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

Overview: Evolutionary advances over lower vascular plants Fully adapted to life on land, even dry places Posses woody tissue made of xylem which transports water, minerals and supports the plant Grows in diameter and height Reproduction involved wind borne pollen and seeds and does not require water

Overview: Transforming the World Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems A seed consists of an embryo and nutrients surrounded by a protective coat What human reproductive organ is analogous to this seed?

Concept 30.1: Seeds and pollen grains are key adaptations for life on land In addition to seeds, the following are common to all seed plants Reduced gametophytes which are retained within the sporophyte, and are dependent on the sporophyte Heterospory – two types of spores produced – megasporangia produce megaspores that give rise to female gametophytes, and microsporangia produce microspores that give rise to male gametophytes Ovules – a structure made up of megasporangium, megaspore, and their integument(s) (a protective layer of tissue ) Pollen – consists of a male gametophyte enclosed within the pollen wall

This is a great review of plant life cycles PLANT GROUP Mosses and other nonvascular plants Ferns and other seedless vascular plants Seed plants (gymnosperms and angiosperms) Reduced, independent (photosynthetic and free-living) Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition Gametophyte Dominant Reduced, dependent on gametophyte for nutrition Sporophyte Dominant Dominant Gymnosperm Angiosperm Sporophyte (2n) Microscopic female gametophytes (n) inside ovulate cone Microscopic female gametophytes (n) inside these parts of flowers Sporophyte (2n) Gametophyte (n) Example Figure 30.2 Gametophyte/sporophyte relationships in different plant groups Microscopic male gametophytes (n) inside these parts of flowers Microscopic male gametophytes (n) inside pollen cone Sporophyte (2n) Sporophyte (2n) Gametophyte (n)

(a) Unfertilized ovule Fig. 30-3-1 Integument Spore wall Immature female cone Megasporangium (2n) Figure 30.3a From ovule to seed in a gymnosperm Megaspore (n) (a) Unfertilized ovule

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 eliminates the need for a film of water and can be dispersed great distances by air or animals 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 Seeds provide some evolutionary advantages over spores: They may remain dormant for days to years, until conditions are favorable for germination They may be transported long distances by wind or animals

Figure 30.3 From ovule to seed in a gymnosperm Seed coat (derived from integument) Integument Female gametophyte (n) Spore wall Egg nucleus (n) Immature female cone Food supply (female gametophyte tissue) (n) Male gametophyte (within a germinated pollen grain) (n) Megasporangium (2n) Discharged sperm nucleus (n) Embryo (2n) (new sporophyte) Megaspore (n) Micropyle Pollen grain (n) Figure 30.3 From ovule to seed in a gymnosperm (a) Unfertilized ovule (b) Fertilized ovule (c) Gymnosperm seed

Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones The gymnosperms have “naked” seeds not enclosed by ovaries and consist of four phyla: (Do not memorize!) Cycadophyta (cycads) Gingkophyta (one living species: Ginkgo biloba) Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) Coniferophyta (conifers, such as pine, fir, and redwood) Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

Fig. 30-5k Figure 30.5 Gymnosperm diversity Sequoia

Fig. 30-5l Figure 30.5 Gymnosperm diversity Wollemi pine

Fig. 30-5m Figure 30.5 Gymnosperm diversity Common juniper

The Life Cycle of a Pine: A Closer Look Three key features of the gymnosperm life cycle are: 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 provides an example The pine tree is the sporophyte and produces sporangia in male and female cones Small cones produce microspores called pollen grains, each of which contains a male gametophyte The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes It takes nearly three years from cone production to mature seed

Surviving megaspore (n) Seedling Fig. 30-6-4 Key Haploid (n) Ovule Diploid (2n) Ovulate cone Megasporocyte (2n) Integument Pollen cone Microsporocytes (2n) Mature sporophyte (2n) Megasporangium (2n) Pollen grain Pollen grains (n) MEIOSIS MEIOSIS Microsporangia Microsporangium (2n) Surviving megaspore (n) Seedling Archegonium Figure 30.6 The life cycle of a pine Seeds Female gametophyte Food reserves (n) Sperm nucleus (n) Seed coat (2n) Pollen tube Embryo (2n) FERTILIZATION Egg nucleus (n)

The Life Cycle of a Pine - Animation http://bcs.whfreeman.com/thelifewire/content/chp30/30020.html (Pine tree life cycle) http://www.youtube.com/watch?v=D9byVQxvMXs

Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms are seed plants with reproductive structures called flowers and fruits They are the most widespread and diverse of all plants Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

Characteristics of Angiosperms Reasons for evolutionary success Superior competitors Coevolution with animals that acted as pollinators The flower Flower parts are modified leaves of the sporophyte generation Pollen grains are immature male gametophytes Female gametophytes are enclosed inside female flower parts Seeds develop inside fruits (the fruit is the mature ovary) Double fertilization is a reproductive feature of flowering plants The endosperm (the seed food supply) results from double fertilization

Flowers The flower is an angiosperm structure specialized for sexual reproduction Many species are pollinated by insects or animals, while some species are wind-pollinated 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 on their terminal anthers Carpels, which produce ovules

Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule Fig. 30-7 Stigma Carpel Stamen Anther Style Filament Ovary Figure 30.7 The structure of an idealized flower Petal Sepal Ovule

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 Various fruit adaptations help disperse seeds Seeds can be carried by wind, water, or animals to new locations http://www.youtube.com/watch?v=bwCpQflmQG4&feature=related Animation: Fruit Development

Tomato Ruby grapefruit Nectarine Hazelnut Milkweed Fig. 30-8 Figure 30.8 Some variations in fruit structure Hazelnut Milkweed

Figure 30.8a–e (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp (e) Walnut, a dry fruit that remains closed at maturity (d) Milkweed, a dry fruit that splits open at maturity

Wings Seeds within berries Barbs Fig. 30-9 Figure 30.9 Fruit adaptations that enhance seed dispersal Barbs

The Angiosperm Life Cycle The flower of the sporophyte is composed of both male and female structures Male gametophytes are contained within pollen grains produced by the microsporangia of anthers The female gametophyte, or embryo sac, develops within an ovule contained within an ovary at the base of a stigma Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of the same species A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary The ovule is entered by a pore called the micropyle Double fertilization occurs when the 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 endosperm The endosperm nourishes the developing embryo Within a seed, the embryo consists of a root and two seed leaves called cotyledons

Mature flower on sporophyte plant (2n) Microsporocytes (2n) Fig. 30-10-4 Key Haploid (n) Diploid (2n) Microsporangium Anther Mature flower on sporophyte plant (2n) Microsporocytes (2n) MEIOSIS Generative cell Microspore (n) Ovule (2n) Tube cell Male gametophyte (in pollen grain) (n) Ovary Pollen grains MEIOSIS Germinating seed Stigma Megasporangium (2n) Pollen tube Embryo (2n) Endosperm (3n) Seed coat (2n) Sperm Seed Megaspore (n) Style Antipodal cells Central cell Synergids Egg (n) Figure 30.10 The life cycle of an angiosperm Female gametophyte (embryo sac) Pollen tube Sperm (n) Nucleus of developing endosperm (3n) FERTILIZATION Zygote (2n) Egg nucleus (n) Discharged sperm nuclei (n)

Angiosperm Life Cycle http://www.youtube.com/watch?v=htMZYR1nCOQ&feature=related Double fertilization in an angiosperm http://www.youtube.com/watch?v=H_UyDtaa8Ow&feature=related Life Cycle of a Flowing Plant

Dog rose (Rosa canina), a wild rose Exploring Angiosperm Diversity Orchid (Lemboglossum fossii) Monocot Characteristics Embryos Leaf venation Stems Roots Pollen Flowers Pollen grain with one opening Root system Usually fibrous (no main root) Vascular tissue scattered Veins usually parallel One cotyledon Two cotyledons netlike usually arranged in ring Taproot (main root) usually present three openings Zucchini (Cucurbita Pepo), female (left) and male flowers Pea (Lathyrus nervosus, Lord Anson’s blue pea), a legume Dog rose (Rosa canina), a wild rose Pygmy date palm (Phoenix roebelenii) Lily (Lilium “Enchant- ment”) Barley (Hordeum vulgare), a grass Anther Stigma California poppy (Eschscholzia californica) Pyrenean oak (Quercus pyrenaica) Floral organs usually in multiples of three Floral organs usually in multiples of four or five Filament Ovary Eudicot MONOCOTS EUDICOTS Figure 30.12

Evolutionary Links Between Angiosperms and Animals Pollination of flowers and transport of seeds by animals are two important relationships in terrestrial ecosystems Co-evolution – reciprocal adaptations http://www.youtube.com/watch?v=0mchJw5Dz7g Hummingbird pollination

Concept 30.4: 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

Table 30-1a Table 30.1

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 At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years

You should now be able to: Explain why pollen grains were an important adaptation for successful reproduction on land Explain how fruits may be adapted to disperse seeds Describe the current threat to plant diversity caused by human population growth