PLANT DIVERSITY II: THE EVOLUTION OF SEED PLANTS CHAPTER 30

Dates for plant group origins: Mosses Ferns 420 Gymnosperms 360 Angiosperms 140

Overview of Seed Plant Evolution Reduction of the gametophyte –Male gametophyte becomes pollen grains –Female develops inside the ovule Gymnosperms –use seeds to disperse offspring –Use pollen for fertilization eliminating the need for water for fertilization Angiosperms –Dispersed seeds inside fruits –evolved flowers to attract pollinators

Origins Angiosperms and Gymnosperms evolved from the progymnosperms –Origins of seeds at about 360 MYA –More suited to a drier climate forming on land at the time Angiosperms with flowers & fruits form much later around 140 MYA

gymnosperms are in four divisions angiosperms are all in one division both are monophyletic groups

Gametophyte reduction

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) Sporophyte (2n), the flowering plant (independent) Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n) (independent) Microscopic female gametophytes (n) inside these parts of flowers (dependent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). (c) Gametophyte (n) Gametophyte (n) Sporophyte (2n) Sporophyte (2n) Gametophyte/sporophyte

Retention of gametophyte Evolution has not eliminated gametophyte Gametophyte may provide a mechanism for “screening” new alleles All sporophyte embryos are dependent, at least to some extent, on tissues of the female gametophyte.

Heterosporous Makes two different types of sporangia. Megasporangia give rise to ovules, which are retained in the sporophyte Microsporangia give rise to pollen

Unnumbered figure page 586 Homosporous spore production Sporangium in sporophyll Single type of spore Typically a bisexual gametophyte Eggs Heterosporous spore production Megasporangium in megasporophyll Megaspore Female gametophyte Microsporangium in microsporophyll Microspore Male gametophyte Sperm Eggs Sperm What about Mosses???

An ovule consists of: integuments (2n), megaspore (n), and megasporangium.

Figure 30.3 From ovule to seed 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.) (a) 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. (b) 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) Integument Spore wall Megasporangium (2n) Megaspore (n) Male gametophyte (within germinating pollen grain) (n) Female gametophyte (n) Egg nucleus (n) Discharged sperm nucleus (n) Pollen grain (n) Micropyle Seed coat (derived from integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte)

What’s new in Gymnosperms? Pollen Seeds Greatly reduced gametophyte

The Pine Life cycle Fig. 30.9

The resistant seed coat is derived from the integuments of the ovule. A seed may remain dormant for years The gymnosperms the seed is released from the cone Angiosperms retain seeds inside fruit

Pollen (Staminate) Cone: Produced yearly Short lived- die after releasing pollen

Pollen are the male gametophytes No longer need film of water to fertilize

Ovulate cone very small and fleshy when young Forms the “Pine Cone” with seeds –often matures over 2-3 years

Ovule Forms inside female (ovulate) cone Zygote develops into embryo, inside nutritive female gametophyte tissue. Embryo goes dormant, integuments harden and ovule now becomes a seed

Gnetum Ephedra Ovulate cones Welwitschia PHYLUM GNETOPHYTA PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta

Douglas fir Pacific yew Common juniper Wollemia pine Bristlecone pine Sequoia PHYLUM CONIFEROPHYTA

A progymnosperm

What’s new in angiosperms? Fruits (3n) endosperm in seeds Flowers Fibers and Vessels

Angiosperms Cones modified into flowers Pollen delivered to flower by pollinator Ovules enclosed inside an ovary Double fertilization Ripened ovary (with seeds inside) is the fruit Fruits disperse seeds Coevolved with pollinators and disperses Have complex xylem with fibers and vessels. Not more independent from water. About 160 MYA

The flower Sepals Petals Stamens (male) –Anther - pollen –Filament Carpel (female) –Stigma –Style –Ovary –Ovule-eggs

Petals attract animal pollinators –Many flowers also have nectaries to reward Stamens consists of: –the filament- –the anther Carpels produce female gametophytes. –stigma. –style –ovary –ovule

Fig Fruit development

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

The life cycle of an angiosperm Pollen tube Sperm Stigma Pollen grains Pollen tube Style Discharged sperm nuclei (n) Egg nucleus (n) Mature flower on sporophyte plant (2n) Key Haploid (n) Diploid (2n) Anther Ovule with megasporangium (2n) Male gametophyte (in pollen grain) Microspore (n) MEIOSIS Microsporangium Microsporocytes (2n) MEIOSIS Generative cell Tube cell Surviving megaspore (n) Ovary Megasporangium (n) Female gametophyte (embryo sac) Antipodal cells Polar nuclei Synergids Egg (n) Pollen tube Sperm (n)

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

Figure A primitive flowering plant? Carpel Stamen Archaefructus sinensis, a 125-million-year- old fossil. (a) Artist’s reconstruction of Archaefructus sinensis (b) 5 cm

Gametophyte formation Fig 38.4

Gametophyte growth

Pollen Tube growth

Double fertilization Fig 38.9 Two sperm nuclei enter the ovule. One fertilizes the egg –becomes a zygote –develops into the embryo The second sperm nucleus fuses with the (2n) polar nuclei. –Forms a triploid (3n) nucleus, the endosperm –Divides by mitosis and takes over remains of ovule forming a large multinucleate cell mass. –Endosperm forms before embryo develops –Is nutritive tissue for embryo (at least initially)

Angiosperm Diversity All in one division Several early groups We’ll look at two classes –Monocots –Eudicots

Origin of the ovary (Goethe) The enclosure of seed within the ovary (the carpel), probably evolved from a seed- bearing leaf that became rolled into a tube.

Fruit – Fig A fruit is a mature ovary. As seeds develop from ovules after fertilization, the wall of the ovary thickens to form the fruit. Fruits protect dormant seeds and aid in their dispersal. This flower has an ovary consisting of a single carpel with multiple ovules /seeds.

Both gymnosperms and angiosperms have tracheids Angiosperms have two additional xylem cell types: –Fibers –Vessels Fig

Pollination

Angiosperm seed formation

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)

Orchid (Lemboglossum rossii) Monocot Characteristics Embryos Leaf venation Stems Root Pollen Flowers Pollen grain with one opening Root system Usually fibrous (no main root) Vascular tissue scattered Veins usually parallel One cotyledonTwo cotyledons Veins usually netlike Vascular tissue usually arranged in ring Taproot (main root) usually present Pollen grain with 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 Characteristics MONOCOTS EUDICOTS

Figure Flower-pollinator relationships 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. (a) 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. (c) 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. (b)

Medicines Derived from Seed Plants

Unnumbered figure page 607 Green algae Mosses Fems GymnospermsAngiosperms