Plant Diversity II: The Evolution of Seed Plants

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

2. Advantages of Seed Production • Seed production enabled plants to become the dominant producers in most terrestrial ecosystems • Make up the majority of plant biodiversity • The reduced gametophytes of seed plants are protected in ovules and pollen grains • They develop within the walls of spores retained within tissues of the parent sporophyte • Enables the gametophyte to obtain nutrition from the sporophyte

3. Sporophyte (2n) Sporophyte (2n) Seedless vascular plants Bryophytes & mosses Gametophyte (n) Gametophyte (n) (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). (b) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). 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) Seed plants Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n), the flowering plant (independent) Sporophyte (2n) (independent) (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). Gametophyte/ Sporophyte Relationships

4. Evolution of Gametophytes • Most seedless plants are homosporous, meaning they produce one type of bisexual spore • Seed plants evolved from plants that had megasporangia • Which produce megaspores that give rise to female gametophytes • And microsporangia • Which produce microspores that give rise to male gametophytes

5. Egg Production • Seed plants retain the megaspore within the parent sporophyte • Integuments envelope and protect it • Gymnosperms have one integument layer • Angiosperms have two integument layers • The ovule includes the whole structure integument and megaspore • Eggs are produced from the megasporangium inside each ovule

6. Integument Spore wall Megasporangium (2n) Megaspore (n) (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.) Egg Production

7. Pollen Production • Microspores develop into pollen grains • Can be carried from the parent by wind, water, or animal • Transfer of the pollen to the plant containing the ovule is called pollination • Durable and can travel long distances • When gets to the egg a pollen tube is made to the ovule and the sperm swin through that

8. 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, allowsentry of a pollen grain. The pollen grain contains amale gametophyte, which develops a pollen tubethat discharges sperm. Pollen Production

9. Seeds • A seed • Develops from the whole ovule • Is a sporophyte embryo, along with its food supply, packaged in a protective coat • Development of seeds allowed plants to withstand harsh environments and distribute their offspring widely • Most seeds will only germinate under good conditions

10. Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) (c) Gymnosperm seed. Fertilization initiatesthe 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. Seeds

11. Gymnosperms • Plants that have “naked seeds” that are not enclosed in ovaries • Exposed on modified leaves that usually form cones • Called conifers;, include pine, fir, and redwood • The gymnosperms include four plant phyla • Cycadophyta- ancient plants, cycads • Gingkophyta- ginkgos • Gnetophyta- welwitschia, and gnetum • Coniferophyta, pines and firs

12. PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta PHYLUM GNETOPHYTA Gnetum Welwitschia Ovulate cones Ephedra Gymnosperm Diversity

13. Gymnosperm Diversity Douglas fir Common juniper Wollemia pine Pacific yew Bristlecone pine Sequoia

14. Gymnosperm Evolution • The first seed-bearing plants appeared around 360 mya • Gymnosperm species • Gymnosperms dominated the Mesozoic terrestrial ecosystems • Many cycads and other progymnosperms Archaeopteris

15. 2 An ovulate cone scale has two ovules, each containing a mega- sporangium. Only one ovule is shown. 5 4 1 3 7 8 6 Key In most conifer species, each tree has both ovulate and pollen cones. Haploid (n) Ovule Diploid (2n) A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. Megasporocyte (2n) Ovulate cone Integument Longitudinal section of ovulate cone Micropyle Pollen cone Microsporocytes (2n) Megasporangium Mature sporophyte (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. Longitudinal section of pollen cone Surviving megaspore (n) Sporophyll Microsporangium 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. Seedling Germinating pollen grain Archegonium Egg (n) Integument Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. Seed coat (derived from parent sporophyte) (2n) 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. Discharged sperm nucleus (n) Pollen tube 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. Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n) Life Cycle of a Pine

16. Angiosperms • Are commonly known as flowering plants • Are seed plants that produce the reproductive structures called flowers and fruit • Seeds are contained in the fruit which is a modified ovaries • Are the most widespread and diverse of all plants • About 90% of all plants, over 250, 000 species

17. Angiosperms • The key adaptations in the evolution of angiosperms • Are flowers and fruits • Flowers are structures specialized for sexual reproduction • Contains male and/or female sexual organs and gametes

18. Carpel Stigma Anther Style Stamen Ovary Filament Petal Sepal Receptacle Ovule Flowers • 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

19. Fruits • Usually consist of a mature ovary • Several different types of fruit • Can be either fleshy or dry • Strawberries, grapes • Beans and peas, nuts • Can be dispersed by wind, water or animal

20. (b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp (a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp (c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp (d) Milkweed, a dry fruit thatsplits open at maturity (e) Walnut, a dry fruit that remains closed at maturity Types of Fruits

21. (a) Wings enable maple fruits to be easily carried by the wind. (b) Seeds within berries and other edible fruits are often dispersed in animal feces. (c) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. Types of Seed Dispersal

22. 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

23. Key 1 Anthers contain microsporangia. Each microsporangium contains micro- sporocytes (microspore mother cells) that divide by meiosis, producing microspores. 2 Microspores form pollen grains (containing male gametophytes). The generative cell will divide to form two sperm. The tube cell will produce the pollen tube. Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell 7 When a seed germinates, the embryo develops into a mature sporophyte. Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary Pollen grains MEIOSIS Germinating Seed Stigma 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 Pollen tube Megasporangium (n) Embryo (2n) Sperm Endosperm (food Supply) (3n) 6 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). Surviving megaspore (n) Seed Pollen tube Seed coat (2n) Style Antipodal cells Polar nuclei Synergids Egg (n) Female gametophyte (embryo sac) Pollen tube Zygote (2n) Nucleus of developing endosperm (3n) Egg Nucleus (n) Sperm (n) After pollina- tion, eventually two sperm nuclei are discharged in each ovule. 4 FERTILIZATION 5 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. Discharged sperm nuclei (n)

24. Carpel Stamen (b) (a) Artist’s reconstruction of Archaefructus sinensis Archaefructus sinensis, a 125-million-year- old fossil. 5 cm Angiosperm Evolution • Angiosperms originated at least 140 million years ago • And during the late Mesozoic, the major branches of the clade diverged from their common ancestor • Primitive fossils of 125-million-year-old angiosperms display both derived and primitive traits Anthers and seeds but no petals or sepals

25. 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 • Only about 100 species • Magnoliids • Share some traits with basal angiosperms but are more closely related to monocots and eudicots • Very old lineage, fossil records of manoliid pollen

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

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