Chapter 26 Seeded Plants Seed Plants aka spermatophytes

Chapter 26 Seeded Plants

Seed Plants • aka spermatophytes • Evolved to be sporophyte dominant • Systematic reduction in gametophyte size • All are heterosporous • Megaspores (female) - develop into female gametophytes that produce eggs • Microspores (male) – develop into male gametophytes that produce sperm

Distinguish between seedless and seeded: 1. Seeds 2. Pollen These are critical adaptations for drought Also necessary for reproduction that doesn’t require water

Earliest seed plants • 350 mya • Progymnosperms – preceded gymnosperms; transitional group that superficially resembled conifers but reproduce like Bryophytes • Gymnosperms – 380 mya • Naked seed plants • Dominated in Triassic and Jurassic periods of Mesozoic era • Angiosperms • Surpassed gymnosperms in middle of the Cretaceous (~100 mya)

Today - Angiosperms are the most abundant plant group in terrestrial biomes Pollen and seeds allowed plants to break their dependence on water for reproduction and embryo development

Pollen • Pollen grains are the male gametophytes; contain sperm • Protective coat prevents desiccation and mechanical damage • Can travel far from original sporophyte – spreading genes • Male gametes reach female gametophyte and egg cell through pollen tube (extension within the pollen grain)

Seed • Protection, nourishment for embryo • Allows it to be dormant for many years • Ensuring germination when conditions are optimal • Contain diploid embryo that will germinate into a sporophyte

Evolution of Gymnosperms • Gymnosperms are 1 st true seed plants because they developed structures called cupules to enclose and protect the ovule • Ovule – female gametophyte; develops into a seed upon fertilization • Seed plants became numerous and diverse in Carboniferous period

Evolution of Angiosperms • Produce flowers containing male and reproductive structures • Diversification of angiosperms in mid Cretaceous also linked with appearance of many modern day groups of insects • It is now believed that angiosperms did not evolve from gymnosperms • Developed in parallel with gymnosperms

• Flowers and fruit are improved reproductive strategies that protect embryo while increasing genetic variation and range • Angiosperms produce their gametes in separate organs – usually in a flower • Fertilization and embryo development are protected inside • Most flowers have a mutualistic pollinator with distinctive features

• Fertilization of egg – ovule grows into a seed • Fruit – results from thickening of surrounding tissues and ovary • Fruit protects the seed and often ensures its dispersal over a wide range • Fruits vary widely: Pineapple, avocado, walnut shells, olives, tomatoes • Fruits also act as an agent of dispersal

Gymnosperms • Means “naked seeds” • Diverse group of seed plants • Paraphyletic

Characteristics of Gymnosperms • Naked seeds • Separate male and female gametes • Pollination by wind • Tracheids • Seeds NOT enclosed in ovary (exposed on cones or modified leaves)

• Sporophylls – specialized leaves that produce sporangia • Strobilus –sporophylls arranged around a central stalk (cones) • Integument – layer of sporophyte tissue surrounds megasporangium

• Gymnosperms are still the prominent phylum in the Taiga where evergreens have an advantage in cold and dry weather • One disadvantage – conifers are more susceptible to infestations because they don’t lose all of their leaves at once; can not shed parasites and restart fresh

Gymnosperm life cycle • Alternation of generations • Sporophyte dominant • Reduced gametophyte • Heterosporus • Reproductive organs form cones and strobili • Monoeclous – male and female sporangia produced on the same plant • Dioecious – male and female sporangia produced on separate plants

Modern day gymnosperms: 4 phyla • Coniferophyta • Cycadophyta • Ginkgophyta • Gnetophyta

1. Conifers (Coniferophyta) • Most variety • Tall trees, needlelike leaves • Harvested for paper pulp and timber

2. Cycads (Cycadophyta) • Mild climates • Mistaken for palms • Large cones • Some pollinated by beetles (rather than wind) • Only ~ 100 species still living

3. Gingkophytes • Single surviving species Gingko biloba • Fan shaped leaves • Unusually resistant to pollution

4. Gnetophyta • Closest relative to angiosperms • Broad leaves • Varied from tropical rainforests to desert environments • Ephedra – source of ephedrine (powerful decongestant)

Angiosperms • Flowering plants • Dominate most terrestrial ecosystems • > 250, 000 species • Success is due to flowers and fruit

Flowers • Modified leaves (sporophylls) • Sepals – together called calyx; base of peduncle, enclose the unopened floral bud • Petals – together called corolla; inside whorl of sepals; vivid colors to attract pollinators • Sepals + petals = perianth

• Sexual organs located at center of flower • Female reproductive organs: Gyoecium (carpel) – consist of style, stigma and ovule • Flower structure is diverse • Pistil – multiple fused carpels • Megaspore and female gametophytes are produced and protected by thick tissues of carpel

• Style – long, thin structure leads from stigma to ovary • Stigma – pollen is deposited • Ovary – enclosed in carpel; houses 1 or more ovules • Ovules will develop into a seed when fertilized

• Male reproductive organs – Androecium (stamen) • Surround the central carpel • Filament – thin stalk • Anther – sac like structure; supported by filament; where microspores are produced through meiosis and develop into pollen grains

Fruit • As seed develops, ovary walls thicken and form fruit • Seed forms in an ovary which also enlarges as seed grows • Fruit – fertilized and fully grown ripened ovary • Fruits contain seeds: eggplant, zucchini, string beans, peppers • Mature fruits can be fleshy or dry • Fleshy – berries, peaches, apples • Dry – rice, wheat, nuts

Angiosperm Life Cycle • Sporophyte dominant • Heterosporous (micro and megaspores) • Double fertilization • 1 sperm – joins with egg and forms diploid zygote (future embryo) • 2 nd sperm – fuses with polar nuclei forming triploid cell; develops into endosperm which serves as a food reserve

2 major groups of angiosperms • Monocots – contain 1 leaflike cotyledon • Dicots – contain 2 cotyledons • Seed food is stored outside embryo in the form of a complex carbs, lipids and proteins • Cotyledons are embryonic leaves that transport broken down food reserves from their storage site inside the seed to developing embryo

• Most flowers are monoecious (bisexual) – both stamen and carpel • But only a few species self-pollinate • These flowers also contain both types of sex organs • Cross pollination increases genetic diversity in a species

Diversity of Angiosperms • Phylum – Anthophyta • Monophyletic • 2 major groups: • Moncots • Dicots (aka eudicots)

Monocots • Single cotyledon • Parallel leaf veins • Adventitious roots • Ex. Palms, rice, corn, bananas

Eudicots • 2 cotyledons • Branched leaf veins • Herbaceous or woody • Taproot • Ex:

Role of seed plants • Herbivory – consumption of plants by insects and other animals • Co-evolution of flowering plants and insects

Pollination • Wind, water, animals • > 80% angiosperms depend on animals for pollination • Nectar – sugary liquid secreted by many animal-pollinated flowers • Fertile pollen (for reproduction) • Sterile pollen (nutrients for pollinators)

Importance of seed plants for Humans • Crops - rice, wheat, potatoes • Perfume • Alcohol • Timber • Fuel • Paper • Clothing • Dyes • Medicine