Plant Diversity II: The Evolution of Seed Plants Chapter 30 Plant Diversity II: The Evolution of Seed Plants

Overview: Feeding the World Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems

Concept 30.1: The reduced gametophytes of seed plants are protected in ovules and pollen grains The following are common to all seed plants: Reduced gametophytes Embryo protected within seed Ovule Pollen

Advantages of Reduced Gametophytes Sporophyte (2n) Gametophyte (n) Sporophyte dependent on gametophyte (mosses and other bryophytes) Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants) Microscopic female gametophytes (n) in ovulate cones (dependent) Microscopic male inside these parts of flowers Sporophyte (2n), the flowering plant (independent) gametophytes (n) in pollen cones Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms) The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte

Ovules and Production of Eggs An ovule consists of a megasporangium, megaspore, and one or more protective integuments Gymnosperm megaspores have one integument Angiosperm megaspores usually have two integuments Integument Megasporangium (2n) Megaspore (n) Unfertilized ovule Fertilized ovule Spore wall Male gametophyte (within germinating pollen grain) (n) Micropyle Female gametophyte (n) Egg nucleus (n) Discharged sperm nucleus (n) Pollen grain (n) Seed coat (derived from integument) Embryo (2n) (new sporophyte) Gymnosperm seed Food supply (female gametophyte tissue) (n)

Ovules and Production of Eggs Diagrammatic ovules. a, Angiosperm ovule. b, Gymnosperm ovule. i, integument (covering); ii, inner integument; m, micropyle (opening); oi, outer integument; s, stalk.

Pollen and Production of Sperm Microspores develop into pollen grains, which contain the male gametophytes If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule Integument Megasporangium (2n) Megaspore (n) Unfertilized ovule Fertilized ovule Spore wall Male gametophyte (within germinating pollen grain) (n) Micropyle Female gametophyte (n) Egg nucleus (n) Discharged sperm nucleus (n) Pollen grain (n) Seed coat (derived from integument) Embryo (2n) (new sporophyte) Gymnosperm seed Food supply (female gametophyte tissue) (n)

The Evolutionary Advantage of Seeds A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat

Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones The gymnosperms include four phyla: Cycadophyta (cycads) Gingkophyta (one living species: Ginkgo biloba) Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) Coniferophyta (conifers, such as pine, fir, and redwood)

LE 30-4aa Cycas revoluta

LE 30-4ab

LE 30-4ac

LE 30-4ad Gnetum. This genus includes about 35 species of tropical trees, shrubs, and vines, mainly native to Africa and Asia. Their leaves look similar to those of flowering plants, and their seeds look somewhat like fruits.

LE 30-4ae Ephedra. This genus includes about 40 species that inhabit arid regions throughout the world. Known in North America as “Mormon tea,” these desert shrubs produce the compound ephedrine, commonly used as a decongestant.

LE 30-4af Welwitschia. This genus consists of one species Welwitschia mirabilis, a plant that lives only in the deserts of southwestern Africa. Its strap like leaves are among the largest known.

LE 30-4ag Ovulate cones

LE 30-4ba Douglas fir. “Doug fir” (Pseudotsuga menziesii) provides more timber than any other North American tree species. Some uses include house framing, plywood, pulpwood for paper, railroad ties, and boxes and crates.

Pacific yew. The bark of Pacific yew (Taxa brevifolia) is a LE 30-4bb Pacific yew. The bark of Pacific yew (Taxa brevifolia) is a source of taxol, a compound used to treat women with ovarian cancer. The leaves of a European yew species produce a similar compound, which can be harvested without destroying the plants. Pharmaceutical companies are now refining techniques for synthesizing drugs with taxol-like properties.

LE 30-4bc Bristlecone pine. This species (Pinus longaeva), which is found in the White Mountains of California, includes some of the oldest living organisms, reaching ages of more than 4,600 years. One tree (not shown here) is called Methuselah because it may be the world’s oldest living tree. In order to protect the tree, scientists keep its location a secret.

LE 30-4bd Sequoia. This giant sequoia (Sequoiadendron giganteum), in California’s Sequoia National Park weighs about 2,500 metric tons, equivalent to about 24 blue whales (the largest animals), or 40,000 people. Giant sequoias are the largest living organisms and also some of the most ancient, with some estimated to be between 1,800 and 2,700 years old. Their cousins, the coast redwoods (Sequoia sempervirens), grow to heights of more than 110 meters (taller than the Statue of Liberty) and are found only in a narrow coastal strip of northern California.

LE 30-4be Common juniper. The “berries” of the common juniper (Juniperus communis), are actually ovule-producing cones consisting of fleshy sporophylls.

Wollemia pine. Survivors of a confer group LE 30-4bf Wollemia pine. Survivors of a confer group once known only from fossils, living Wollemia pines (Wollemia nobilis), were discovered in 1994 in a national park only 150 kilometers from Sydney, Australia. The species consists of just 40 known individuals in two small groves. The inset photo compares the leaves of this “living fossil” with actual fossils.

Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems Living seed plants can be divided into two clades: gymnosperms and angiosperms

A Closer Look at the Life Cycle of a Pine Key features of the gymnosperm life cycle: Dominance of the sporophyte generation Development of seeds from fertilized ovules The transfer of sperm to ovules by pollen The life cycle of a pine is an example Animation: Pine Life Cycle

LE 30-6_3 Key Haploid (n) Diploid (2n) Ovule Ovulate cone Megasporocyte (2n) Integument Longitudinal section of ovulate cone Micropyle Pollen cone Megasporangium Mature sporophyte (2n) Microsporocytes (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS Longitudinal section of pollen cone Surviving megaspore (n) Sporophyll Microsporangium Seedling Germinating pollen grain Archegonium Integument Egg (n) Seeds on surface of ovulate scale Female gametophyte Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n)

Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms are flowering plants These seed plants have reproductive structures called flowers and fruits They are the most widespread and diverse of all plants

Characteristics of Angiosperms All angiosperms are classified in a single phylum, Anthophyta The name comes from the Greek anthos, flower

Flowers The flower is an angiosperm structure specialized for sexual reproduction A flower is a specialized shoot with up to four types of modified leaves: Sepals, which enclose the flower Petals, which are brightly colored and attract pollinators Stamens, which produce pollen Carpels, which produce ovules

Video: Flower Blooming (time lapse) LE 30-7 Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule Receptacle Video: Flower Blooming (time lapse)

Animation: Fruit Development Fruits A fruit typically consists of a mature ovary but can also include other flower parts Fruits protect seeds and aid in their dispersal Mature fruits can be either fleshy or dry Animation: Fruit Development

LE 30-8 Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp Tomato, a fleshy fruit with soft outer and inner layers of pericarp Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp Milkweed, a dry fruit that splits open at maturity Walnut, a dry fruit that remains closed at maturity

LE 30-9 Wings enable maple fruits to be easily carried by the wind. Seeds within berries and other edible fruits are often dispersed in animal feces. The barbs of cockleburs facilitate seed dispersal by allowing these fruits to hitchhike on animals.

The Angiosperm Life Cycle In the angiosperm life cycle, double fertilization occurs. ovum food-storing endosperm nourishes the developing embryo

LE 30-10a Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on Sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (n) Antipodal cells Female gametophyte (embryo sac) Polar nuclei Pollen tube Synergids Eggs (n) Sperm (n)

LE 30-10b Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Male gametophyte (in pollen grain) Tube cell Ovary Pollen grains MEIOSIS Stigma Pollen tube Megasporangium (n) Sperm Surviving megaspore (n) Pollen tube Style Female gametophyte (embryo sac) Antipodal cells Polar nuclei Pollen tube Synergids Eggs (n) Sperm (n) Eggs nucleus (n) Discharged sperm nuclei (n)

LE 30-10c Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Male gametophyte (in pollen grain) Tube cell Ovary Pollen grains Germinating seed MEIOSIS Stigma Pollen tube Megasporangium (n) Embryo (2n) Sperm Surviving megaspore (n) Endosperm (food supply) (3n) Seed Pollen tube Seed coat (2n) Style Female gametophyte (embryo sac) Antipodal cells Polar nuclei Pollen tube Synergids Eggs (n) Zygote (2n) Sperm (n) Nucleus of developing endosperm (3n) Eggs nucleus (n) FERTILIZATION Discharged sperm nuclei (n)

Animation: Plant Fertilization Animation: Seed Development Video: Flowering Plant Life Cycle (time lapse)

Angiosperm Evolution Clarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologists Angiosperms originated at least 140 million years ago During the late Mesozoic, the major branches of the clade diverged from their common ancestor

Archaefructus sinensis, a 125-million-year-old fossil LE 30-11 Carpel Stamen 5 cm Archaefructus sinensis, a 125-million-year-old fossil Artist’s reconstruction of Archaefructus sinensis

Angiosperm Diversity The two main groups of angiosperms are monocots and eudicots Basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots

BASAL ANGIOSPERMS Amborella trichopoda Water lily (Nymphaea LE 30-12aa BASAL ANGIOSPERMS Amborella trichopoda Water lily (Nymphaea “Rene Gerald”) Star anise (Illicium floridanum)

HYPOTHETICAL TREE OF FLOWERING PLANTS LE 30-12ab HYPOTHETICAL TREE OF FLOWERING PLANTS Star anise and relatives Amborella Water lilies Magnoliids Monocots Eudicots MAGNOLIIDS Southern magnolia (Magnolia grandiflora)

LE 30-12ba MONOCOTS EUDICOTS Orchid California (Lemboglossum Monocot rossii) Monocot Characteristics Eudicot Characteristics California poppy (Eschscholzia california) Embryos One cotyledon Two cotyledons

LE 30-12bb MONOCOTS EUDICOTS Pyrenean oak Leaf (Quercus venation pyrenaica) Leaf venation Veins usually netlike Veins usually parallel Stems Pygmy date palm (Phoenix roebelenii) Vascular tissue usually arranged in ring Vascular tissue scattered

LE 30-12bc MONOCOTS EUDICOTS Lily (Lilium “Enchantment”) Roots Dog rose (Rosa canina), a wild rose Root system usually fibrous (no main root) Taproot (main root) usually present

LE 30-12bd MONOCOTS EUDICOTS Barley (Hordeum vulgare), a grass Pea (Lathyrusner vosus, Lord Anson’s blue pea), a legume Pollen Pollen grain with one opening Pollen grain with three openings Flowers Anther Zucchini (Cucurbita Pepo), female (left), and male flowers Floral organs usually in multiples of three Stigma Floral organs usually in multiples of four or five Filament Ovary

Evolutionary Links Between Angiosperms and Animals Pollination of flowers by animals and transport of seeds by animals are two important relationships in terrestrial ecosystems A flower pollinated by honeybees. hummingbirds. A flower pollinated by nocturnal animals. Video: Bee Pollinating Video: Bat Pollinating