Transcript Chapter 29
Plant Diversity: How Plants Colonized Land More than 280,000 species of plants inhabit Earth today Most plants live in terrestrial environments Referred to as “land” plants Evolved from green algae called charophyceans There are four main types of land plants Bryophytes Most common are mosses Pteridophytes ferns Gymnosperms Pines and conifers (cone-bearing plants) Angiosperms Flowering plants Bryophytes are distinguished from algae by several features derived during evolutionary adaptations to living on land Many are reproductive: offspring develop from a multicellular embryo that remains attached to the “mother” plants, which protects and nourishes it Bryophytes have not evolved to have vascular tissue Pteridophytes, angiosperms, and gymnosperms are called vascular plants Cells are joined into tubes that transport water and nutrients throughout the plant body Pteridophytes are called “seedless” plants because there is no seed stage in their life cycles Gymnosperms and angiosperms are seed plants Seed consists of a plant embryo packaged along with a food supply within a protective coat First evolved 360 million years ago Early seeds were not enclosed in any specialized chambers Gave rise to the present day gymnosperms Gymno = naked; sperma = seed Flower evolution led to further diversification of plants beginning about 130 million years ago Flowers are complex reproductive structures that bear seeds within protective chambers called ovaries Gave rise to modern day angiosperms Angion = container Origin of bryophytes from algae 2. Origin of vascular plants 3. Origin of seeds 4. Origin of flowers 1. All plants are multicellular, eukaryotic, photosynthetic autotrophs Cell walls made of cellulose, and presence of chlorophyll a and b Plasma membranes are equipped with rose shaped arrays of proteins that synthesize the cellulose of cell walls Rosette-cellulose-synthesizing complexes Several characteristics are present in all land plants that are absent in their closest algal relatives Apical meristems Multicellular, dependent embryos Alternation of generation Walled spores Multicellular gametangia Because plants cannot move from place to place, the elongation and branching of their shoots and roots maximize their exposure to environmental resources This growth in length occurs because of apical meristems Localized regions of cellular division at the tips of shoots and roots Cells produced differentiate into the plants’ various tissues Develop within tissues of the female parent Parental tissues provide the developing embryo with nutrients, such as sugars and amino acids Have specialized placental transfer cells which enhance the transfer of nutrients from parent to embryo Known as embryophytes because of this characteristic During the life cycle of all land plants, two multicellular body forms alternate, each form producing the other Gametophytes are haploid and produce gametes Fertilization of gametes forms diploid zygotes that divide and create sporophytes Diploid, and produce haploid cells called spores Reproductive cell that can develop into a new organism without fusing with another cell Plant spores are haploid reproductive cells that have the potential to grow into multicellular, haploid gametophytes by mitosis A polymer called sporopollenin, the most durable organic matter known, makes the walls of spores very tough an resistant to harsh environments Makes it possible for wind-carried spores to disperse through dry air without harm Multicellular organs called sporangia produce spores Within it, spore mother cells undergo meiosis and generate the spores The gametophyte forms within multicellular organs called gametangia Females are called archegonia that is vase shaped and produces a single egg cell and retains the egg within the base of the organ Males are called antheridia produce sperm cells that are released in the environment when mature Sperm bear flagella and swim through water droplets or water film to eggs Eggs are fertilized within the archegonia, where the zygote begins to develop into an embryo Water conservation Water transport Secondary compounds The epidermis of leaves and other aerial parts of most land plants is coated in a cuticle, which consists of waxes Helps protect the plant from microbial attack and acts as waterproofing that helps prevent excessive water loss from the aboveground plant parts Has pores called stomata that support photosynthesis by allowing the exchange of carbon dioxide and oxygen Also the main avenues that water exits by evaporation Changes in the shapes of the cells that border the stomata can close the pores to minimize water loss Except for bryophytes, land plants have true roots, stems and leaves which are defined by the presence of vascular tissue Two types of vascular tissue are: Xylem carry water and minerals up from the roots These conducting cells are actually dead, with only their walls remaining to provide a system of microscopic water pipes Phloem is living with nutrient-conducting cells arranged in tubes that distribute sugars, amino acids, and other organic products throughout the plant Land plants produce many unique molecules called secondary compounds They are named because they are products of “secondary” metabolic pathways that produce lipids, carbohydrates, and other compounds Alkaloids, tannins, and phenolics such as flavonoids Have bitter tastes, strong odors, or toxic effects that help defend agains herbivores Flavonoids absorb UV radiation Phenolics deter attack by pathogenic microbes Lignin hardens the cell walls of “woody” tissues Lignin Stone: a threat to security? Homologous chloroplasts Homologous cell walls Homologous peroxisomes Homologous sperm Molecular systematics Compared key nuclear genes Fossils place plants on land over 500 million years ago Oldest known are encased in rocks about 550 million years old Petrified wood: There are three separate phlya of bryophytes: Liverworts Hornworts Mosses “Wort” means herb, named so because they are small nonwoody plants named for their shapes Spores are dispersed to a favorable habitat, such as moist soil or tree bark, they may germinate and grow into gametophytes by mitosis Germinating spores produce a mass of green, branched, one-cell thick filaments known as protonema Sometimes mistaken as algae Have a large surface area that enhances absorption Produce buds that generate the gametophore which is the mature, gamete producing structure Tissues are only a couple cells thick because they lack the vessels to aid in water and nutrient transport Most grow close to the ground, anchored by rhizoids, delicate and colorless strands Mosses are able to exist in very harsh environments, and because of this, they are crucial to many ecosystems One moss genus creates deposits of undecayed material known as peat; also called “peat moss” Reduce bacterial activity Once used as wound packing Now used as soil conditioner World’s peatlands store an estimated 400 billion tons of organic carbon Stabilize atmospheric CO2 concentrations and climate Evolved over 408 million years ago; only was 50 centimeters tall Diploid version of plant is larger, and remains for most of the time Homosporous plants produce a single type of spore Produces bisexual gametophytes Heterosporous plants produce two kinds of spores: Megaspores develop female gametophytes Microspores develop male gametophytes Many are tropical epiphytes- plants that live on other plants but are not parasites Others grow close to the ground on forest floors in temperate regions Groups of specialized leaves called sporophylls are clustered to form club-shaped cones When they are mature, the spores are released in clouds Early photographers loved lycophytes back in the day! Their spores were oil-rich and flammable Photographers would ignite the spores to provide a flash of light 12,000 species Most widespread and diverse pteridophytes today Leaves are known as fronds Produce clusters of sori on the underside of the leaves Groups of spore producers Arrangement is useful in identification Equipped with spring-like devices that catapult spores several meters Most of the coal used today was from a time period where there were no known animals, only early plants and the earth was flooded by shallow swamps Swamps were later covered by the sea and sediment piled on top of it Heat and pressure converted this peat to coal, a fossil fuel