Chapter 35: Reproduction in Flowering Plants

Angiosperms can reproduce sexually and asexually.

Flowers function in sexual reproduction to increase the likelihood that pollen is transferred to another plant of the same species.

• Meiosis makes gametes (eggs and sperm) that unite in fertilization to form zygote within the flower’s ovary.
• Seeds are then produced inside fruit.
• Sexual reproduction leads to genetic diversity
• New gene combinations from independent assortment of chromosomes during meiosis.

Asexual reproduction gives offspring similar to parents.

• The vegetative part of the plant grows and separates.
• Rhizomes, tubers and corms are stems that reproduce asexually, also roots with suckers or runners.

Alternation of generations means plants have multicellular haploid and multicellular diploid forms. Angiosperm sporophyte is dominant. Gametophyte is dependent on sporophyte.

Flower parts

• Receptacle - enlarged tip of stalk
• Sepals are outermost, protect bud.
• All sepals are the calyx
• petals are broad, flat, thin with various shapes and colors. May fuse into tube
• All petals are the corolla
• Stamen is male reproductive organ. Stalk is the filament, anther is the sac of pollen.
• Pollen transferred to female carpel, produces 2 cells with outer coat. One cell forms nonflagellated sperm, other cell forms pollen tube.
• Center of flower has carpel, female reproductive organ. Female part of flower also called pistil (one or many carpels).
• Stigma is where pollen lands
• Style is neck through which pollen tube grows
• Ovary has one or more ovules. Ovule has embryo sac that makes egg, and 2 polar nuclei.

Details

• Ovule has diploid cell, which undergoes meiosis to form haploid cells.
• One haploid cell is megaspore. Mitosis produces the embryo sac, the female gametophyte.
• One of the cells in the embryo sac has 2 separate polar nuclei.
• Pollen sacs have diploid microsporocytes. Meiosis produces haploid microspores.
• Microspore mitosis produces male gametophyte, the pollen grain, with a tube cell and generative cell.
• Generative cell will divide into 2 nonmotile sperm.

Pollination is transfer of pollen grains from anther to stigma.

• Self pollinated if pollination of same flower or another flower on same plant
• Cross pollinated if pollen goes to flower on another plant.
• Insects, other animals like birds, wind.

1. Flowers want to attract animal pollinators with visually showy petals or scent.

• Animals get food (nectar, pollen grains)
• 70% flowers pollinated by bugs. Insects see UV radiation, and some flowers have UV bullseye.
• Insects have strong sense of smell: sweet, carrion for flies.
• Birds like hummingbirds see red, orange, yellow, no strong scent.
• Bat pollinated flowers bloom at night, and have strong scent of fermented fruit.
• Coevolution means the 2 species (plant and animal) interact so closely that they become increasingly adapted to each other.
• Plants evolved animal attractants, and animals evolved specialized body parts.
• Animal behavior has coevolved, too. Some orchids look like female insects. Males pseudocopulate with orchid flowers.

1. Wind pollinates grasses, ragweed, maples, oaks.

• many small, inconspicuous flowers
• Maybe feathery stigma to catch pollen
• Vast amounts of pollen produced (less likely that animal pollination).

Pollen on stigma, tube cell grows tube down through style, into an ovule in ovary. 2nd pollen cell divides into 2 sperm cells.

• Double fertilization means both sperm fertilize.
• One with the egg,
• Other. with the 2 polar nuclei to form the 3n endosperm.
• Endosperm nourishes embryonic plant
• Ovule develops into seed
• Ovary develops into fruit.

Dicot embryonic development

• Fertilized egg divides into 2 cells
• Bottom cell is suspensor, anchors embryo and helps with food intake
• Top cell is embryo
• Top cell forms a chain of proembryo cells, then a small ball of globular embryo
• Tissues start to develop in globular embryo.
• Cotyledons start to form: heart stage
• Torpedo stage has elongating cotyledons and larger embryo.

Seed has embryonic plant, food in cotyledons or endosperm, and seed coat for protection

• Radicle is embryonic root
• coyledons are seed leaves (one in monocots, 2 in dicots)
• Hypocotyl is the embryonic shoot that connects the cotyledon(s) and radicle.
• Plumule is terminal bud above cotyledons.
• Dormant, food from cotyledons or endosperm.

Fruit protects seed and aids dispersal. From ovule and ovary.

1. Simple fruit - develops from a single pistil ( one carpel or several fused carpels)

• berry - tomato, grape, banana - fruit wall is soft
• drupe - peach, plum, almond - inner wall is hard
• follicle - milkweed - splits along one seam
• legume - green bean, pea - splits along 2 seams
• capsule - iris, pansy, poppy - splits along 3 + seams
• grain - wheat, corn - fruit wall fused to seed coat, don’t spilt open
• achene - sunflower - fruit wall separate from seed coat, attach at one place
• nut - acorn - stony wall, doesn’t split open

1. Aggregate

• fruit develops from a single flower with several pistils (carpels not fused)
• Blackberry, raspberry

1. Multiple fruit

• fruit develops from ovaries of a group of flowers
• pineapple

1. Accessory fruit

• fruit mainly tissue other than ovary
• apple, pear, strawberry

Seed, fruit or entire tumbleweed plant may be dispersed by

1. wind: winged fruits, light feathery plumes.
1. animals: burr spines, edible fruit with nondigestible seed, acorns buried by squirrel
1. water - coconut has corky floats.
1. explosive dehiscence.

• Fruit bursts open suddenly ands forces seeds out.
• Turgor pressure or drying out make it burst.

Asexual reproduction with stems, leaves, roots

1. Rhizome is horizontal underground stem, with scalelike leaves, buds, nodes, internodes.

• Rhizome branches off, old part dies, new parts becomes separate.
• Iris, ginger, bamboo, grasses

1. Tubers are thickened ends of rhizomes

• Function in food storage
• tuber breaks off parent to form new plant
• white potato, elephant ear
• potato eyes are axillary buds

1. Bulb is modified underground bud with fleshy storage leaves on short stem

• paperlike bud scales modified from leaves.
• axillary buds break off from parent bulb.
• lilies, tulip, onion, daffodils

1. Corm - short, upright underground stem

• food stored in thick underground stem, not leaves
• Axillary daughter corms split when parent dies.
• Crocus, gladiolus

1. Stolons or runners are horizontal, above-ground stems that grow along ground.

• Long internodes
• buds along stolon develops new shoots and roots
• stolon dies, new plant on its own.
• Strawberry, St. Augustine

1. Plantlets are tiny plants formed along edges of kalanchoe leaves. Drop off and grow into new plant.
1. Suckers are above ground stems that grow from adventitious buds on roots. Aspen trees grown from suckers cover 106 acres in Utah.
1. Apomixis is production of embryos in seeds without meiosis or gametes (sex).

• Embryo can develop from diploid cell in ovule
• Genetically the same as parent
• Dispersed by seeds and fruit
• dandelion, citrus seeds, garlic, some grasses.

Sexual reproduction = genetic diversity

• advantageous in changing environment, or to exploit new environment.
• Lots of energy to make gametes and get them to meet.
• Diversity means some offspring won’t be as well adapted as parents.

Asexual reproduction = genetically identical

• advantage if plant well adapted to environment.
• Mostly asexual plants sometimes have sexual repro: pressures like predators and parasites, food availability, competition means environment is changing.