Plant Evolution and Classification PDF

Plant Evolution and Classification Ancestor and Derived Traits of Land Plants The most widely accepted ancestor of plants were the green algae. Charophyceae were the closest relatives of the land plants. - The have very similar plastids, both structurally and in terms of chloroplastic DNA...

Plant Evolution and Classification Ancestor and Derived Traits of Land Plants The most widely accepted ancestor of plants were the green algae. Charophyceae were the closest relatives of the land plants. - The have very similar plastids, both structurally and in terms of chloroplastic DNA. - Their cells walls are also composed of cellulose and their anti- photorespiration enzymes are both packaged in peroxisomes. - Both their sperms have similar flagellated structures and their phragmoplasts are structurally similar. - DNA and RNA sequences support the close relationship of plants to the charophytes especially to the genera Chara and Coleochaete Characteristics that are evolutionary derived from and common among all four groups of plants but absent in charophyceans include the following: 1. Apical meristem, a cluster of embryonic cells found in root and shoot tips. 2. Alternation of generations, a characteristic life cycle present in all land plants. 3. Spores are produced in sporangia, haploid reproductive cells become multicellular haploid gametophyte by mitosis. These spores form from the meiosis of sporocytes in the sporangium. 4. Multicellular gametangia produce the gametes. In algae gametes are produced in unicellular gametangia. Gametes Chrophyceae Spores  Plant diversification and floral changes were seen in Ordovician and Silurian periods. Paleobotany, the study of fossil plants, contribute mainly to the study of plant evolution by taking into consideration dispersed spores and megafossils. Plant Evolution Across Geological Time Scale Green algae, widely regarded as the ancestor of living plants remained abundant in ocean. Small terrestrial plants began their migration to wet terrestrial environment during the Ordovician Period. These were non vascular plants with no water conducting vessels. Stomata, appeared during the Silurian Period, following the evolution of the first vascular plants with water conducting tubes. No differentiation of leaves, stems and roots were seen. Organ differentiation was seen during the early Devonian Period. By the end of this period, Carbon dioxide levels in the atmosphere dropped following the diversification of land plants.  Horse tails, club mosses and scale trees appeard during the Carboniferous period, followed by primitive conifers and ferns. Advanced conifers dominated the dry climate during the Permian Period. The importance of Cycads and Ginkgos continued to the Triassic and Jurrasic periods. Seed plants started to dominate the land. Flowering plants appeared during the Jurrasic period but became widespread during the Cretaceous period. Modern trees we know today appeared during Cretaceous period. Grasses appeared during the Cenozoic Period. Ginkgo Cycads Conifers Three major epochs define the evolution and diversification of terrestrial plants 1. Eoembryophytic (476 Myr – 432 Myr) – In this period, spore tetrads and a decay resistant wall were widespread over broad geographical areas which provided the first good evidence of land plants. Liverwort-like plants were most abundant during this time. 2. Eotracheophytic (432 Myr – 402 Myr) – Plants employed simle spores instead of tetrads. Early assemblage of plants in this epoch included the clubmosses and plantsvwith no known affinities. Only few plants we know today trace their origins to this epoch. 3. Eutracheophytic (398 Myr – 256 Myr) – A substantial increase in vascular plant diversity happened during this period, including the appearace of many important plant groups we know today. Evidence for this includes the dramatic increase in spore and megafossil diversity across many geographical regions. Definition of Botanical terms Plastids – a membrane bound organelle found in cells of plants, algae and some eukaryotic organisms which divided into 3 types – chloroplasts, chromoplasts and leukoplasts. Organelle – tiny organs of the cell that performs a specific function. Phragmoplast – a structure that builds a cell wall called a “cell plate” which consists of two daughter cells that form during late cytokinesis. Peroxisome – small vesicles, small membrane-enclosed organelle that plays important roles in metabolism and oxidative reactions. Apical meristem – specialized portion or zone of growth at the tip of plant. Gametangia –specialized organs in which gametes are formed in algae, fungi, protists, ferns and some other plants. Gamete – reproductive cell of an animal or plant. Conifers – group of cone-bearing seed plants, a subset of gymnosperms. Different plant groups evolved at different times in geologic time scale. 1. Permian 2. Triassic 3. Jurassic 4. Cretaceous 5. Ordovician 6. Silurian 7. Devonian 8. Carboniferous 9. Cenozoic ADAPTATIONS TO THE TERRESTIAL ENVIRONMENT Plant Migration to terrestrial environments altered plant`s morphology and physiology. The evolution of multicellularity from single-celled algae paved the way for more complex tissues necessary for growth. A root system necessary for anchored was formed. The vascular system for transport of water and nutrients, biosynthesis of lignin and the appearance of lateral meristems allowed the increase in girth among trees. Leaves were able to capture energy from light. Stomata evolved to allow conductance of carbon dioxide while cutinized leaves prevented dessication. Spores and seeds had thicker walls to prevent them from drying out. PLANT CLASSIFICATION DIVISION CHARACTERISTICS EXAMPLE Non-vascular plants with a flattened Marchantiophyta leafless thallus; species resemble Liverworts the shape of a liver, also called hepatics Anthoceratophyta With a sporophyte that resembles a Hornworts horn Bryophyta Plants with simple leaves with one Moses cell layer attached to a stem that function for conducting water Lycophyta Vascular plants with microphylls, a Club mosses type of leaf with one single, unbranched leaf vein. Pteridophyta Vascular plants that reproduce via spores and have neither seeds nor Ferns, whisk ferns and horsetails flowers; leaves are complex called megaphylls  DIVISION CHARACTERISTICS EXAMPLE Cycadophyta Produce naked seeds (gymnosperms) Cycads Gingkophyta Monotypic taxon with only one Ginkgo extant (living) member, Ginkgo biloba Pinophyta Cone-bearting seed plants, Conifers perennial woody plants with secondary growth. Gnetophyta Contain vessel elements similar to Gnetophytes flowering plants; only include 70 known members Seed producing plants with flower Magnoliophyta and endosperms Flowering plants