Land Plant Evolution and Reproduction

• Plants are defined as multicellular eukaryotic organisms with cells containing plastids.
• Kingdom Plantae is composed of several hundred thousand modern plant species.
• Plants primarily live on land with algal ancestors evolved from green in aquatic habitats.
• Plants are distinguished from algal relatives through adaptations for terrestrial life.

Significance of the Plant Embryo

• Charophyceans (a type of algae) lack plant embryos.
• Plant embryos represent one of the first critical innovations of land plants.
• Plant embryos are young sporophytes, which develop from zygotes.
• The key features: multicellular, diploid zygotes, and embryos retained in maternal tissue.
• Depends on transferred organic and mineral materials from mother plant via placental tissues

Placental Transfer Tissue

• Often in gametophyte tissues closest to embryos
• Cells are specialized to promote the movement of solutes from gametophyte to embryo
• Finger-like ingrowths of cell wall increase surface area of plasma membrane for transport proteins

Ancestry of Plants

• Plantae originated from a photosynthesis protist ancestor classified among the streptophyte algae.
• Modern plants and their closest green algal relatives are known as streptophytes.
• Complex streptophyte green algae share several traits with land plants.
• Traits include a distinctive type of cytokinesis, intracellular connections, plasmodesmata, and sexual reproduction using egg and smaller sperm

Land Plants Changed the Earth By

• Development of substantial soils.
• Rising modern levels of atmospheric oxygen.
• Evolving modern plant communities.
• Colonization of land by animals.
• Land was bare a billion years ago except for some cyanobacteria crusts

Plants Transformed Earth's Atmosphere

• Photosynthesis uses CO2 from the atmosphere to produce carbon-containing organic molecules.
• Oxygen (O2) is a byproduct of photosynthesis.
• Plants influenced Earth's climate by reducing CO2 and increasing O2 concentrations in the atmosphere.
• Plants produce decay-resistant materials, including sporopollenin, cutin, and lignin.
• Some organic molecules of ancient plants were buried in sediments and eventually transformed into rock.

The Coal Age

• Dead plants would fall into the water.
• Low oxygen levels in swamp sediments inhibited decomposition.
• Organic carbon accumulated in sediments later formed to coal.
• Much of today's coal is from the remains of Coal Age plants.
• Plant organic matter storage in sediments decreased atmospheric CO2 and raised oxygen levels.
• The Coal Age had very large insects such as giant dragonflies, whose increased size created a need for oxygen.

Distinctive Features of Land Plants

• Land plants can be distinguished from their close algae relatives by several adaptive features.
• Their bodies are composed of three-dimensional tissue.
• Increased ability to avoid water loss.
• Tissues arise from apical meristems, which are fast-dividing cells at growing tips.
• Able to produce thick, robust bodies.
• Tissues and organs with specialized functions.

Reproduction Features

• Adaptations that aid in survival in terrestrial habitats.
• Alternation of generations: two types of multicellular bodies alternate in time.
• The diploid (2n) sporophyte produces spores by meiosis; the haploid (1n) gametophyte produces gametes by mitosis.
• The sporophyte embryo is nourished and protected by maternal tissues.
• Land plants produce many more spores per zygote than streptophyte algae.
• Land plant sporophytes produces tough-walled nonflagellate spores that survive dispersal through dry air.
• Allows plants to spread widely across land surfaces.
• Other advantages are Matrotrophy which includes zygotes remain sheltered and fed within gametophyte tissue
• All groups of land plants possess matrotrophic embryos, so they are known as Embryophytes.
• Mature spores are produced in protective enclosures known as sporangia.
• Sporopollenin is a tough material in plant spore cell walls that prevents cellular damage.
• During evolution, plant sporophytes became larger and more complex.

Evolution of Reproductive Features in Land Plants

• Multicellular diploid sporophyte generations are advantageous because it allows a single plant to disperse widely.
• Meiosis is used to produce numerous, genetically variable haploid spores.
• Each spore has the potential to grow into a gametophyte.

Gametophytes

• The role is to produce haploid gametes, which is done by mitosis.
• Gametangia protects developing gametes from drying out and microbial attack.
• Antheridia: spherical or elongate gametangia produce sperm.
• Archegonia: flask-shaped gametangia enclosing an egg.
• Sperm swim to egg and fuse to form diploid zygote

Zygote Grow into Sporophytes

• Dominate stage in mosses.
• Mosses has male and female reproductive structures.
• The male antheridia produce sperm, while the female archegonia contain egg cells.
• Water is essential because sperm must swim to the egg and forms a diploid zygote.
• The zygote grows by mitosis into a sporophyte, which is the foot (anchors to gametophyte), seta (stalk), and capsule (sporangium) where spores form.
• Inside the capsule, meiosis occurs, producing haploid spore that are genetically diverse due to recombination.
• Spores are released and dispersed by wind or water, then germinate into a protonema, which develops into a gametophyte, restarting the cycle.

Plant Phyla

• Liverworts: Hepatophyta
• Hornworts: Anthocerophyta
• Cycads: Cycadophyta
• Ginkgos: Ginkophyta
• Conifers: Coniferophyta
• Angiosperms (flowering plants): Anthophyta
• Lycophytes: Lychophyta
• Pteridophytes: Pteridophytes
• Mosses: Bryophyta (Bryophytes)
• Includes liverworts, hornworts, and mosses that share common structural, reproductive, and ecological features and each forms a monophyletic phylum descended from common ancestor
• Mostly small in stature and most common and diverse in moist habitats

Bryophyte Generations

• Gametophytes are the dominant generation of bryophytes.
• Sporophytes are dependent on gametophytes.
• Sporophytes are small and short-lived.
• Lycophytes: Lycophyta
• Pteridophytes: Pteridophytes
• Both seedless vascular plants.
• Possess vascular tissue for support and transport.
• Do not possess seeds.
• Known as tracheophytes: name taken from tracheids, a type of specialized cell that conducts support.
• Lycophytes are more numerous and larger in the past but now about 1000 relatively small species
• Lycophytes roots and leaves differ from those of other tracheophytes.
• Pteridophytes are diversified more recently than lycophytes
• 12,000 modern species of Pteridophytes include horsetails, whisk ferns, and other ferns.

Lycophyte & Pteridophyte Generations

• Limited by dry reproduction conditions, Like bryophytes.
• Lychophytes and pteridophytes can produce many more spores larger sporophyte generation if fertilization occurs.
• Vascular plant sporophytes are dependent upon maternal gametophytes for only a short time during early embryo development.
• Stems of vascular plant sporophytes are able to produce branches, forming relatively large adult plants with many leaves

Overview of the Fern Life Cycle

• Diploid Sporophyte (2n) is the Dominant Generation, aka the leafy plant structure most recognize.
• Mature ferns develop sporangia (small structures in clusters called sori) on leaf undersides.
• Specialized cells undergo meiosis inside the sporangia, producing spores that are dispersed by wind.
• When they land spores germinate and grow into gametophytes that are small, are heart-shaped, and anchored by root-like rhizoids.
• Mature gametophytes create the male and female reproductive structures called male gametangia (antheridia) and female gametangia (archegonia)
• Sperm swim from the antheridia to the archegonia for fertilization.
• The diploid zygote formed undergoes into an embryo within a multicellular embryo.
• The developing embryo continues to grow into a mature sporophyte, which results in the gametophyte withering away.

Roots, Stems and Leaves of Vascular Plants

• Stem produces leaves and sporangia and contains a specialized conducting tissue called pholem (specialized stem) and xylem (contains lgin and tracheids)
• Roots are specialized for uptake of water and minerals from the soil.
• Leaves have a photosynthetic function.
• Provides a high surface area that helps leaves to effectively capture sunlight for use in photosynthesis

Leaf Types

• Lycophytes produce the simplest, most ancient leaves called lycophylls (small leaf) or microphylls.
• Other vascular plants have leaves with extensively branched euphylls (large leaf) or megaphylls.
• Larger size provides considerable advantage and evolved in a series of steps

Ovule

• Sporangium with single spore and a small egg-producing gametophyte inside enclosed by integuments.
• Seed plants produce two distinct types of spores in two different types of sporangia.
• Microsporangia produce small microspores that give rise to male gametotypes (pollen grains).
• Megasporangia produced larger megaspores that give rise to female gametophytes that produce eggs.

Pollination

• Pollination must first occur in order for Embryos and Seeds to Develop.
• Is defined as the process by which pollen comes into contact with ovules.
• Typically occurs by wind or animal transport; pollen will then germinate, producing sperm.

Seed Development

• Double fertilization happens during seed development.
• One sperm fertilizes egg to become an embryo.
• Other sperm fuses with different gametophyte tissue to form endosperm. is
• Ovules develop into a protective, hard and tough seed coat.
• Seeds allow embryos access to food supplied by the older sporophyte generation.

Seeds Reproductive Advantages

• Seeds are a key adaptation to reproduction in a land habitat.
• Able to remain dormant in the soil, under favorable conditions.
• Undergo Adaptations to improve dispersal
• Seeds can store food and allow sperm to reach egg without swimming.
• Seed plants have not "replaced" spores with seeds, but rather added to them.
• Ovules and seeds added to life history including spores.
• Releasing one spore and gametophyte type is homospory.
• Whereas as a product of microspores and megaspores is heterospory.
• Increasing cross-fertilization is an advantage of heterospory.
• Protected gametophytes grow inside microspore and megaspore walls.

Water Content

• Waxy cuticle present on most surfaces of vascular plant sporophytes prevents dessication.
• Cutin is found in cuticle and contains wax, which prevents water loss and helps block pathogens.
• Stomata are specialized cells that associate with pores allowing gas exchange while minimizing water loss.
• Pores aid in the drying process; allow intake of CO2.

The Age of the Gymnosperms

• The removal of large amounts of CO2 from the atmosphere by Coal Age plants had a colling effect which led to the climate becoming dry.
• Cooler, drier conditions favored diversification of first seed plants, gymnosperms
• Diverse phyla of gymnosperms dominated Earth's vegetation through the Mesozoic era (248-65mya).

Gymnosperms

• Ancient seed plant classified within cycads, ginkgos, and confiers.
• Reproduce using spores and seeds (like angiosperms).
• Seeds protect and provide energy for young sporophyte, that are not enclosed by fruit.
• Gymnosperms and early angiosperms were probably major sources of food for early mammals as well as herbivorous dinosaurs.

Rise of Angiosperms

• A meteorite that struck Yucatan diminished sunlight to kill plants.
• The severely reduced food supply doomed dinosaurs, however flowering plants diversified, leaving Extinction space for the dinosaurs left room for birds and mammals to adapt.
• Angioserms are distinguished by the presence of flowers and endosperm, which enhances seed production and allows fruits to grow after seed dispersal.
• Endosperm has increased storage efficiency as a nutritive seed tissue
• In summary the adaptation to land was required the evolutionary lineage of seed plants, where these seed plants have also played a role in shaping Earth's ecosystems.

Explore land plant evolution, focusing reproductive advantages and structures. Learn about sporophytes, gametophytes, gametangia, and spore protection. Understand moss and fern life cycles, including fertilization and dominance.