Transcript Chapter 19
Chapter 19 Lecture Outline Introduction to Seed Plants: Gymnosperms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outline Introduction Phylum Pinophyta – The Conifers Other Gymnosperms • Phylum Ginkgophyta – Ginkgo • Phylum Cycadophyta – The Cycads • Phylum Gnetophyta – The Gnetophytes Human Relevance of Gymnosperms • Conifers • Other Gymnosperms Introduction Oldest known seeds - Late Devonian, more than 350 million years ago Seeds provide a significant adaptation for plants on land. • Protective seed coat • Supply of food for embryo • Capable of dormancy in unfavorable environmental conditions First seed plants fernlike in appearance = pteridosperms (seed ferns) - Reclassified as gymnosperms Introduction Gymnosperm refers to the exposed nature of the seeds. • Seeds produced on surface of sporophylls or similar structures, instead of enclosed within a fruit as in flowering plants. • Seed-bearing and pollen-bearing sporophylls often arranged in cones. Introduction Pollen cones produce pollen grains. Female gametophyte - Produced inside an ovule that contains nucellus • Nucellus enclosed in integument. – Integument becomes seed coat after fertilization. • Female gametophyte even more reduced in cell number than are ferns and their relatives. • Does not grow independently, but develops within sporophyte structures. Introduction Four living phyla • Pinophyta – Bear distinct strobili (cones), needlelike leaves arranged in clusters – About 600 species-Pines, furs, spruces, cedars • Ginkgophyta – Notched, broad, fan-shaped leaves with evenly forked veins – Plum like seeds enclosed in fleshy covering. – Has single living species, Ginkgo biloba • Cycadophyta – Cross between tree fern and palm, strobili in center – About 300 species • Gnetophyta – Vessesls and tracheids in xylem – About 70 species Phylum Pinophyta – The Conifers Pines (Pinus)-largest genus • Dominant trees in coniferous forests of Northern Hemisphere – Include world’s oldest known living organisms - Bristlecone pines • Structure and form: – Leaves needlelike and arranged in clusters of two to five leaves. – Cluster = fascicle – Fascicles are short shoots Have restricted growth Phylum Pinophyta – The Conifers Pines - Structure and form: – Have modifications that enable them to survive harsh conditions o Hypodermis located below the epidermis. « One to two layers of thick-walled cells o Thick cuticle o Recessed or sunken stomata o Resin canals « Resin is antiseptic and aromatic, prevents development of fungi, and deters insects. o Mycorrhizal fungi associated with roots of most conifers. Phylum Pinophyta – The Conifers Pines - Structure and form: • Wood consists entirely of tracheids. – – Conifer wood = softwood - Thick-walled cells absent. Broadleaf tree (dicot) wood = hardwood - Thick-walled vessels and fibers present. Phylum Pinophyta – The Conifers Pines • Reproduction: – Two kinds of spores produced. – Pollen cones (male strobili) consist of papery or membranous scales. o Microsporangia in pairs toward bases of scales. Phylum Pinophyta – The Conifers Pines • Reproduction: – Meiosis produces microspores that then develop into pollen grains. o Pollen grain consists of four cells and a pair of air sacs. « Air sacs add buoyancy in wind. Phylum Pinophyta – The Conifers Pines • Reproduction: – Megaspores in megasporangia within ovules. o – Pair of ovules at bases of seed cone scales. Seed cones larger than pollen cones. o Have woody scales with inconspicuous bracts between Phylum Pinophyta – The Conifers Pines - Reproduction: • Ovule contains a megasporangium containing a nucellus and a single megasporocyte. • Megasporangium surrounded by integument. – • Integument has a pore called micropyle. Megasporocyte undergoes meiosis, producing four megaspores. – Three megaspores degenerate. – Remaining megaspore develops into female gametophyte with archegonia at micropyle end. Phylum Pinophyta – The Conifers Pines - Reproduction: – Seed cones take two years to mature. – First year: o Pollen grains catch on sticky pollen drops oozing out of micropyle. o Pollen grain produces pollen tube that grows through nucellus. « Two sperms produced in pollen tube. « Mature male gametophyte = germinated pollen grain with pollen tube and two sperm « Sperm have no flagella and no antheridium is formed. o Megaspore develops. Phylum Pinophyta – The Conifers Pines - Reproduction: – Second year: o Female gametophyte and archegonium mature. o Pollen tube arrives at archegonium. o One sperm unites with egg, forming zygote. « Other sperm degenerates. – Embryo nourished by female gametophyte. – Integument becomes seed coat. Phylum Pinophyta – The Conifers Pines - Reproduction: Phylum Pinophyta – The Conifers Other Conifers: • Yew (Taxus) and California nutmeg (Torreya) produce ovules singly at tips of shoots. – Each ovule at least partially surrounded by fleshy, cuplike aril. Taxus • Southern hemisphere conifer - Podocarpus – Fleshy-coated seeds with large appendage at base • Junipers - Seed cones fleshy. Podocarpus Other Gymnosperms Phylum Ginkgophyta – Ginkgo (maidenhair trees) • Only one living species – Only exists in cultivation • Notched, broad, fan-shaped leaves – Leaves on short, slow-growing spurs. o No midrib or prominent veins o Hair-like veins branch dichotomously. o Deciduous Other Gymnosperms Phylum Ginkgophyta • Life cycle similar to pines. – Dioecious - Male and female structures on separate trees. – Seeds enclosed in fleshy seed coat with nauseating odor. Seeds and leaves Male strobili Other Gymnosperms Phylum Cycadophyta – The Cycads • Slow-growing plants of tropics and subtropics • Tall unbranched trunks • Crown of large pinnately divided leaves • Life cycle similar to conifers. – Pollination sometimes by beetles. – Dioecious – Has pollen and seed strobili Male cycad Female cycad Other Gymnosperms Phylum Gnetophyta – The Gnetophytes Part of plant • Unique among the gymnosperms in having vessels in the xylem. Male strobili • Joint firs (Ephedra) - Shrubby plants of drier regions of southwestern North America Female strobilus Ephedra Other Gymnosperms Phylum Gnetophyta – The Gnetophytes • Gnetum - Vine-like plants with broad leaves – In tropics Other Gymnosperms Phylum Gnetophyta – The Gnetophytes • Welwitschia - Only one species, confined to deserts of southwestern Africa – Short stem – Long taproot – Only two straplike leaves that become tattered and split – Dioecious – Has male and female strobili Human Relevance of Gymnosperms Conifers • Edible inner bark and needles of white pine, and seeds of nearly all pines • Masts in sailing vessels • Crates, boxes, matchsticks, furniture • Telephone poles, railroad ties, mine timbers • Turpentine and rosin (both from resin) • Fuel • Pulpwood • Construction lumber • Ornamentals • Pharmaceuticals (taxol for ovarian cancer from yew trees) Human Relevance of Gymnosperms Other Gymnosperms • Ginkgo: – Seeds for food (after seed coat removal) – Ginkgo extracts to increase blood circulation • Ephedra - Mormon tea – Drug ephedrine for respiratory problems from a Chinese species Review Introduction Phylum Pinophyta – The Conifers Other Gymnosperms • Phylum Ginkgophyta – Ginkgo • Phylum Cycadophyta – The Cycads • Phylum Gnetophyta – The Gnetophytes Human Relevance of Gymnosperms • Conifers • Other Gymnosperms