Transcript Ginkgo

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