte . 1.5 Pollination Pollination is the transfer of pollen grains from the anther to the stigma . It can occur by: • wind • water • gravity • animals (insects, birds, bats, etc.) Types of Pollination • Cross - pollination : between different plants (most common) • Self - pollination : within the same plant Flower shapes and colors are often adapted to help pollination happen efficiently.

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    Study Guide 1.6 Fertilization and Double Fertilization If pollen lands on a compatible stigma : 1. It absorbs moisture. 2. A pollen tube grows through the style toward the ovule. 3. The pollen tube enters the embryo sac through the micropyle . 4. One synergid is destroyed during entry. Double Fertilization (Unique to Angiosperms) Two fertilization events occur: • One sperm + egg → zygote (2n) → develops into the embryo • One sperm + two polar nuclei → endosperm (usually 3n) → provides nutrition This process is called double fertilization , a key feature of flowering plants. 1.7 Seed and Fruit Formation • The ovule becomes a seed • The ovary becomes a fruit • The endosperm nourishes the developing embryo If fertilization does not occur: • The ovule aborts • The ovary may wither and fall off Key Takeaway s Angiosperms have: • Alternation of generations • Highly reduced gametophytes • Pollination followed by double fertilization • Seeds protected inside fruits

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