Chapter 30: Plant Diversity II: The Evolution of Seed Plants - Feeding the World Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems

The reduced gametophytes of seed plants are protected in ovules and pollen grains In addition to seeds, the following are common to all seed plants: Reduced gametophytes – seed development is protected Heterospory – male and female gametophytes develop separately Ovules Pollen

Sporophyte (2n) Sporophyte (2n) The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte – BIG ADVANTAGE Gametophyte (n) 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) Sporophyte (2n), the flowering plant (independent) Microscopic male gametophytes (n) in inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Microscopic female gametophytes (n) in inside these parts of flowers (dependent) Sporophyte (2n), (independent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

Heterospory: The Rule Among Seed Plants Seed plants evolved from plants with megasporangia, which produce megaspores that give rise to female gametophytes Seed plants evolved from plants with microsporangia, which produce microspores that give rise to male gametophytes

Ovules and Production of Eggs An ovule consists of a megasporangium, megaspore, and one or more protective integuments Phylum Coniferphyta - Gymnosperm megaspores have one integument (covering) Phylum Anthophyta - Angiosperm megaspores usually have two integuments These two phylum are most closely related to each other than the other 2 because they both form seeds, NOT spores Seed coat (derived from integument) Integument Female gametophyte (n) Spore wall Egg nucleus (n) Food supply (female gametophyte tissue) (n) Male gametophyte (within germinating pollen grain) (n) Megasporangium (2n) Discharged sperm nucleus (n) Embryo (2n) (new sporophyte) Megaspore (n) Pollen grain (n) Micropyle Unfertilized ovule Fertilized ovule Gymnosperm seed

Pollen and Production of Sperm Microspores develop into pollen grains, which contain the male gametophytes Pollination is the transfer of pollen to the part of a seed plant containing the ovules Pollen can be dispersed by air or animals, eliminating the water requirement for fertilization If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

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 ADVANTAGES OF A SEED: A seed aids in dispersal of the embryo to different locations in an ecosystem Supplies nutrients to the embryo Allows the embryo to remain dormant, or not growing, until conditions are right for growth Allows dessication resistance, or keeps the embryo from drying out Living seed plants can be divided into two groups: gymnosperms and angiosperms

Gymnosperms bear “naked” seeds, typically on cones – NOT in fruits 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)

Gymnosperms Cycas revoluta

Gymnosperms 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. 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.

Gymnosperms 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. 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.

Gymnosperms 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.

Gymnosperms 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.

Gymnosperms 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.

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

Gymnosperms 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 two small groves. The inset photo compares the leaves of this “living fossil” with actual fossils.

Gymnosperms bear “naked” seeds Ovulate cones

Gymnosperm Evolution Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems

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

A Closer Look at the Life Cycle of a Pine 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)

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 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

Structure of Flowers Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule Receptacle

Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp Fruits Tomato, a fleshy fruit with soft outer and inner layers of pericarp A fruit typically consists of a mature ovary but can also include other flower parts Fruits protect seeds and aid in their dispersal Seeds are made from the ovule, while the fruit part comes from the ovary Mature fruits can be either fleshy or dry 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

Dispersal of Fruits Various fruit adaptations help disperse seeds Wings enable maple fruits to be easily carried by the wind. Dispersal of Fruits Various fruit adaptations help disperse seeds Seeds can be carried by wind, water, or animals to new locations 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 when a pollen tube discharges two sperm into the female gametophyte within an ovule One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing, triploid endosperm The endosperm nourishes the developing embryo

The Angiosperm Life Cycle Key Haploid (n) Diploid (2n) The Angiosperm Life Cycle 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)

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 class diverged from their common ancestor Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

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

An “Evo-Devo” (Evolution/Development) Hypothesis of Flower Origins Scientist Michael Frohlich hypothesized how pollen-producing and ovule-producing structures were combined into a single flower He proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

Angiosperm Diversity The two main groups of angiosperms are monocots and eudicots A third group, the basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages A fourth group, the Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots BASAL ANGIOSPERMS Amborella trichopoda Water lily (Nymphaea “Rene Gerald”) Star anise (Illicium floridanum)

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

Monocots vs. Eudicots MONOCOTS EUDICOTS MONOCOTS EUDICOTS Orchid (Lemboglossum rossii) Monocot Characteristics Eudicot Characteristics California poppy (Eschscholzia california) Embryos One cotyledon Two cotyledons MONOCOTS EUDICOTS Pyrenean oak (Quercus pyrenaica) Leaf venation Veins usually netlike Veins usually parallel Stems Pygmy date palm (Phoenix roebelenii) Vascular tissue usually arranged in ring Vascular tissue scattered

MONOCOTS EUDICOTS 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 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

Differences between monocots and eudicots Monocot embryos have one cotyledon, eudicots have two Monocot leaf veins are usually parallel, while eudicot leaves usually have netlike meshwork of veins Vascular tissue in monocots is scattered, while eudicot vascular tissue is usually arranged in a ring Monocots usually have a fibrous root system, while eudicots usually have a main taproot Monocot pollen grains usually have one opening, while eudicot pollen grains usually have three openings Monocot flower organs are usually in multiples of three, while eudicot flower organs are usually in multiples of four or five

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. A flower pollinated by hummingbirds. A flower pollinated by nocturnal animals.

Human welfare depends greatly on seed plants No group of plants is more important to human survival than seed plants Plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical Most of our food comes from angiosperms Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% of the calories consumed by humans Modern crops are products of relatively recent genetic change resulting from artificial selection Many seed plants provide wood Secondary compounds of seed plants are used in medicines

LOTS of medicine comes from seed plants

Threats to Plant Diversity Destruction of habitat is causing extinction of many plant species Loss of plant habitat is often accompanied by loss of the animal species that plants support