Chapter 30 Flashcards
Seed plant evolution
Evolved from progymnosperms (spore-bearing). Gymnosperm and progymnosperms are similar with vascular tissue (including secondary xylem to increase girth), leaves, grew tall and produced wood. Progymnosperms produce spores.
Evolved 350 MYA (305-465 MYA)
Rapid diversification around 309 MYA. Whole genome duplications (4n) occurred at origin of both gymnosperm and angiosperm. Genes have evolutionary pressures: Mutations (Pseudogenes no proteins produced), novel functions (resistance), reinforce existing functions.
Seed plant characteristics
Seeds
Sporophyte is dominant generation (protects small gametophyte generation). Male and female gametophytes are highly reduced - (protected from UV radiation and desiccation).
Vascular tissues - wood =secondary xylem
Water not required to move male gametophyte
Heterosporous - 2 morphology types Microspores (male) and megaspores/macrospores (female)
Microspores vs Macrospores/Megaspores
Microspores - Male, generally smaller, diversity in shape and size. Don’t need water for movement. Pollen grains. Microsporocyte.
Microspore mother cell (2n) to microspore (1n) to microgametophyte
Macrospores/Megaspores - Female, larger. Develop within ovules and remain attached to the sporophyte. Immature female cone holds megaspore, surrounded mostly by integument but allows a gap for male sperm to enter.
Megaspore mother cell (2n) to megaspore (1n) to megagametophyte
Seed plant anatomy
Embryo, food for embryo and seed coat.
Allows “clock to be stopped” to survive harsh periods. Spores have to grow immediately, seeds can wait.
Naked seeds (Gymnosperms) first, then later development of fruits to enhance dispersal.
1 seed coat - Integument (gymnosperms). Can withstand harsh conditions.
2 Embryo - (2n) baby plant, sustained by nutritive tissue for energy found inside seed
3 Similar structures in angiosperms as well, theses allow for dormant stages, Opens up larger range of habitats.
Pollination vs Fertilization
Pollination is sperm getting into pollen tube (female parts) but has not meet the egg.
Fertilization is sperm and egg being fused.
5 Phyla of existing seed plants
Coniferophyta - gymnosperms
Cycadophyta - gymnosperms
Gnetophyta - gymnosperms
Ginkgophyta - gymnosperms
Anthophyta - Angiosperm, seeds enclosed within carpel (tissue)
Gymnosperm-naked seeds, ovules not enclosed by an ovary but exposed. Lack flowers, fruit. Ovule exposed on a scale (thick cone)
Conifers phylum
Coniferophyta
Most diverse and familiar gymnosperm. Pines, spruces, firs, cedars, yews, and others. Found in colder and sometimes dry regions. Conifers are source of important products. (Timber, paper, resin, taxol (yew anti-cancer chemical)).
Pines
100 spp. in All in northern hemisphere. Produce tough needlelike leaves in clusters. Leaves have thick cuticle and recessed stomata to retard water loss. Leaves have canals with resin to deter insect and fungal attacks.
Pine Reproduction
Male gametophytes (pollen grains in male cones) develop microspores by meiosis, lower branches, smaller. Pollen releases 2 sperm, only one reaches egg. Female cones (ovules) form on the upper branches of same tree. Have woody scales, larger, two ovules develop on each scale and contain megasporangium, each will become female gametophyte.
Seed and maturing
Female cones usually take 2 or more seasons to mature. During the first spring pollen grains drift down between open scales, drawn into micropyle (opening on seeds). Scales then close. 1 year later female gametophyte matures. 15 months after pollination, pollen tube reaches archegonium and discharges contents. One sperm unites with egg to form zygote, other degenerates.
Cycads (Cycadophyta)
300ish spp. Slow growing gymnosperm of tropical or subtropical regions. Sporophytes resemble palm trees. Female cones can weigh 45 kg. Have largest sperm cell of all organisms.
Sago palm
Gnetophytes (Gnetophyta)
Only gymnosperms with vessels in their xylem. Additional vessels for drawing in water. Desert, arid, semiarid. Usually smaller, not tall
Contain 3 genera Welwitschia (SW Africa), Ephedra (SW US and Mex)(Treats respiratory illness)(Large leafy shrub), Gnetum (vines & tropical trees) (Taproots, shallow cupped stem)
Ginkgophytes (Ginkgophyta)
1 living spp Ginkgo biloba. Flagellated sperm, uses pollen or wind. Dioecious, usually vegetative propagation.
Angiosperms
Carpels
Flowering plants. Ovules contain eggs, and become seeds after egg fertilization.
Carpels- A modified leaf that covers seeds, develops into fruit. Sporophyte tissue (Fruit tissue) covering gametophyte. Holds the seed.
As early as 145-208 MYA. Oldest known angiosperm is fossil Archaefructus. Diversified rapidly.
Flower morphology
All flower parts have evolved from either stems or leaves. Primordium that develops into bud at end of stalk.
Flower parts are organized in circles called whorls.
Flower = modified stems bearing modified leaves.
Flower parts and whorls
Outermost whorl - sepal (green 3-5)
Second whorl - petals (colorful to attract pollinators. 3-5. Could be missing fused or seperate)
Third whorl - stamens (androecium) (pollen is male gametophyte) each stamen has an anther (where pollen is) and a filament (stalk)
Forth whorl (center) - gynoecium (Female gametophyte) Consists of one or more carpels
Carpel 3 major regions
Ovary - contains the ovules. Later develops into fruit and ovules inside becomes the seeds.
Stigma - Tip where pollen lands
Style - neck or stalk
Pollination
Mechanical transfer of pollen from anther to stigma, mostly between different flowers/plants.
May or may not be followed by fertilization
Pollen grains develop a pollen tube that is guided to the embryo sac.
The generative cell divides to produce two sperm cells. No flagella on angiosperm sperm
Double fertilization
One sperm unites with egg to form the diploid zygote (New sporophyte embryo).
Other sperm unites with 2 polar nuclei to form the triploid endosperm (3n). Provides nutrients to new sporophyte embryo and surrounds embryo.
Seed may remain dormant for many years. Germinate (Grow by mitosis) when conditions are favorable.
Seeds in angiosperms
Development of embryo stops soon after meristems and cotyledons differentiate. Integuments develop into a relatively impermeable seed coat. Encloses the seed with its dormant embryo and stored food.
Once seed coat forms most of the embryo’s metabolic activities cease/greatly reduced. Germination cannot take place until water and oxygen reach the embryo.
4 important adaptations of seeds
1 Maintain dormancy under unfavorable conditions.
2 They protect the young plant when it is most vulnerable.
3 Provide food fort the embryo until it can produce its own food (Leaves and roots, perform photosynthesis)
4 They facilitate dispersal of the embryo (Pass through digestive system, wind blown)
Fruits
Most are simply define as mature ovaries (carpels) Ovary’s develop into fruit. At the same time seeds are forming the flower ovary begins to develop into a fruit. (It is possible for fruits to develop without seed development).
Types of fruits
True berries
Legumes
Drupes
Samaras
Aggregate fruits
Multiple fruits
True berries
Multiple seeds in 1 or more ovaries. Tomatos
Legumes
2 carpel edges with seeds attached to edges. Dry out and then pop open.
Pea and beans
Drupes
Single seed in hard pit. Pericarp 3 layers - Exocarp (Skin), Mesocarp (fruit), endocarp (pit)
Peach, plum, cherry
Samaras
Seed attaches to pericarp wing
Maple, elm, ash
Aggregate fruits
Ovaries of single flower with multiple lobes. Ovary not fused.
Blackberry, strawberry, raspberry.
Multiple fruits
Individual flower forms fruit around single stem. Fruit fused together.
Pineapple pericarp fuse together.
Fruit Dispersal
Occurs through a wide array of methods and allows to colonize large areas.
Ingestions and transportation by birds or other vertebrates. Consume fruit, seed survives guts.
Hitching a ride with hooked spines or birds and mammals
Burial in caches by herbivores.
Wind blown
Floating and drifting on water.
