Gymnosperms

Chapter 24

Gymnosperms

• “naked seed”

• Seeds are beneath cone scales or other specialized structures

• Not enclosed inside a fruit

• Nonflowering seed plants

• Present throughout world– Particularly prominent in cool, temperate

forests

Gymnosperms

• Living examples– Cycads (Cycas, Dioon, Zamia)– Maidenhair tree (Ginkgo)– Cedar (Cedrus)– Juniper fir (Juniperus)– Pine (Pinus)– Redwood (Sequoia, Metasequoia, Sequoiadendron) – Spruce (Picea)– Mormon tea or joint fir (Ephedra)

Origin of Seeds, Pollen, and Wood

• Fossils from Devonian and early Carboniferous periods provide information about origin of seed plants

• Progymnosperms– Reveal transition or intermediate form

between spore-releasing vascular plants and seed plants

Origin of Seeds, Pollen, and Wood

• Plants with true seeds appeared in Devonian period

• Oldest known seed plant, Elkinsia polymorpha– Small tree with fernlike foliage– Ring of separate vascular bundles– Fossils found in West Virginia and Belgium

Origin of Seeds, Pollen, and Wood

• Steps involved in evolution of seeds– Heterospory

• Creates division of labor, enhances sedentary nature of one type of gametophyte and mobility of the other

– Retention of megaspore inside megasporangium

– Development of pollen to deliver sperm to egg cell

Features of Gymnosperms

• Seeds and pollen

• Megaphylls reduced to simple leaves, needles, or scales

• Primary stem vascular system – Ring of separate bundles with phloem toward

the outside and xylem toward the center

Features of Gymnosperms

• Secondary growth from lateral cambium– Secondary xylem to the inside and secondary

phloem to the outside

• Main stem with lateral branching

• Almost all are woody

• Most are trees

Relationships Among Gymnosperms

• Four gymnosperm groups traditionally classified as divisions– Each is monophyletic (possible exception is conifers)– Exact relationships are problematic– Groups

• Cycads – possess series of ancestral features• Ginkgo – retains ancestral features, shares similarities with

conifers• Conifers – bulk of gymnosperm species• Gnetophytes – vegetative and reproductive traits similar to

flowering plants

Gymnosperm Dominance

• Mesozoic era– 245 million to 65 million years ago– Age of gymnosperms, dinosaurs, and moving

continents

• Today fewer species and growth forms

• Occupy fewer habitats than during Mesozoic era

Gymnosperm Vascular System

• Primary stem vascular system composed of a ring of bundles

• Distinct pith and cortex regions

• Vascular cambium produces secondary tissues

Gymnosperm Vascular System

• Some differences in secondary growth patterns– Cycads produce light wood with abundance of

living parenchyma– Ginkgo and other conifers produce dense

wood primarily composed of cells that are dead at maturity

Gymnosperm Vascular System

• Trunks have secondary xylem, phloem, rays, and bark

• Absence of vessels– Gnetophytes may have vessels in their wood

• Seasonal variation in tracheid size results in annual rings in wood

• Phloem– Sieve cells, fibers, ray parenchyma, axial

parenchyma

Gymnosperm Vascular System

• Many produce resin– Mix of organic byproducts of tree’s

metabolism– Accumulates and flows in resin ducts– Inhibits insects– Stressed trees produce less resin making

them more susceptible to insect infestation

Gymnosperm Vascular System

• Adapted to survive drought– Needles (leaves)

• Thick cuticle • Sunken stomata• Fibrous epidermis• Closely packed mesophyll without intercellular air

spaces• Veins only in center of leaf• Thick rather than thin• Lifespan ranging from 3 to 30 years (do not

continue growing during life span)

Cycads

• Contains only 11 genera and about 125 species

• Mainly grow in tropics

• Zamia integrifolia only species occurring naturally in United States (Florida and southern Georgia)

• Contain potent toxins– Can be eaten if prepared correctly

Cycads

• Stem pith yields edible starch

• Many are palmlike in appearance

• Slow growing – Specimen 2m in height could be as old as

1,000 years

Cycads

• Distinctive features– Large compound leaves– Rarely branch– Wood has a lot of parenchyma in xylem

• All are dioecious– Seed producing cone large and often

protected by sharp prickles or woody plates– Pollen strobili large and upright

• Vectors of pollen transfer are beetles or wind

Cycads

• Seeds– Often covered with fleshy, brightly colored

seed coat to attract animal dispersers

Ginkgo

• Single living representative maidenhair tree (Ginkgo biloba)

• Grows wild only in warm-temperate forests of China

• Planted throughout world as urban street tree because of pollution tolerance

• Tree is symbol of longevity

Ginkgo

• Uses for herbal supplements made from leaves– Brain dysfunction– Cardiovascular fitness

• Seeds– Important food in Asia – Mildly toxic– Poisoning rare, but more likely to occur in

children

Ginkgo

• Dense wood similar to conifer wood• Fan-shaped leaves

– Often divided into two lobes– Turn brilliant golden color in fall before

dropping

• Dioecious– Due to foul-smelling, fleshy seed coat,

planting of pollen-producing trees is preferred as ornamental

Conifers

• Most widely known and economically important gymnosperms

• Approximately 650 species

• “cone bearer”

• Conifers that lack woody cones– Junipers, podocarps, yews, plum yews

Conifer Clades

• One clade includes:– Araucariaceae– Podocarpaceae

• Other clade includes:– Taxaceae– Cupressaceae– Sciadopityaceae– Cephalotaxaceae

Pinaceae

• Includes pines, firs, spruces

• Economically important for wood, pulp, turpentine, resin, and as ornamentals

• Constitutes bulk of conifer forests

• Leaves needlelike single or in clusters called fascicles

• Most are monoecious

Pinaceae

• Pines (Pinus) – largest genus in family – 93 species– Large, long-lived trees with asymmetrical

shape– Bristlecone pines (Pinus longaeva) – oldest

living organisms• May be more than 5,000 years of age

Pinaceae

• Pines– Cones usually shed once seeds have

matured and spilled out– Closed-cone pines keep scales closed until

heated by fire

Pinaceae

• Firs (Abies)– Symmetrical, cylindrical, or pyramidal in

shape– Annual growth marked by symmetrical whorl

of branches– Seed cones shatter at maturity rather than

falling as a unit– About 40 species

• Restricted to cooler parts of Northern Hemisphere

Pinaceae

• Spruces (Picea)– About 40 species

• All in Northern Hemisphere

– Resins • Once collected and chewed by Native Americans

and European immigrants• Spruce Gum

– Gum manufacturers eventually shifted to paraffin and then chicle latex from an angiosperm tree

Pinaceae

• Hemlocks (Tsuga)– Pyramidal with slender, horizontal branches

and drooping tops– About 10 species in North America and Asia

Pinaceae

• Douglas firs (Pseudotsuga)– 5 species– Pseudotsuga menziesii

• Most heavily cut timber tree in United States• Dominates Pacific Northwest, Cascade, and Rocky

Mountain regions

Pinaceae

• Larches and tamaracks (Larix)– Unusual among conifers in being deciduous– American larch frequently found at edge of

bogs

• Cedars (Cedrus sp.)– Native to North Africa and Asia

• Widely planted as ornamentals in North America

– Asymmetrical to pyramidal in shape– Important timber trees since biblical times

Cupressaceae

• Includes junipers, cypresses, redwoods– Can be monoecious or dioecious– More than 130 species with world-wide

distribution– Juniper cones can be eaten and are used to

flavor gin– Cone scales

• Woody in cypress• Fleshy in juniper

Cupressaceae

– Dawn redwood and bald cypress deciduous– Coast redwood possibly tallest tree in world– Sierra redwood most massive tree in world

• Largest living tree, the General Sherman tree, estimated to weigh 625 metric tons

– Dawn redwood• Known only from fossil record until living trees

were discovered in China’s Szechwan Province

Taxaceae

• Includes yews– Shrubs or trees with dark-colored, broadly

linear, sharp-pointed leaves– Dioecious– Only conifers that lack cones– English yew (Taxus baccata)

• Famous for bows made of its wood

– Pacific yew (Taxus brevifolia)• Found to contain anticancer compound, TAXOL

Cephalotaxaceae

• Includes plum yews– Consists of less than 10 species of Chinese

shrubs and trees– Dioecious

Podocarpaceae

• About 140 species– Most are restricted to Southern Hemisphere – Most are dioecious– Cones are attractive to birds (vector for seed

distribution) – Some excellent lumber trees are in this

lineage – Many species widely planted as ornamentals

Araucariaceae

• More than 30 species exclusively native to Southern Hemisphere– Genera Araucaria, Agathis, Wollemia– Relatively large trees– Cones disintegrate when ripe– Some, such as Norfolk Island pine, widely

planted as ornamentals throughout world

Life Cycle of Pinus

• Pine sporophytes are trees (except Pinus mugo, which is a shrub)

• Heterosporous• Pollen produced in strobili• Female gametophytes produced in

ovulate or seed cones• Strobili and ovulate cones differ in size,

architecture, longevity, and location on tree

Life Cycle of Pinus

• Pollen strobili– Average 1 cm in length and 5 mm in diameter– Produced in groups, usually on lower

branches of trees– Each strobilus composed of many

microsporophylls spirally attached to an axis

Life Cycle of Pinus

– Two microsporangia develop on underside of each sporophyll

• Microsporangium lined with layer of nutritive cells called the tapetum

• Inside microsporangium, microsporocytes undergo meiosis, form haploid microspores

• Resulting pollen grain (immature male gametophyte) contains two nuclei that undergo further divisions

Life Cycle of Pinus

• Pollen grains– Yellow and lightweight (wind dispersal)– Generally released in spring– Have two inflated wings that help orient pollen

grain on the pollination droplet of ovule

Life Cycle of Pinus

• Ovulate cone– Familiar cone associated with pines and other

conifers– Composed of many woody scales spirally

attached to axis beneath it– Young ovulate cone often reddish and softer

and smaller than male strobilus – Develop singly in early spring at tips of young

branches in upper part of tree

Life Cycle of Pinus

• Ovulate cone– Two ovules develop on upper surface of each

scale– Megasporangium and its integument layer

form ovule of pine– Megasporocyte divides by meiosis producing

four megaspores– Only one of the megaspores develops into

megagametophyte

Life Cycle of Pinus

• Ovulate cone– Megaspore slowly grows into female gametophyte

(several months to just over a year)– Two or more archegonia develop at micropylar end of

gametophyte (digest nucellus for energy)– Mature ovule consists of integument, thin layer of

remaining nucellus, female gametophyte that is undifferentiated except for several archegonia at one end

– Each archegonium has enclosed egg– Space between micropyle and nucellus micropylar

chamber (where pollen grains begin to grow pollen tubes)

Life Cycle of Pinus

• Pollination – Transfer of pollen from male strobilus to

ovulate cone– Usually undergo cross-pollination

• More likely due to placement of ovulate cones above pollen strobili

– Occurs when ovulate cone is about nine months old

Life Cycle of Pinus

• Pollination – Pollination drop

• Exuded from micropyle• Chemically similar to flower nectar• Passively traps pollen grains that touch it

– Chemical signal• Diffuses from trapped pollen to ovule• Triggers absorption of liquid, draws pollen grains

through micropyle into micropylar chamber

Life Cycle of Pinus

• Pollination– Pollen grain germinates– Grows through nucellus toward egg– Pollen grain undergoes nuclear division in

tube– Final division produces two sperm nuclei– Pollen tube, containing two sperm nuclei and

several vegetative nuclei, is mature microgametophyte

Life Cycle of Pinus

• Fertilization– Sperm nuclei discharged directly around egg

cell• One sperm nucleus enters egg and fuses with egg

nucleus

– Fertilized egg becomes diploid zygote– Divides and forms proembryo

• Apical cells of proembryo develop into embryo

Life Cycle of Pinus

• Seeds– Cone scales open and seeds fall out when

seeds are mature– Wing attached to seed aids in dispersal – Seeds usually spread immediately on

ripening, in late summer or early fall

Life Cycle of Pinus

• Seeds– Usually lie dormant until the next spring– Dormancy broken by exposure to cold, wet

winter conditions• Prevents pines from germinating on warm fall days

– Germination • Radicle emerges first• Hypocotyl elongates, takes cotyledons and epicotyl

above surface

Life Cycle of Pinus

– Germination • Cotyledons become photosynthetic when light

strikes them• Cotyledons may remain attached into second year• Young pine grows slowly during first year, most

growth occurs below ground in root system

Gnetophytes

• Three living genera– Ephedra– Gnetum– Welwitschia

Gnetophytes

• Ephedra – Also called joint fir or Mormon tea– Only gnetophyte found in North America– Vine or shrub with opposite or whorled leaves

and prominent joints– Source of the drug ephedrine

• Alkaloid• Constricts swollen blood vessels• Mild astringent • Overdose can cause death

Gnetophytes

• Ephedra – Asian species contain more ephedrine

• Called ma huang• Important in Chinese herbal medicine

– Native American groups• Used tea to cure venereal diseases

Gnetophytes

• Gnetum – Tropical genus of 30 species– Includes lianas (climbing vines), shrubs, or

trees– Leaves resemble those of broad-leafed

flowering plants – Difficult to identify unless reproductive

structure are present

Gnetophytes

• Welwitschia– Found in Namib Desert– Characterized by two long, leathery, straplike

leaves that trail along soil surface– Darwin described it as “platypus of plant

kingdom”– Slow growing plants– Individual plants can attain ages of 1,000 to

2,000 years

Ecological and Economic Importance of Gymnosperms

• Ecological importance– Foliage rich in organic acid

• Decomposition makes soil acidic and relatively low in nutrients

– Strongly affects plant species that can grow – Acidity hinders bacteria but favors fungi

• Foliage and wood high in secondary compounds that inhibit grazing animals

Ecological and Economic Importance of Gymnosperms

• Economic importance– Major source of lumber, paper pulp,

turpentine, resins– Useful as fuel for heat

• Provides energy for smelting, heating, cooking

– Widely used in landscaping