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Chapter 19
Lecture Outline
Introduction to
Seed Plants:
Gymnosperms
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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
• 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