Unit 2: Land Plant Evolution

Cladogram: diagram that represents the phylogenetic relationships among categories Be able to map land plant cladogram phylogeny & identify parts of cladogram      

Label: Monilophytes, lycophytes, gymnosperm, angiosperm, bryophytes Seeds, stomata, flowers, megaphylls, rhizoids, vascular tissue, mycorrhizal associations, sporophyte dominance, pollen, fruit, roots, sporopollenin, wood, protected embryo, microphyll, megaphyll, cuticle  

Label:  Node, branch, root, taxon, clade, tip, timescale (more to less recent)

What makes a plant? Multicellular: composed of eukaryotic cells that have a nucleus, large central vacuole, and rigid, cellulose-containing cell walls Photosynthetic: capable of making their own food via chloroplasts, using light as energy source Adapted to terrestrial habitats: where they gather diffuse resources (light, carbon dioxide, water, minerals Stationary: non-mobile Indeterminate growth: type of growth that continues through plant's entire life due to areas of perpetually dividing and differentiating cells called meristems Protected embryo: nurtured by parent plant as part of its sexual reproductive stage Alternation of generations: two distinct phases: sporophyte which has 2+ sets of chromosomes and gametophyte which has one set of chromosomes and is used in sexual reproduction    

Gametophyte vs Sporophyte  Land plants alternate between the two generations Sporophyte: spore-producing phase Diploid: 2N sets of chromosomes Biological advantage over gametophyte because having two sets of genes makes masking bad mutations possible Adapt better to unfavorable conditions Allows for aerial spore dispersal, greater genetic variation potential Over time, sporophyte became larger, more complex, better adapted than gametophyte because of branching ability in vascular tissue  More opportunities to produce spores and more cushion if some branches were damaged) Gametophyte: gamete-producing phase Haploid: 1N set of chromosomes Produces sexual gametes (eggs & sperm) within the gametangia organs Originally the dominant phase

Origin of Terrestrial Adaptations Fossil records of plants are hard to come by since they are only fossilized if they were next to a body of water or preserved in tree sap First fossil plant remains date back 475 million years Embryophytes: land plants Evolved from ancestral green algae  Embrophytes received many characteristics from green algae Photosynthesize using chloroplasts Store sugars produced as starch Cellulose-rich cell walls However, there are some differences. The algae weren't always multicellular didn't produce embryo exclusively exist in aquatic environments Predators may have helped land plant evolution because any plant that could adapt to the shoreline was beyond the reach of herbivores Advantages to terrestrial life Unfiltered sunlight More concentrated carbon dioxide levels Soil rich in minerals Originally less competition for resources and fewer herbivores/pathogens Disadvantages to terrestrial life Gravity Radiation Limited water supply Existing in two different environments Terrestrial Challenges & Embryophyte Solutions   Adaptations Desiccating atmosphere: air is drying & water is usually confined to soil making desiccation-resistance necessary for aerial plant parts  Stomata - special epidermal openings that can open and close, regulating gas exchange and helping to restrict water loss Sporopollenin - protective coating of spores and pollen that protects these reproductive cells from drying out most decay-resistant biological material known Cuticle - waxy coating on outer surface of plant that prevents drying out exuded by epidermal cells in contact with atmosphere gas exchange is limited so there are small openings for absorbing carbon dioxide from air these pores eventually became separate guard cells that open and close to control water escaping form stomata Relative scarcity of water: sea life ensures algae is surrounded by water but water is scarce on land & is usually confined to soil Protected embryo - early embryonic growth is protected and nurtured on the parent plant Roots/rhizoids - specialized organs that extract water and minerals from soil Vascular system - internal tissue that transports water and dissolved minerals throughout plant, otherwise known as veins as land crowded, competition for light increased productivity was increased by increasing surface area for photosynthesis required improvements to absorbing and transporting water true roots could now form from rhizoid counterparts along with true leaves grew taller because xylem and phloem tissues helped transport water and distribute nutrients between roots and shoots roots were delving deeper into soil, could no longer photosynthesize, and needed sugar transported to them shoots and true leaves formed further away from soil so they needed water & minerals transported to them Structural support against gravity: aquatic plants get structural support from buoyancy and external pressure of water Lignin - rigid, woody compound that reinforced the cell walls of vascular tissue and acts as an internal skeleton for plants necessary as plants grew taller reason for woodiness in land plants not found in aquatic plants Uptake of carbon dioxide and absorption of nutrients: algae's whole bodies were surrounded by gas and mineral-containing water. Land has higher concentrations of these but is separated into air & soil and necessitated specialization in plant parts exposed to them Stomata/pores - special openings in epidermis that allow gas exchang e necessary since cuticle blocks gas flow Roots/rhizoids - specialized organs that extract water & minerals from soil Mycorrhizal associations - symbiotic fungi that interact with roots and increase absorption of water and nutrients from soil ancestral land plants developed mutualistic relationships with mycorrhizal (growing in association with roots) fungi in the soil speculated that a fungus attacked a plant but failed to kill it and began feeding it with minerals from soil while extracting the plant's sugars today they aid in water & nutrient uptake by increasing surface area of plant roots which in turn give them sugars    

Land Plant Groups

Five major groups of Embryophytes categorized by features referred to as 'key speciation events' 1. Bryophytes - liverworts, mosses, hornworts Most similar to ancestor that evolved from algae, most primitive land plants today Gametophyte-dominant Range in form from thalloid (simple, flattened bodies) to leafy Rhizoids - hair-like projections that help hold their diminutive bodies against the ground lack water-conducting vascular tissue Rhizoids & leaf-life parts aren't considered true roots or leaves Non-vascular nature means they're found in wet environments and stay small, allowing them to absorb water through their bodies Liverworts have open pores Mosses & hornworts exhibit stomata reproduce by dispersing small spores form their sporangium (spore capsule)    

2. Lycophytes - clubmosses, quillworts, spike mosses Reproduce via spores, but bear small cones of sporangium called strobili Vascular tissue allowed them to become taller & more branches True roots Branching Sporophyte-dominant lifecycle Leaves are microphylls since they only have a single strand of vascular tissue

3. Monilophytes - ferns, horsetails, whisk ferns Most diverse of the spore-producing plants Sporophyte-dominant Vascular tissue True roots Leaves are megaphylls since they have multiple veins running through a leaf, allowing them to become larger In ferns, sporangia produced in large numbers on leaves, grouped into small clusters called sori Horsetails produce cone-like strobili Whisk ferns have specialized naked sporangia produced in tight clusters at their tips Widely distributed around the world in moist, shaded environments

4. Gymnosperms - cycads, ginkgo, gnetophytes, conifers Ability to produce seeds and develop wood (secondary growth) Produce pollen as their male gametophyte Non-flowering Seeds develop on surface of scale or leaf-life appendages of cones or at end of short stalks Pollen & seeds allowed them to decrease dependency on water for reproduction Largest extant group are conifers (pines, cypresses) Smallest is ginkgo Vascular True roots Megaphylls Sporophyte-dominant

5. Angiosperms - dicots (eudicots), monocots Flowering True fruit Most diverse & abundant land group plants, occupying diverse environments including marine Many involve animals in pollen & seed dispersal Sporophyte Vascular True roots Megaphylls Produce wood Most reduced gametophyte of all land plants Huge variety of physical forms - trees, herbs, submerged aquatics, bulbs, epiphytes Pollen & seed allow them to decrease water dependency for reproduction