Introduction to Seed Plants: Gymnosperms

Chapter 22

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, >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

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

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

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

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

Phylum Pinophyta – The Conifers• Structure and form cont’d.:

−Modifications to survive harsh conditionsHypodermis located below epidermis»1-2 layers of thick-walled cells

Thick cuticleRecessed or sunken stomataResin canals»Resin antiseptic and aromatic, prevents

development of fungi, and deters insectsMycorrhizal fungi associated with roots of most

conifers

Phylum Pinophyta – The Conifers• Structure and form cont’d.:– Wood consists entirely of tracheids

Conifer wood = softwood - thick-walled cells absentDicot wood = hardwood - thick-walled vessels and

fibers present

Phylum Pinophyta – The Conifers• Reproduction:

−2 kinds of spores

−Pollen cones (male strobili) - papery or membranous scalesMicrosporangia

in pairs toward bases of scales

Phylum Pinophyta – The Conifers• Reproduction cont’d.:

−Microspores develop into pollen grainsPollen grain

consists of 4 cells and pair of air sacs aiding in wind dispersal

Phylum Pinophyta – The Conifers• Reproduction cont’d.:

−Megaspores in megasporangia within ovulesPair of ovules at

bases of seed cone scales

−Seed cones larger than pollen conesHave woody scales

with inconspicuous bracts between

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

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

Phylum Pinophyta – The Conifers• Reproduction cont’d.:−2nd year:

Female gametophyte and archegonium mature

Pollen tube arrives at archegonium1 sperm unites with egg, forming zygote»Other sperm degenerates

−Embryo nourished by female gametophyte−Integument becomes seed coat

Phylum Pinophyta – The Conifers

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

Other Gymnosperms Phylum Ginkgophyta – Ginkgo

(maidenhair trees)• 1 living species only existing in

cultivation• Notched, broad, fan-shaped

leaves−Leaves on short, slow-

growing spursNo midrib or prominent

veinsHair-like veins branch

dichotomouslyDeciduous

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

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• 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

Other Gymnosperms Phylum Gnetophyta – The Gnetophytes• Gnetum - vine-like plants with broad leaves

− In tropics

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

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

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