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

1 st 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 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

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

Phylum Pinophyta – The Conifers
• Reproduction:
− 2 kinds of spores
− Pollen cones (male
strobili) - papery or membranous scales
 Microsporangia in pairs toward bases of scales

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

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

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 archegonium
 1 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, slowgrowing spurs
 No midrib or prominent veins
 Hair-like veins branch dichotomously
 Deciduous

Other Gymnosperms
• Life cycle similar to pines
− Dioecious - male and female structures on separate trees
− Seeds enclosed in fleshy seed coat with nauseating odor
Male strobili
Seeds and leaves

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
Ephedra
Part of plant
Male strobili
Female strobilus


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


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)


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