Bryophytes/Seedless Vascular & Seed Plants

Alternation of generations (All plants have life cycles with an alternation of generations)

-A haploid gametophyte generation produces and alternates with a diploid sporophyte generation. The sporophyte, in turn, produces gametophytes. -The life cycles are heteromorphic; sporophytes and gametophytes differ in morphology. -The sporophyte is larger than the gametophyte in all plants except mosses and their relatives.

Ferns

- Appearing in the Devonian, ferns radiated into diverse species that coexisted with tree lycopods and horsetails in the great Carboniferous forests. 1. Ferns are the most well represented seedless plants in modern floras.

Immature male gametophytes:

-Are pollen grains, which develop within the anthers of stamens -Each pollen grain has two haploid nuclei that will participate in double fertilization characteristic of angiosperms.

Adaptations to shallow water preadapted plants for living on land 2

-Natural selection may have favored shallow-water plants tolerant to periodic drying. Adaptations to shallow water may also have been preadaptive for terrestrial life. (Examples: Waxy cuticles, Protection of gametes, Protection of developing embryos.) -Eventually, accumulated adaptations made it possible for ancestral plants to live permanently above the water line, opening a new adaptive zone with: Sunlight unfiltered by water and algae, Soil rich in minerals, Absence of terrestrial herbivores

Some species of Lycopodium are epiphytes, plants that use another organism as a substratum, but are not parasites.

-The sporangia of Lycopodium are borne on sporophylls, leaves specialized for reproduction. In some, sporophylls are clustered at branch tips into club-shaped strobili - hence the name club moss. -Spores develop into inconspicuous gametophytes. The non-photosynthetic gametophytes are nurtured by symbiotic fungi. -Most are homosporous. -Genus Selaginella is heterosporous.

The gametophyte is the dominant generation in the life cycles of bryophytes.

-The sporophyte (2n) produces haploid spores by meiosis in a sporanigium; the spores divide by mitosis to form new gametophytes. -Contrary to the life cycles of vascular plants, the haploid gametophyte is the dominant generation in mosses and other bryophytes. Sporophytes are generally smaller and depend on the gametophyte for water and nutrients.

Emergence of flowering plants during the early Cretaceous, about 130 mya.

-Unlike gymnosperms, flowering plants bear seeds within the flower's protective ovaries. -Most contemporary plants are flowering plants or angiosperms.

Structural, chemical, and reproductive adaptations enabled plants to colonize land

1. Some characteristics of plants 2. Plants as embryophytes 3. Alternation of generations. 4. Some highlights of plant phylogeny

Angiosperms (Flowering Plants)

1. The flower is the defining reproductive adaptation of angiosperms 2. Fruits help disperse the seeds of angiosperms 3. The life cycle of an angiosperm is a highly refined version of the alternation of generations common to all plants

Hornworts (Anthocerophyta)

-Hornworts resemble liverworts, but sporophytes are horn-shaped, elongated capsules that grow from the matlike gametophyte. -Their photosynthetic cells have only one large chloroplast, unlike the many smaller ones of other plants.

A sporophyte-dominant life cycle evolved in seedless vascular plants.

-The sporophyte (diploid generation) emerged as the larger and more complex plant from the time of early vascular plants.

Pteridophytes (Pteridophyta)

1. Horsetails 2. Whisk ferns 3. Ferns

Fruit = A ripened ovary that protects dormant seeds and aids in their dispersal; some fruits (like apples) incorporate other floral parts along with the ovary

Aggregate fruits = Several ovaries that are part of the same flower (e.g., raspberry) Multiple fruit = One that develops from several separate flowers (e.g., pineapple) Modifications of fruits that help disperse seeds include: -Seeds within fruits that are shaped like kites or propellers to aid in wind dispersal -Burr-like fruit that cling to animal fur -Edible fruit with tough seeds which pass through the digestive tract of herbivores unharmed, dispersing seeds miles away

Seed Plants

Angiosperms and Gymnosperms

The bryophytes include plants found in three groups:

Bryophyta (mosses) Hepatophyta (liverworts) Anthocerophyta (hornworts)

An outline of the angiosperm life cycle follows:

-Pollen from the anther lands on the sticky stigma at the carpel's tip; most flowers do not self-pollinate, but have mechanisms to ensure cross-pollination. -The pollen grain germinates on the stigma by growing a pollen tube down the style of the carpel. -When it reaches the ovary, the pollen tube grows through its micropyle and discharges two sperm cells into the embryo sac. -Double fertilization occurs as one sperm nucleus unites with the egg to form a diploid zygote; the other sperm nucleus fuses with two nuclei in the embryo sac's central cell to form triploid (3n) endosperm. -After double fertilization, the ovule matures into a seed

Whisk ferns

-Psilotum nudum -Once thought to be evolutionarily ancient descendants of anatomically simple ancestors. -DNA sequence data finally settled the question in favor of a more modern origin of whisk ferns from fern like ancestors -The whisk ferns are a clade of highly specialized plants that evolved fairly recently from anatomically more complex ancestors by loss or reduction of leaves and true roots.

Characteristics of plants: chemical adaptations

-Secretion of a waxy cuticle, helps prevent desiccation. -Cuticle waxes are secondary products (because they arise through metabolic pathways not common to all plants.) -Other secondary products include lignin (cell wall component of "woody" plants) and sporopollenin (a resilient polymer in the walls of spores and pollen grains).

Origin of the seed near the end of the Devonian about 360 mya.

-Seed = Plant embryo packaged with a store of food within a resistant coast. -Early seed plans bore seeds as naked structures and evolved into gymnosperms including conifers. -Conifers and ferns coexisted in the landscape for more than 200 million years.

Adaptations to shallow water preadapted plants for living on land

-Many modern charophytes live in shallow water, and some ancient charophytes may have also lived in shallow-water habitats subject to occasional drying. -About 440 million year ago, during the transition from Ordovician to Silurian, repeated glaciations and climatic changes caused fluctuations in the water levels of lakes and ponds.

All seed plants are heterosporous in that they possess two different kinds of sporangia, each producing a different type of spore.

-Megasporangia produce megaspores that give rise to female egg-containing gametophytes. -Microsporangia produce microspores that give rise to male sperm-containing gametophytes.

Mosses (Bryophyta)

-A tight pact of many moss plants forms a spongy mat that can absorb and retain water. -Each plant grips the substratum with rhizoids, elongate cells or cellular filaments. -Photosynthesis occurs mostly in the small stem-like and leaflike structures found in upper parts of the plant; these structures are not homologous with stems and leaves in vascular plants. -Mosses cover about 3% of the land surface, and they contain vast amounts of organic carbon.

The relatively harsh terrestrial environment led to the development of resistant structures for the dispersal of offspring.

-Bryophytes (nonvascular) and seedless vascular plants produce and release hardy single-celled spores. -Seeds are more hardy because of their multicellular nature.

The green algae are likely the photosynthetic organisms most closely related to land plants. This conclusion is based on homologies in:

-Cell wall composition -Structure and pigmentation of chloroplasts

Origin of plants from aquatic ancestors (probably green algae) in the Ordovician about 475 million years ago (mya).

-Cuticle and jacketed gametangia evolved which protected gametes and embryos. -Vascular tissue evolved with conducting cells that transport water and nutrients throughout the plant.

A pollen grain near an ovule will extend a tube through sperm cells into the female gametophyte within the ovule.

-Cycad's sperm cells are flagellated (ancestral condition). -Other gymnosperms (including conifers) and angiosperms do not have flagellated sperm cells.

Characteristics of plants: structural adaptations

-Developed structural specializations to extract the resources needed for photosynthesis (water, minerals, carbon dioxide, light). -Gas exchange occurs via stomata, special pores on the surfaces of leaves.

Female gametophytes:

-Do not produce an archegonium -Are located within an ovule -Consist of only a few cells: an embryo sac with eight haploid nuclei in seven cells (a large central cell has two haploid nuclei) -One of the cells is the egg

Alternation of generations (Comparison of life cycles among plant divisions)

-Dominance of the diploid sporophyte. -Flagellated sperm replaced by pollen.

Bryophytes display a pivotal adaptation that made the move onto land possible: the embryophyte condition.

-Gametangia protect developing gametes. a. Antheridium produces flagellated sperm cells. b. Archegonium produces a single egg; fertilization occurs within the archegonium, and the zygote develops into an embryo within the protective jacket of the female organ (embryophyte condition).

Plants as embryophytes (With the move from an aquatic to terrestrial environment, a new mode of reproduction was necessary to solve two problems.)

-Gametes must be dispersed in a non-aquatic environment. Plants produce gamete within gametangia, organs with protective jackets of cells that prevent gametes from drying out. -Embryos must be protected against desiccation. The zygote is fertilized and develops into an embryo within the female gametangia's jacket of protective cells.

Homologous chloroplasts

-Green algae and plants both have the accessory pigments, chlorophyll b and beta-carotene. -Green algae and plants both have chloroplasts with thylakoid membranes stacked as grana. -Compared to chloroplast DNA of various green algae, plant chloroplast DNA most closely matches that of charophytes.

Lycophytes (Lycophyta)

-The Lycophyta includes the club mosses and ground pines. -Lycophytes survived through the Devonian period and dominated land during the Carboniferous Period (340-280 million years ago). -Some are temperate, low-growing plants with rhizomes and true leaves.

The sporophyte-dominant life cycle is exemplified by ferns, one group of the seedless vascular plants:

-The familiar leafy plant is the sporophyte. -Gametophytes are quite small and grow on or below the surface of the soil.

Horsetails

-The only existing genus is Equisetum -Lives in damp locations and has flagellated sperm -Is homosporous -Has a conspicuous sporophyte generation ie. Equisetum hyemale

Liverworts (Hepatophyta)

-Liverworts are less conspicuous than mosses. -They sometimes have bodies divided into lobes. -They have a life cycle similar to mosses. Their sporangia have elaters, coil-shaped cells, that spring out of the capsule and disperse spores. -They can also reproduce asexually from gemmae (small bundles of cells that can bounce out of cups on the surface of the gametophyte when hit by rainwater). -They display their greatest diversity in tropical forests.

Life cycles of angiosperms are heterosporous (in common with all seed plants) and the two types of sporangia are found in the flower:

-Microsporangia in anthers produce microspores that form male gametophytes. -Megasporangia in ovules produce megaspores that develop into female gametophytes.

The conifers are the largest group of gymnosperms.

-Most are evergreens: pines, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods all belong to this division. -Includes some of the tallest (redwoods and some eucalyptus); largest (giant sequoias); and oldest (bristlecone pine) living organisms. -Most lumber and paper pulp is from conifer wood.

Biochemical similarity

-Most green algae and plants contain cellulose in their cell walls. -Charophytes are the most plantlike in was composition with cellulose making up 20% - 26% of the wall material. -Charophyte peroxisomes are the only algal peroxisomes with the same enzyme composition as plant peroxisomes.

The development of the seed is associated with the megasporangia.

-The megasporangium of seed plants is not a chamber, but a fleshy structure called a nucellus. -Additional tissues called integuments surround the megasporangium (contribute to the protective coat). -Megaspore, megasporangium, and integuments—is called an ovule. -The female gametophyte develops within the wall of the megaspore and is nourished by the nucellus. -If the egg cell of a female gametophyte is fertilized by a sperm cell, the zygote develops into a sporophyte embryo. -The resulting sporophyte-containing ovule develops into a seed.

The life cycle of pine, a representative conifer, is characterized by the following:

-The multicellular sporophyte is the most conspicuous stage; the pine tree is a sporophyte, with its sporangia located on cones. -The multicellular gametophyte generation is reduced and develops from haploid spores that are retained within sporangia. 1. The male gametophyte is the pollen grain; there is no antheridium. 2. The female gametophyte consists of multicellular nutritive tissue and an archegonium that develops within an ovule.

The microspores develop into pollen grains, which in turn, mature to form the male gametophores (antheridia) of seed plants.

-Pollen grains are coated with a resilient polymer, sporopollenin. -Pollen grains can be carried away by wind or animals (e.g., bees) following release from microsporangia.

Ferns are homosporous and the conspicuous leafy fern plant is the sporophyte.

-Specialized sporophylls bear sporangia on their undersides; many ferns have sporangia arranged in clusters called sori and are equipped with spring-like devices (annulus) that catapult spores into the air, where they can be blown by the wind far from their origin. -The spore is the dispersal stage. -The free-living gametophyte is small and fragile, requiring a moist habitat. -Water is necessary for fertilization, since flagellated sperm cells must swim from the antheridium to the archegonium, where fertilization takes place. -The sporophyte embryo develops protected within the archegonium.

It takes nearly three years to complete the pine life cycle, which progresses through a complicated series of events to produce mature seeds. 2

-The pine embryo, or new sporophyte, has a rudimentary root and several embryonic leaves. It is embedded in the female gametophyte, which nourishes the embryo until it is capable of photosynthesis. The ovule has developed into a pine seed, which consists of an embryo (2n), its food source (n), and a surrounding seed coat (2n) derived from the integuments of the parent tree. -Scales of the ovulate cone separate, and the winged seeds are carried by the wind to new locations. Note, that with the seed plants, the seed has replaced the spore as the mode of dispersal. -A seed that lands in a habitable place germinates, its embryo emerging as a pine seedling.

Vascular plants display two distinct reproductive strategies:

-The sporophyte of a homosporous plant produces a single type of spore (e.g., ferns); each spore develops into a bisexual gametophyte with both male (antheridia) and female (archegonia) sex organs. -The sporophyte of a heterosporous plant produces two kinds of spores: 1. Megaspores develop into female gametophytes possessing archegonia. 2. Microspores develop into male gametophytes possessing antheridia

A seed consists of a sporophyte embryo together with a food supply surrounded by a protective coat.

-The sporophytes do not release their spores, but retain them in their sporangia, as a result, the sporophyte also contains a gametophyte.

Flowering plants are the most widespread and diverse; 250,000+ species are now known.

-There are two groups; Monocots and Eudicots -Most use insects and animals for transferring pollen, and therefore, are less dependent on wind and have less random pollination

Bryophytes are not totally free from their ancestral aquatic habitat.

-They need water to reproduce. Their flagellated sperm cells must swim from the antheridium to the archegonium to fertilize the egg. -Most have no vascular tissue to carry water from the soil to aerial plant parts; they absorb water and distribute it throughout the plant by the relatively slow processes of diffusion, capillary action, and cytoplasmic streaming.

Needle-shaped conifer leaves are adapted to dry conditions

-Thick cuticle covers the leaf. -Stomata are in pits, reducing water loss. -Despite the shape, needles are megaphylls, as are leaves of all seed plants.

Conifer life cycles are heterosporous; male and female gametophytes develop from different types of spores produced by separate cones

-Trees of most pine species bear both pollen cones and ovulate cones, which develop on different branches. -Pollen cones have microsporangia; cells in these sporangia undergo meiosis producing haploid microspores, small spores that develop into pollen grains—the male gametophytes. -Ovulate cones have megasporangia; cells in these sporangia undergo meiosis producing large megaspores that develop into the female gametophyte. Each ovule initially includes a megasporangium (nucellus) enclosed in protective integuments with a single opening, the micropyle

It takes nearly three years to complete the pine life cycle, which progresses through a complicated series of events to produce mature seeds.

-Windblown pollen falls onto the ovulate cone and is drawn into the ovule through the micropyle. -The pollen grain germinates in the ovule, forming a pollen tube that begins to digest its way through the nucellus. -A megaspore mother cell in the nucellus undergoes meiosis producing four haploid megaspores, one of which will survive; it grows and divides repeatedly by mitosis producing the immature female gametophyte. -Two or three archegonia, each with an egg, then develop within the multicellular gametophyte. -More than a year after pollination, the eggs are ready to be fertilized; two sperm cells have developed and the pollen tube has grown through the nucellus to the female gametophyte. -Fertilization occurs when one of the sperm nuclei unites with the egg nucleus. All eggs in an ovule may be fertilized, but usually only one zygote develops into an embryo.

Characteristics of plants

-multicellular eukaryotes that are photosynthetic autotrophs. -Chloroplasts with photosynthetic pigments: chlorophyll a, chlorophyll b, and carotenoids. -Cell walls containing cellulose. -Food reserve that is starch stored in plastids.

Seedless Vascular Plants

1. A sporophyte-dominant life cycle evolved in seedless vascular plants. 2. The two groups of seedless vascular plants are lycophytes and pteridophytes.

The seed is a mature ovule, consisting of:

1. Embryo. The zygote develops into an embryo with a rudimentary root and one (in monocots) or two (in dicots) cotyledons or seed leaves. 2. Endosperm. The triploid nucleus in the embryo sac divides repeatedly forming triploid endosperm, rich in starch and other food reserves. 3. Seed coat. This is derived from the integuments (outer layers of the ovules).

Available evidence supports the hypothesis that plants and green algae called charophytes both evolved from a common ancestor. Researchers have found the following homologies between charophytes and plants:

1. Homologous chloroplasts 2. Biochemical similarity 3. Similarity in sperm ultrastructure. Charophyte sperm ultrastructure is more similar to certain plants than to other green algae. 4. Genetic relationship. DNA and rRNA similarities in charophytes and plants provides additional evidence for the hypothesis that charophytes are the closest relatives of plants.

The two groups of seedless vascular plants are:

1. Lycophytes 2. Pteridophytes

Highlights of plant phylogeny

1. Origin of plants from aquatic ancestors (probably green algae) in the Ordovician about 475 million years ago (mya). 2. Diversification of seedless vascular plants, such as ferns, during the early Devonian about 400 mya. 3. Origin of the seed near the end of the Devonian about 360 mya. 4. Emergence of flowering plants during the early Cretaceous, about 130 mya.

The Origin of Land Plants

1. Plants probably evolved from a common ancestor shared with the green algae called Charophytes (charales). 2. Adaptations to shallow water preadapted plants for living on land

Three life cycle modifications contributed to terrestrial seed plant success:

1. Reduction of the gametophyte. They were retained in the moist reproductive tissue of the sporophyte generation (not independent). 2. Origin of the seed -Zygotes developed into embryos packaged with a food supply within a protective seed coat. -Seeds replaced spores as main means of dispersal. 3. Evolution of pollen. Plants were no longer tied to water for fertilization.

Gymnosperms

1. The Mesozoic era was the age of gymnosperms. 2. The four groups of extant gymnosperms are the cycads, the ginkgo, the gnetophytes, and the conifers. 3. The life cycle of a pine demonstrates the key reproductive adaptations of seed plants.

Bryophytes (Non Vascular Plants)

1. The embryophyte adaptation evolved in bryophytes. 2. The gametophyte is the dominant generation in the life cycles of bryophytes. 3. The three groups of bryophytes are mosses, liverworts, and hornworts. Bryophytes lack woody tissue and cannot support tall plants on land; they may sprawl horizontally as mats, but always have a low profile.

Overview of Reproductive Adaptations of Seed Plants

1. The gametophytes of seed plants became even more reduced than the gametophytes of seedless vascular plants 2. In seed plants, the seed replaced the spore as the main means of dispersing offspring 3. Pollen became the vehicles for sperm cells in seed plants

Plant Evolution

Appeared on land about 475 million years ago.

The gametophytes of seed plants became even more reduced than the gametophytes of seedless vascular plants

Gametophytes of seed plants are reduced in size and retained within the moist reproductive tissue of the sporophyte generation. -Trend reverses the gametophyte-sporophyte relationship observed in bryophytes (non-vascular plants). -Diploid generation dominance may afford protection from solar radiation induced mutations of the genome (damaging radiation is more extensive on land than in aquatic habitats).

The Mesozoic era was the age of gymnosperms

Gymnosperms appear in the fossil record much earlier than flowering plants. Gymnosperms most likely descended from Devonian progymnosperm and were seedless. -Seeds evolved by the end of the Devonian. Gymnosperms lack enclosed chambers (ovaries) in which seeds develop.

Flower = The reproductive structure of an angiosperm which is a compressed shoot with four whorls of modified leaves.

Sepals - Sterile, enclose the bud. Petals - Sterile, aid in attracting pollinators. Stamen - Produces the pollen. Pistil (Carpel) - Evolved from a seed-bearing leaf that became rolled into a tube. Stigma - Part of the carpel that is a sticky structure that receives the pollen. Ovary - Part of the carpel that protects the ovules, which develop into seeds after fertilization.