Chapter 28 Reproduction in Plants

Figure 28.1

Life Cycle of Flowering Plants In contrast to animals with one type of adult generation, flowering plants exhibit an alternation of generations includes a diploid & a haploid generation. Sporophyte is diploid generation Sporophyte produces haploid spores (meiosis) Spores develop into a haploid gametophyte. Gametophyte is haploid generation Gametophyte produces haploid gametes (mitosis) gametes fuse to form diploid zygote. Zygote develop into the sporophyte.

Sporophyte-Flower A flower is the reproductive organ of a flowering plant; it develops within a bud. Shoot apical meristem stops forming leaves to form flowers; axillary buds can become flowers directly. Flower structures are modified leaves attached to a stem tip (receptacle) Monocot flower parts = 3’s or multiples Dicot flower parts = 4’s, 5’s or multiples

Sporophyte-Flower Not all flowers have sepals, petals, stamens, and a pistil. Complete flowers have sepals, petals, stamens, and a pistil; incomplete flowers do not. Perfect flowers have both stamens and a pistil. Staminate flowers have only stamens. Pistillate flowers have only pistils. Monoecious - If staminate and pistillate flowers are on same plant Dioecious - If staminate and pistillate flowers are on different plants

Life Cycle of a Flowering Plant Floral Structure & Function Egg/Pollen Fertilization

Animation – Double Fertilization Figure 28.5 Animation – Double Fertilization

Pollination Pollination and fertilization are separate events. Pollination is strictly transfer of pollen from anther to stigma of a pistil. Pollination occurs by wind or with assistance of particular animal pollinators. Self-pollination is transfer of pollen from anther to stigma of the same plant. Cross pollination is transfer of pollen from anther of one plant to stigma of another plant; it is evolutionarily advantageous because of genetic recombination resulting in new and varied plants.

Fertilization Pollen tube grows through micropyle and releases both sperm cells into the ovule. sperm nucleus + the egg nucleus = 2n zygote sperm nucleus + 2 polar nuclei = 3n endosperm nucleus. This is double fertilization Zygote divides mitotically to become the embryo; endosperm nucleus divides mitotically to become endosperm Embryo - is a young sporophyte. Endosperm - tissue that will nourish embryo and seedling as they undergo development.

Animation

Embryonic Development Embryo continues to differentiate into three parts. Epicotyl between cotyledons & 1st leaves; it contributes to shoot development Hypocotyl is below cotyledon and contributes to stem development Radicle is below hypocotyl and contributes to root development

Monocot embryo = one cotyledon Dicot embryo = two cotyledons cotyledon rarely stores food; rather, it absorbs food molecules from endosperm and passes them to embryo. After differentiation into embryo and suspensor, one or two cotyledons develop. Cotyledon (seed leaf) provides nutrient for a developing plant before it photosynthesizes. Dicot embryo = two cotyledons cotyledons usually store nutrients the embryo uses, obtaining those nutrients from endosperm.

Seeds and Fruits As a zygote develops into embryo, integuments of the ovule harden and become seed coat. Ovule matures into the seed sporophyte embryo stored food Ovary (and sometimes other floral parts) develops into fruit (mature ovary that usually contains seeds).

Seed Dispersal For plants to be widely distributed, seeds have to be dispersed away from parent plant. Hooks, spines & bur attach to fur of animals Birds & mammals eat fruits & defecate them Squirrels gather seeds & bury them Dispersal by water Dispersal by wind Touch-me-nots & Oxalis has seed pods burst

Animations Germination in Dicots & Monocots

Germination in Dicots Prior to germination, embryo consists of the following: 2 Cotyledons - supply nutrients to embryo & seedling; soon shrivels and disappears Plumule—epicotyl bearing young leaves Hypocotyl - which becomes the stem Radicle - which develops into roots. As dicot seedling emerges, the shoot is hook-shaped to protect delicate plumule. As seed germinates in darkness, it etiolates (stem length increases; leaves remain small) Phytochrome pigment = red and far-red light induces normal growth in light.

Germination in Monocots Endosperm is food-storage tissue; cotyledon does not have a storage role. Monocot “seed” is actually the fruit; outer covering is the pericarp. Prior to germination, embryo consists of one cotyledon, a plumule, and a radicle. Plumule and radicle are enclosed in protective sheaths: coleoptile and coleorhiza, respectively. Plumule and radicle burst through these coverings when germination occurs.

Plants:Asexual Reproduction Plants contain nondifferentiated meristem tissue and reproduce asexually by vegetative propagation. In asexual reproduction, offspring arise from a single parent and inherit genome of that parent only. Vegetative propagation utilizes meristematic tissue of a parent plant. Nodes of stolons will produce strawberry plants. Eyes of a potato plant tuber is a bud;produces new plant Sweet potatoes can be propagated from modified roots. Many trees can be started from small “suckers.” Discovery that auxin will cause roots to develop has expanded ability to use stem cuttings.