Flowering plants

Early steps in the evolution of angiosperms

1. *evolution of vessels*: Angiosperms have efficient water conduction through vessel elements and *libriform fibers* (fibers of dead sclerenchyma cells with thick lignifies walls, proves support & protection) evolved from tracheids. • These vessels differ from tracheids because they are shorter, wider, and each has an open ending that allows it to stack upon other to conduct wanter. 2. *Carpels fused by tissue connection*: The carpels of mesangiosperms are fused by a tissue connection. This fusion occurred when a sticky assisting fluid seals the carpel together. Later in development, a tissue connection deals the carpel shut.

The Mesangiosperms

Includes the three most recently evolved groups of angiosperms: the magnoliids, monocots, and eudicots.

Magnoliids

Includes trees and shrubs with net-veined, aromatic leaves. • Sister to monocots and eudicots. • They have flowers with non-fused parts • They have *monosulcate pollen* with a single, linear, thinned, furrow-like area on the grain. • Have many petals, stamens, and carpels which vary from plant to plant. • Flat carpels with a long stigma that has a broad surface. This differs from the carpel of later angiosperms that have a narrow surface. • Flat stamens with the microsporangia spread along the surface. • Magnolias have large white fragrant flowers

Amborella trichopoda

Believed to be the earliest diverging angiosperm. • Only found in Madagascar. • Water lilies and star anise were later to develop.

Pollen

Contains two haploid nuclei • Can be delivered to the ovule via water, wind or animal. • It is often dry until it lands on the female stigma, and receives a signal to hydrate itself.

Secondary growth

Occurs when a plant gets larger in diameter due to the thickening of the stem and roots. • Some woody plants have a waterproof cork cambium that inhibits gas exchange. • This is especially true for birch bark, which repels water • *Lenticels* are small openings that lie on the stems of woody eudicots to assist in gas exchange

Fruits

One of the synapomorphies of angiosperms is the presence of fruit • A *fruit* is simply a swollen ovary that contains seeds. • Diversity in fruits related to the diversity in dispersal mechanisms

Stamen

The part of a slower that is the male reproductive unit of the plant, holding the microsporangia. • It contains a *filament* (or stalk) and an *anther* • The anther is part of the plant that produces pollen

Receptacle

The portion of the flower where the floral organs are attached.

Pollination after double fertilization

• After double fertilization, all of the other nuclei in the developing ovule disintegrate, so all that is left are the 3n primary endosperm nucleus and the 2n zygote. • The primary endosperm rapidly divides through mitosis to fill the space around the zygote. This tissue nourishes the embryo. • The integuments completely grow over to form the seed coat. • The ovule will therefore develop into a seed, and the ovary will develop into the fruit that encloses the seed. • Double fertilization thus allows the plant to delay energy investment into nutritive tissue until after fertilization has occurred. • After the endosperm nucleus is formed, it divides rapidly to form the nutritive tissue for the developing embryo

Development of the Game Gametophyte (Microgametophyte) (seeded plants)

• In flowering plants, the male gametophyte begins to develop at the anther. • The anther has several chambers of multiple microspore mother cells, which are all diploid (2n). • These cells undergo meiosis, producing four haploid (n) microspores for every diploid cell that undergoes meiosis. As this occurs, the cell walls disintegrate. • In one of these haploid (n) microspore nuclei, mitotic division occurs to produce two haploid nuclei. • The wall of the nucleus hardens to become a *pollen grain*, containing two haploid nuclei. • One of these haploid nuclei is the *generative nucleus*, which will eventually form sperm • The other nucleus is the *tube nucleus* which will eventually migrate to the tip of the pollen tube. • At this point the male gametophyte is mature

Eudicots vs Monocots : germination

• Monocots and eudicots exhibit different patterns of germination due to their structure. For eudicot germination, the seed will extend the embryotic rood (the *radicle) down and shoot tip up in a process called *aerial germination*. • Many dicots break the surface by extending the radical downward and forming a hook that is "pulled" up to the surface. The cotyledons emerge as seed leaves, but eventually shrivel and wall away. • Monocots, such as maize and other grasses, germinate differently. They "push" the coleoptile (the protective tissue that covers the apical meristem) upward through the soil, and the shoot tip grows straight up. • This process is known as *subterranean germination*

Cotyledon

A leaf-life structure inside the seed that serves as an absorptive structure between the endosperm and the embryo • As the seed develops, the cotyledon absorbs some of the nutrition from the endosperm much that the endosperm gets smaller and the cotyledon gets larger. • Eventually, the energy stored in the cotyledon is used in the development of the embryo

Mesangiosperms

A monophyletic group with the most recent angiosperms to develop that include magnoliids, monocots and eudicots.

Monoecious plant

A plant that has both staminate and carpellate flowers. • They are thus able to self fertilize.

Dioecious plant

A plant that has only staminate or carpellate flowers. • Exists on separate plants, and cannot self fertilize.

Parts of a flower

A typical flower has four parts: sepals, carpels, stamens and petals

Differences between Gymnosperm seeds and Angiosperm seeds

Both are seed bearing plants. Two major differences: 1. The nutritive tissue in a gymnosperm seed is the megagametophyte tissue. In an angiosperm, the nutritive tissue is the triploid endosperm. 2. The seed coats in gymnosperms are derived from a single integument, while the seed coats in angiosperms are derived from two integuments.

Double fertilization

In plant reproduction, two sperm perform different functions: • One sperm fuses with the egg to form the diploid (2n) zygote • One sperm fuses with the two polar nuclei to form a triploid (3n) nucleus. This is the primary endosperm nucleus. • Double fertilization thus allows the plant to delay energy investment into nutritive tissue until after fertilization has occurred.

Angiosperms

Flowering plants • Plants with seeds that are surrounded by the ovule. • They quickly became the dominant plants on earth, with reproductive advantages over other plants

Taproot system

Gymnosperms and eudicots have a taproot system in which one main vertical taproot develops from the embryonic root. • The taproot branches off and gives rise to lateral roots. • Penetrates deep into the group to access water and nutrients.

Pollination in monoecious plants

Monoecious plants contain both the carpellate and the staminate. • The male and female gametophytes coming from the same plant hinders genetic variability. • Many monoecious angiosperms have developed mechanisms that make it either impossible or difficult to self fertilize. • For example, *self-incompatibility* refers to the plant's ability to reject its own pollen: When a grain of the flower's own pollen lands on the stigma, it is prevented from fertilizing the egg by a biochemical block that prevents the growth of a pollen tube. • Self-incompatibility is the most common mechanism by which a plant is prevented from fertilizing itself.

Pollination

Pollination occurs when pollen is transfered from an anther to a stigma. • This can be achieved by abiotic agents (wind and water) or by biotic agents (animals such as butterflies or bees). • Flower often attract pollinators by developing special color patterns or structural characteristics, such as alternative colors when placed under UV light.

Adventitious roots

Seedless plants and monocots have a fibrous root system in which the embryonic root does not give rise to a taproot. • Instead,the roots of the seedling are replaced by roots arising from the stems or leaves. • There is no main root, so the root system does not penetrate deep in the ground.

Female parts of the flower

The *ovary, style and stigma* are the female parts of the plant • They are collectively called the *carpel*. • This is where the megasporangia are housed. • The sigma is sticky to trap pollen. Mature ovaries develop into a fruit. Inside the ovary are embryonic seeds called *ovules*. These eventually develop into the seeds of the fruit. • Some flowers have a single carpel as a female unit.

Nonreproductive organs of a flower

The *sapals* and *petals* are nonreproductive organs. • Sapals generally have a greenish color, whereas the petals have a different color. • Sapals protect the flower, and petals attract pollinators. • The sepals collectively form a whorl called the *calyx*, and the petals collective form a whorl called the *corolla*. • Together, the corolla and calyx form the *perianth

Development of the female gametophyte (macrogametophyte)

The ovary comes from parental tissue, meaning that it is diploid (2n). • Within the ovary are a number of differentiated tissues called ovules. • Initially, each of these ovules consists of an embryonic seed coat called an *integument* and a large diploid megaspore mother cell. • As the ovule develops, it divides through meiosis to form four haploid nuclei called *megaspores* • Three of these haploid nuclei degenerate, and one remains. At this point, the integument (the embryonic seed coat) begins to grow over the ovule. • The remaining megaspore is what forms from the megagametophyte (the ovule), which produces the aregonia and eggs. • It undergoes mitosis and divides three times, forming a total of eight nuclei in the embryo sac. The nuclei position themselves in specific locations in the developing embryo sac and are thus assigned individual names. • All eight are genetically identical, but they will be used for different purposes. • The antipodal cells and synergids will eventually degenerate, leaving only the egg cell and the two polar nuclei . • this ovule contains a mature female gametophyte. • The structure is nutritionally dependent on the sporophyte (cannot make food on its own)

Shifts in Gametophyte and Sporophyte dominance

There have been dramatic shift in the dominance of the gametophyte and sporophyte generation in the history of the evolution of embryophytes • In bryophytes (earliest land plants), the gametophyte is the dominant generation, and the sporophyte is nutritionally dependent of the gametophyte • In the monilophytes and lycophytes, the sporophyte and gametophyte generation are independent from one another • In seed plants, the sporophyte is the dominant generation and the gametophyte is nutritionally dependent on the sporophyte.

Monocots and eudicots

Type of Mesangiosperm • Sister taxon to magnoliids. • There are a number of differences between the monocots and eudicots: 1. *number of cotyledons*: Monocots develop from seeds with a single cotyledon. Eudicots develop from seeds that have tow cotyledons. 2. *leaf venation*: The netlike veins of the Mesangiosperms contain the vascular tissue (xylem and phloem). Monocots have leaves with parallel veins, eudicots have netlike veins (called reticulate venation 3. *Root system*: Monocots have the a fibrous root system called adventitious roots (roots arising from stems or leaves), while eudicots have a taproot system. 4. *Secondary growth*: Secondary growth is more common in eudicots that in monocots. Therefore, monocots are generally herbaceous, whereas eudicots can either be woody or herbaceous. 5. *Stem anatomy*: The vascular bundles of monocots are scattered throughout the stem, so the cortex and pith are indistinguishable. The vascular bundles of eudicots, are arranged in a ring and the cortex (on the outside) is distinguishable from the pith (on the inside) 6. *Pollen characteristics*: Monocots have pollen grains with a single germination aperture (pores or slits). Eudicots has pollen with three germination aperture. 7. *Germination* 8. *Flower parts*: Monocot flower parts are in groups of three or multiples of three. Eudicots typically have flower parts in groups of four or five.

Perfect versus imperfect flower

• A *perfect flower* has functional stamens and a carpel. As a result it is hermaphroditic (bisexual) • An *imperfect flower* is unisexual because it either has stamens, or a carpel, but not both.

Major synapomorphies of angiosperms

• *Flowers*: highly visual reproductive parts of angiosperms. Flowers emerge from modified stems, and flower parts develop from modified leaves • *Double fertilization*: They take place in a process in which two fertilizations take place. One sperm fertilizes the egg to form a diploid zygote, while the other sperm fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm. • This results in the production of a triploid nutritive tissue called the *endosperm* which nourishes the sporophyte during early development • *Fruits*: Mature ovary turns into a protective fruit with embryonic seeds called *ovules*. Protects seeds and promotes dispersal. • Embryonic seeds called ovules are enclosed in the female part of the plant called the *carpel*, which consists of an ovary, style and stigma.

Nuclei positions in female gametophyte (seeded plants)

• *synergids* are located near the *micropyle* (small opening in the surface of the ovule. • *polar nuclei* are located in the middle • *antipodal cells* are located farthest away from the micropyle

Pollination and fertilization in flowering plants (until double fertilization)

• The female gametophyte remains inside the ovule. It has seven cells and eight nuclei (the synergids, antipodal, and polar nuclei) • The male gametophyte is in the pollen grain. It has two cells (the tube cell and the generative cell) • When a pollen grain lands on the stigma, it germinates, extending a tube called a *pollen tube* • The haploid tube cell migrates to the tip of the pollen tube, and the generative cell divides through mitosis to form two sperm cells. • The pollen tube continues growing to the micropyle of the ovule. • when it connects the pollen grain to the ovule, both sperms cells enter the female gametophyte. • It is important that both sperm cells enter the female gametophyte to reproduce in what is called *double fertilization*