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 Figure 30.1

In addition to seeds, the following are common to all seed plants Concept 30.1: 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 Heterospory Ovules Pollen

Advantages of Reduced Gametophytes The gametophytes of seed plants Develop within the walls of spores retained within tissues of the parent sporophyte

Gametophyte/sporophyte relationships Sporophyte dependent on gametophyte (mosses and other bryophytes). (a) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). (b) Microscopic female gametophytes (n) in ovulate cones (dependent) Microscopic female gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n), the flowering plant (independent) Sporophyte (2n) (independent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). (c) Figure 30.2a–c

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

Ovules and Production of Eggs An ovule consists of A megasporangium, megaspore, and protective integuments Figure 30.3a (a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.) Integument Spore wall Megasporangium (2n) Megaspore (n)

Pollen and Production of Sperm Microspores develop into pollen grains Which contain the male gametophytes of plants Pollination Is the transfer of pollen to the part of a seed plant containing the ovules

If a pollen grain germinates It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule Female gametophyte (n) Egg nucleus (n) Spore wall Male gametophyte (within germinating pollen grain) (n) Discharged sperm nucleus (n) Pollen grain (n) Micropyle (b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of a pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm. Figure 30.3b

Pollen, which can be dispersed by air or animals Eliminated the water requirement for fertilization

The Evolutionary Advantage of Seeds A seed Develops from the whole ovule Is a sporophyte embryo, along with its food supply, packaged in a protective coat Gymnosperm seed. Fertilization initiates the transformation of the ovule into a seed, which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument. (c) Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) Figure 30.3c

Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones Among the gymnosperms are many well-known conifers Or cone-bearing trees, including pine, fir, and redwood

The gymnosperms include four plant phyla Cycadophyta Gingkophyta Gnetophyta Coniferophyta

Exploring Gymnosperm Diversity Gnetum Ephedra Ovulate cones Welwitschia PHYLUM GNETOPHYTA PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta Figure 30.4

Exploring Gymnosperm Diversity Douglas fir Pacific yew Common juniper Wollemia pine Bristlecone pine Sequoia PHYLUM CYCADOPHYTA Figure 30.4

Gymnosperm Evolution Fossil evidence reveals that by the late Devonian Some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants Figure 30.5

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

A Closer Look at the Life Cycle of a Pine Key features of the gymnosperm life cycle include Dominance of the sporophyte generation, the pine tree The development of seeds from fertilized ovules The role of pollen in transferring sperm to ovules

The life cycle of a pine Figure 30.6 Microsporocytes pollen cone An ovulate cone scale has two ovules, each containing a mega- sporangium. Only one ovule is shown. 2 Figure 30.6 Ovule Megasporocyte (2n) Integument Longitudinal section of ovulate cone Ovulate cone Pollen Mature sporophyte (2n) pollen cone Microsporocytes grains (n) (containing male gametophytes) MEIOSIS Micropyle Germinating pollen grain Megasporangium Sporophyll Microsporangium Surviving megaspore (n) Archegonium Egg (n) Female gametophyte pollen grain (n) Discharged sperm nucleus (n) tube Egg nucleus (n) FERTILIZATION Seed coat (derived from parent sporophyte) (2n) Food reserves (gametophyte tissue) (n) Embryo (new sporophyte) Seeds on surface of ovulate scale Seedling Key Diploid (2n) Haploid (n) In most conifer species, each tree has both ovulate and pollen cones. 1 A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. 4 While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. 5 A pollen cone contains many microsporangia held in sporophylls. Each microsporangium contains microsporocytes (microspore mother cells). These undergo meiosis, giving rise to haploid microspores that develop into pollen grains. 3 The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. 6 Fertilization usually occurs more than a year after pollination. All eggs may be fertilized, but usually only one zygote develops into an embryo. The ovule becomes a seed, consisting of an embryo, food supply, and seed coat. 8 By the time the eggs are mature, two sperm cells have developed in the pollen tube, which extends to the female gametophyte. Fertilization occurs when sperm and egg nuclei unite. 7

Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Are commonly known as flowering plants Are seed plants that produce the reproductive structures called flowers and fruits Are the most widespread and diverse of all plants

Characteristics of Angiosperms The key adaptations in the evolution of angiosperms Are flowers and fruits

Flowers The flower Is an angiosperm structure specialized for sexual reproduction

A flower is a specialized shoot with modified leaves Sepals, which enclose the flower Petals, which are brightly colored and attract pollinators Stamens, which produce pollen Carpels, which produce ovules Anther Filament Stigma Style Ovary Carpel Petal Receptacle Ovule Sepal Stamen Figure 30.7

Fruits Fruits Typically consist of a mature ovary 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

Can be carried by wind, water, or animals to new locations, enhancing seed dispersal Wings enable maple fruits to be easily carried by the wind. (a) Seeds within berries and other edible fruits are often dispersed in animal feces. (b) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. (c) Figure 30.9a–c

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 center cell of the female gametophyte and initiates development of food-storing endosperm The endosperm Nourishes the developing embryo

The life cycle of an angiosperm Key Mature flower on sporophyte plant (2n) Ovule with megasporangium (2n) Female gametophyte (embryo sac) Nucleus of developing endosperm (3n) Discharged sperm nuclei (n) Pollen tube Male gametophyte (in pollen grain) Sperm Surviving megaspore (n) Microspore (n) Generative cell Tube cell Stigma Ovary MEIOSIS Megasporangium grains Egg Nucleus (n) Zygote (2n) Antipodal cells Polar nuclei Synergids Egg (n) Embryo (2n) Endosperm (food Supply) (3n) Seed coat (2n) Seed FERTILIZATION Haploid (n) Diploid (2n) Anther Style Microsporangium Microsporocytes (2n) Germinating Anthers contain microsporangia. Each microsporangium contains micro- sporocytes (microspore mother cells) that divide by meiosis, producing microspores. 1 Microspores form pollen grains (containing male gametophytes). The generative cell will divide to form two sperm. The tube cell will produce the pollen tube. 2 When a seed germinates, the embryo develops into a mature sporophyte. 7 In the megasporangium of each ovule, the megasporocyte divides by meiosis and produces four megaspores. The surviving megaspore in each ovule forms a female gametophyte (embryo sac). 3 The zygote develops into an embryo that is packaged along with food into a seed. (The fruit tissues surround- ing the seed are not shown). 6 After pollina- tion, eventually two sperm nuclei are discharged in each ovule. 4 Double fertilization occurs. One sperm fertilizes the egg, forming a zygote. The other sperm combines with the two polar nuclei to form the nucleus of the endosperm, which is triploid in this example. 5 Figure 30.10

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

Fossil Angiosperms Primitive fossils of 125-million-year-old angiosperms Display both derived and primitive traits Carpel Stamen Archaefructus sinensis, a 125-million-year- old fossil. (a) Artist’s reconstruction of Archaefructus sinensis (b) 5 cm Figure 30.11a, b

An “Evo-Devo” Hypothesis of Flower Origins In hypothesizing how pollen-producing and ovule-producing structures were combined into a single flower Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

The two main groups of angiosperms 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

HYPOTHETICAL TREE OF FLOWERING PLANTS Exploring Angiosperm Diversity Amborella trichopoda Water lily (Nymphaea “Rene Gerard”) Star anise (Illicium floridanum) BASAL ANGIOSPERMS HYPOTHETICAL TREE OF FLOWERING PLANTS MAGNOLIIDS Amborella Water lilies Star anise and relatives Magnoliids Monocots Eudicots Southern magnolia (Magnolia grandiflora) Figure 30.12

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 by animals and transport of seeds by animals Are two important relationships in terrestrial ecosystems (a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the bee and dusts it with pollen, some of which will rub off onto the stigma of the next flower the bee visits. (c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate. (b) A flower pollinated by hummingbirds. The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive. Figure 30.13a–c

Concept 30.4: Human welfare depends greatly on seed plants No group is more important to human survival than seed plants

Products from Seed Plants Humans depend on seed plants for Food Wood Many medicines Table 30.1

Threats to Plant Diversity Destruction of habitat Is causing extinction of many plant species and the animal species they support