Transcript Introduction to Seed Plants: Gymnosperms

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 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
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
gymnosperms in having
vessels in 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 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