Biology 200 Chapter 23: Angiosperms

Introduction to Angiosperms

• Angiosperms are defined as flowering plants. • Seeds are enclosed within a structure called a carpel. • The carpel resembles a leaf that has folded over and fused at the margins. • A pistil may be composed of a single carpel, or it may consist of two or more united carpels. • The seed develops from an ovule located within the carpel. • The ovary matures and becomes a fruit. • Notes credited to Kingsley Stern.

Phylum Magnoliophyta Classification and Primitive Characteristics

• All angiosperms are grouped within the Phylum Magnoliophyta. • This phylum is divided into two large classes:     • Magnoliopsida: Commonly known as Dicots. DNA and cladistic evidence suggest that two distinct groups of dicots should be recognized.     • Liliopsida: Commonly known as Monocots. • The flower is considered a modified stem that bears modified leaves. • Characteristics of the most primitive flowers include:     • A long receptacle.     • Many flower parts that are spirally arranged.     • Flower parts that are separate and not differentiated into sepals and petals.     • Stamens and carpels that are flattened and numerous.

Diversity of Flowering Plants

• Phylum Magnoliophyta exhibits great variation in overall size. • Approximately $4,000$ species of flowering plants are parasitic.     • Parasitic angiosperms intercept food and water within the xylem and phloem of a host plant. • Some angiosperms are saprophytes, obtaining nutrients from dead organic matter.     • Examples include certain types of orchids and the Snowplant. • Illustrations and photographs provided by Phil Schermeister/Getty Images.

Sporophytes and Gametophytes

• Angiosperms are heterosporous. • The sporophyte generation is dominant. • Female gametophytes are wholly enclosed within sporophyte tissue and are reduced to only a few cells. • Mature male gametophytes consist of a germinated pollen grain containing $3$ nuclei.

Development of the Female Gametophyte

• A diploid ($2n$) megasporocyte differentiates within the ovule. • The megasporocyte undergoes meiosis to produce $4$ haploid ($n$) megaspores. • Three of the megaspores degenerate. • The remaining cell enlarges and its nucleus undergoes division to produce $8$ nuclei (without cell walls initially). • The outer two layers of the ovule differentiate into integuments, which later develop into the seed coat. • The micropyle is an opening at one end of the ovule. • The $8$ nuclei form two groups, with $4$ located near each end of the cell. • One nucleus from each group migrates to the center of the cell to form the central cell. • Cell walls eventually form around the remaining $6$ nuclei:     • The egg and two synergids are located closest to the micropyle.     • Three antipodals are located at the opposite end; these have no apparent function. • The female gametophyte is also known as the megagametophyte or embryo sac. It is a large sac containing $8$ nuclei and $7$ cells.

Development of the Male Gametophyte

• Preparation and formation of male gametophytes take place in the anthers. • Four patches of microsporocyte cells differentiate within the anther, corresponding to pollen sacs. • Each microsporocyte undergoes meiosis to produce a quartet of haploid microspores. • Microspores undergo three significant changes:     • They divide once via mitosis to form a small generative cell inside a larger tube cell.     • The nucleus of the tube cell is referred to as the vegetative nucleus.     • The members of each quartet of microspores separate from one another.     • The wall becomes two-layered.         • The outer layer is the exine, which is finely sculptured and contains chemicals that may react with chemicals in the stigma. • The generative nucleus will later divide to produce $2$ sperm.

Pollination and Fertilization

• Pollination is the transfer of pollen grains from the anther to the stigma. • Self-pollination occurs when pollen grains germinate on the stigma of the same flower. • Fertilization is the union of sperm and egg. • Pollination agents include insects, wind, water, animals, or gravity. • After pollination, the male gametophyte may only continue development if the pollen grain is from a different plant of the same species or from a different variety than the receiving flower. • The pollen tube grows between the cells of the stigma and style until it reaches the ovule micropyle. • The vegetative nucleus remains at the tip of the pollen tube, while the generative cell lags behind and divides into $2$ sperm. • The pollen tube enters the female gametophyte, destroying a synergid in the process, and discharges the sperm. • A mature male gametophyte is a germinated pollen grain with its vegetative nucleus and $2$ sperm within the tube cell.

Double Fertilization and Seed Development

• Double fertilization is a characteristic process where:     • One sperm unites with the egg, forming a zygote ($2n$), which then develops into the embryo.     • The other sperm unites with the two central cell nuclei, producing a triploid ($3n$) endosperm nucleus. • This nucleus develops into endosperm tissue, which serves as nutritive tissue for the embryo. • In some monocots, such as corn and other grasses, the endosperm becomes an extensive part of the seed. • Wheat, rice, and corn are major sources of human nutrition due to the quality of the endosperm. • In most dicots, the endosperm is absorbed into the cotyledons. • The ovule matures into a seed, the ovary matures into a fruit, and the integuments harden into a seed coat.

Variations in Gametophyte Development

• Female gametophytes can have between $4$ and $16$ nuclei or cells at maturity. • Endosperm ploidy may vary, resulting in $5x$, $9x$, or $15x$ tissue. • Example in Lilies (James E. Bidlack):     • All $4$ haploid megaspore nuclei produced from the megasporocyte remain functional.     • Three of the nuclei unite to form a $3x$ nucleus, while the fourth remains haploid ($1x$).     • This results in a female gametophyte with four $3x$ nuclei and four $1x$ nuclei.     • One central cell nucleus is $3x$ and the other is $1x$.     • Fertilization by a $1x$ sperm results in a $5x$ endosperm nucleus ($3x + 1x + 1x = 5x$).

Apomixis and Parthenocarpy

• Apomixis: Occurs without the fusion of gametes, though normal structures are otherwise involved. Found in some grasses. The embryo develops from a diploid nutritive cell or another diploid cell of the ovule rather than from a zygote. This results in a vegetatively propagated plant. • Parthenocarpy: Fruits develop from ovaries containing unfertilized eggs. This results in seedless fruits, such as navel oranges and bananas.

Trends of Specialization and Classification

• Historical classifications were based on convenience, but modern botanists group plants according to natural relationships based on evolution. • The fossil record suggests flowering plants first appeared approximately $160$ million years ago during the late Jurassic. • Flowering plants developed further during the Cretaceous and Cenozoic and are the dominant plants today. • Evolutionary trends in flower morphology:     • Flower parts have become fewer in number.     • Some parts have fused.     • Spiral arrangements have been compressed into whorls.

Primitive vs. Specialized Flowering Plants

• Primitive Flowering Plants:     • Simple leaves.     • Flowers with numerous, spirally arranged parts that are not fused to each other.     • Radially symmetrical (regular) flowers.     • Flowers with both stamens and pistils (complete and perfect).     • Superior ovary (hypogynous flower). • Specialized Flowering Plants:     • Flower parts are fewer and definite in number.     • Spiral arrangements are compressed into whorls.     • Bilaterally symmetrical (irregular) flowers.     • Reduction and fusion of floral parts.     • Incomplete or imperfect flowers.     • Inferior ovary.

Pistil Evolution and Ovary Position

• The first pistil developed from a leaflike structure with ovules along the margins, called a carpel. • The edges of the blade rolled inward and fused together. • Separate carpels of primitive flowers fused to form a compound pistil consisting of several carpels. • Ovary positions include:     • Inferior ovary (epigynous flower): The receptacle or other flower parts are fused to the ovary and have grown up around it. The calyx and corolla appear to be attached to the top of the ovary.     • Superior ovary (hypogynous flower): The ovary is produced on top of the receptacle. Other flower parts are attached around the base of the ovary.     • Perigynous flowers: Flower parts are attached to a corolla tube made of fused petals, creating a floral tube that is not attached to the ovary.

Flower Completeness and Sexuality

• Complete flower: Possesses a calyx, corolla, stamens, and pistil. • Incomplete flower: Missing a corolla or other flower parts. • Perfect flower: Both stamens and pistil are present. • Imperfect flower: Either stamens or pistil are missing. • Monoecious species: Male and female imperfect flowers are found on the same plant. • Dioecious species: The plant bears only male flowers, while other plants of the same species bear only female flowers.

Pollination Ecology and Coevolution

• Pollinators and plants have coevolved. There are approximately $20,000$ bee species among current pollinators. • Bee-pollinated flowers:     • Generally brightly colored (mostly blue or yellow).     • Often feature lines or distinctive markings acting as honey guides to lead bees to nectar.     • Markings may be visible only in UV light, which bees can see but humans cannot. • Beetle-pollinated flowers:     • Strong, yeasty, spicy, or fruity odor.     • White or dull in color.     • May not secrete nectar, instead furnishing pollen or food in special storage cells on petals. • Fly-pollinated flowers:     • Smell like rotten meat (e.g., Stapelia species).     • Dull red or brown in color. • Butterfly- and Moth-pollinated flowers:     • Often possess sweet fragrances.     • White or yellow for night-flying moths; red, blue, yellow, or orange for butterflies.     • Nectaries located at the base of corolla tubes or spurs for long tongues. • Bird-pollinated flowers (Hummingbirds and Sunbirds):     • Often bright red or yellow.     • Little to no odor (birds have a poor sense of smell).     • Large size, part of sturdy inflorescences.     • Copious nectar production for highly active birds.     • Characterized by long floral tubes. • Bat-pollinated flowers:     • Primarily in the tropics.     • Open at night during foraging hours.     • Dull in color.     • Large enough for a bat to insert its head or consist of ball-like inflorescences containing many small flowers.

Orchid Flowers and Pollinating Mechanisms

• Orchids have specialized pollinators across all categories. • Pollen grains are produced in sacs called pollinia (singular: pollinium) with sticky pads at the base. • Specific mechanisms include:     • Mechanism 1: Stigma is covered with pointed bumps to brush sticky pollen from an insect's head. A flap yields to an insect's weight, forcing it to exit via an opening near the stigma and anthers.     • Mechanism 2: An insect follows a nectar-secreting furrow. Pollinia are removed when they adhere to the insect after an "explosion" triggered by the insect bumping a sensitive point below an anther.     • Mechanism 3: Orchids pollinated by moths/butterflies may have a clamp at the base of the pollinia. When an insect touches a sensitive area with its tongue, the clamp attaches the pollinia to the tongue.     • Mechanism 4: In showy orchids, pollinia twist after attachment to the insect so they are positioned to deposit into separate stigma patches on the next flower.     • Mechanism 5 (Ophrys): The petal resembles a female bumble bee or wasp. Male insects attempt to copulate with the flower, resulting in pollinia being deposited on their heads (pseudocopulation).     • Mechanism 6 (Bucket Orchids): Faucet glands fill a bucketlike petal with fluid. Insects fall into the fluid and must escape through a trapdoor, ensuring contact with the stigma and anthers containing pollinia.

Herbaria and Plant Preservation

• Herbaria (singular: herbarium) are libraries of dried, pressed plants, algae, and fungi that are arranged and labeled. • Preservation methods:     • Fungi and bryophytes are dried and stored in small packets.     • Vascular plants are dried using a plant press.     • Vascular plant specimens are mounted on $100\%$ rag herbarium paper. • Specimens are stored to allow for easy retrieval and study.