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Introduction to Seed Plants: Gymnosperms
Chapter 22
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Outline Introduction Phylum Pinophyta – The Conifers Other Gymnosperms
Phylum Ginkgophyta – Ginkgo Phylum Cycadophyta – The Cycads Phylum Gnetophyta – The Gnetophytes Human Relevance of Gymnosperms Conifers
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Introduction Oldest known seeds - Late Devonian, >350 mya
Seeds = significant adaptation for plants on land Protective seed coat Supply of food for embryo Capable of dormancy in unfavorable environmental conditions 1st seed plants fernlike in appearance = pteridosperms (seed ferns) - reclassified as gymnosperms
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Introduction Gymnosperm refers to exposed nature of seeds
Seeds produced on surface of sporophylls or similar structures, instead of enclosed within fruit as in flowering plants Seed-bearing and pollen-bearing sporophylls often arranged in cones
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Introduction Pollen cones produce pollen grains
Female gametophyte - produced inside ovule containing nucellus Nucellus enclosed in integument Integument becomes seed coat after fertilization Female gametophyte more reduced in cell # than ferns and their relatives Does not grow independently, but develops within sporophyte structures
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Introduction 4 living phyla Pinophyta - pines, firs, spruces, cedars
Fossils date back to late Carboniferous, 290 mya Ginkgophyta - single living representative, Ginkgo Tree with fan-shaped leaves Seeds enclosed in fleshy covering Cycadophyta - leaves superficially palm-like Gnetophyta - 3 genera Wood with vessels
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Phylum Pinophyta – The Conifers
Pines (Pinus) 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 2-5 leaves Cluster = fascicle Fascicles = short shoots with restricted growth
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Phylum Pinophyta – The Conifers
Structure and form cont’d.: Modifications to survive harsh conditions Hypodermis located below epidermis 1-2 layers of thick-walled cells Thick cuticle Recessed or sunken stomata Resin canals Resin antiseptic and aromatic, prevents development of fungi, and deters insects Mycorrhizal fungi associated with roots of most conifers
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Phylum Pinophyta – The Conifers
Structure and form cont’d.: Wood consists entirely of tracheids Conifer wood = softwood - thick-walled cells absent Dicot wood = hardwood - thick-walled vessels and fibers present
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Phylum Pinophyta – The Conifers
Reproduction: 2 kinds of spores Pollen cones (male strobili) - papery or membranous scales Microsporangia in pairs toward bases of scales
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Phylum Pinophyta – The Conifers
Reproduction cont’d.: Microspores develop into pollen grains Pollen grain consists of 4 cells and pair of air sacs aiding in wind dispersal
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Phylum Pinophyta – The Conifers
Reproduction cont’d.: Megaspores in megasporangia within ovules Pair of ovules at bases of seed cone scales Seed cones larger than pollen cones Have woody scales with inconspicuous bracts between
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Phylum Pinophyta – The Conifers
Reproduction cont’d.: Ovule contains megasporangium containing nucellus and single megasporocyte Megasporangium surrounded by integument Integument has pore = micropyle Megasporocyte undergoes meiosis, producing 4 megaspores 3 megaspores degenerate Remaining megaspore develops into female gametophyte with archegonia at micropyle end
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Phylum Pinophyta – The Conifers
Reproduction cont’d.: Seed cones take 2 years to mature 1st year: Pollen grains catch on sticky pollen drops oozing out of micropyle Pollen grain produces pollen tube that grows through nucellus 2 sperms produced in pollen tube Mature male gametophyte = germinated pollen grain with pollen tube and 2 sperm Sperm have no flagella and no antheridium Megaspore develops
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Phylum Pinophyta – The Conifers
Reproduction cont’d.: 2nd year: Female gametophyte and archegonium mature Pollen tube arrives at archegonium 1 sperm unites with egg, forming zygote Other sperm degenerates Embryo nourished by female gametophyte Integument becomes seed coat
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Phylum Pinophyta – The Conifers
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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 Southern hemisphere conifer - Podocarpus Fleshy-coated seeds with large appendage at base Junipers - seed cones fleshy Taxus Podocarpus
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Other Gymnosperms Phylum Ginkgophyta – Ginkgo (maidenhair trees)
1 living species only existing in cultivation Notched, broad, fan-shaped leaves Leaves on short, slow-growing spurs No midrib or prominent veins Hair-like veins branch dichotomously Deciduous
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Other Gymnosperms 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
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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
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Other Gymnosperms Phylum Gnetophyta – The Gnetophytes
Unique among gymnosperms in having vessels in xylem Joint firs (Ephedra) - shrubby plants of drier regions of southwestern North America Part of plant Male strobili Female strobilus Ephedra
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Other Gymnosperms Phylum Gnetophyta – The Gnetophytes
Gnetum - vine-like plants with broad leaves In tropics
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Other Gymnosperms Phylum Gnetophyta – The Gnetophytes
Welwitschia - only 1 species, confined to deserts of southwestern Africa Short stem Long taproot Only 2 straplike leaves that become tattered and split Dioecious Has male and female strobili
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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)
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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 Chinese species
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Review Introduction Phylum Pinophyta – The Conifers Other Gymnosperms
Phylum Ginkgophyta – Ginkgo Phylum Cycadophyta – The Cycads Phylum Gnetophyta – The Gnetophytes Human Relevance of Gymnosperms Conifers
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