The Plant Kingdom: Gymnosperms

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The Plant Kingdom Gymnosperms

• Chapter 24

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Ponderosa pines (Pinus ponderosa), Yosemite
National Park.
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LEARNING OBJECTIVE 1

• Compare the features of seeds with those of
  spores
• Discuss the adaptive advantages of plants that
  reproduce by seeds

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KEY TERMS

• SEED
• A reproductive body consisting of a young,
  multicellular plant and food reserves, enclosed
  by a seed coat

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Spores

• A spore is a single cell with minimal food
  reserves to sustain the plant that develops from
  a germinating spore
• Seeds are reproductively superior to spores

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What are the significant selective advantages of
Seeds?

• The dormant state of seeds enables seed plants to
  survive extended periods of cold winter or
  drought.
• The seed coat serves as a barrier against
  bacterial or fungal decay.
• The integument protects the embryo from
  desiccation.
• Seeds attract seed-eating animals, which destroy
  some seeds but distribute others.
• Seeds include food for the developing embryo and
  germinating seedling.
• The pollen tube delivers sperm directly to eggs
  and eliminates the need for fresh water for
  sexual reproduction.

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Gymnosperm naked seeds and angiosperm seeds in
fruit. Gymnosperm seeds are exposed on the
surface of modified branches or leaves. The
enclosed seeds of flowering plants, or
angiosperms, form inside fruits.
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Comparing the sporophyte-gametophyte relationship
in seedless plants and seed plants. In contrast
with the two groups of seedless plantsbryophytes
and seedless vascular plantsseed plant
gametophytes, such as those in gymnosperms, are
dependent on the mature sporophyte.
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Seeds
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Embryonic leaves
Seed coat parental sporophyte tissue
Embryo daughter sporophyte (diploid)
Haploid gametophyte (food supply)
Embryonic root
(a) Cross section through a pine seed.
Fig. 24-1a, p. 468
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Scale
Ovule (future seed)
(b) Gymnosperm seed. Longitudinal section through
a female pine cone, showing the ovules (which
develop into seeds) borne on scales. Note the
absence of an ovary wall.
Fig. 24-1b, p. 468
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Fruit (ovary wall)
Seed
(c) Angiosperm seed. Longitudinal section through
an avocado fruit, showing the seed surrounded by
ovary tissue of the maternal sporophyte.
Fig. 24-1c, p. 468
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KEY TERMS

• OVULE
• Structure in seed plants that develops into a
  seed following fertilization
• INTEGUMENT
• Outer layer of an ovule that develops into a seed
  coat following fertilization

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LEARNING OBJECTIVE 3

• Summarize the features that distinguish
  gymnosperms from seedless vascular plants

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KEY TERMS

• GYMNOSPERM
• Any of a group of seed plants in which the seeds
  are not enclosed in an ovary

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Gymnosperm Seeds

• Unlike seedless vascular plants, gymnosperms
  produce seeds
• Gymnosperm seeds are either totally exposed or
  borne on scales of cones

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Yew Seeds
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Gymnosperm Pollen

• Gymnosperms produce wind-borne pollen grains
• Seedless vascular plants do not produce pollen
  grains

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KEY TERMS

• POLLEN GRAIN
• Structure in seed plants that develops from a
  microspore into a male gametophyte

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LEARNING OBJECTIVE 4

• Name and briefly describe the four phyla of
  gymnosperms

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4 Phyla of Gymnosperms

• 1. Conifer
• Woody trees and shrubs with needlelike, mostly
  evergreen leaves, and seeds in cones

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Bristlecone Pine
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4 Phyla of Gymnosperms

• 2. Cycads
• Palmlike or fernlike in appearance
• Pollen and seeds in conelike structures
• Relatively few living members

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Cycads
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4 Phyla of Gymnosperms

• 3. Ginkgoes
• Ginkgo biloba, only surviving species in phylum
• A deciduous tree
• Female ginkgoes produce fleshy seeds directly on
  branches

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Ginkgo tree.
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Ginkgo biloba
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4 Phyla of Gymnosperms

• 4. Gnetophytes
• Share traits with angiosperms
• More efficient water-conducting cells (vessel
  elements) in xylem

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Gnetophytes
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Gnetophytes
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Gnetophytes
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Welwitschia mirabilis.
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Gymnosperm Evolution
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Gymnosperms
Angiosperms
Gnetophytes
Ginkgoes
Conifers
Cycads
Evolution of flowering plants
Evolution of seeds
Fig. 24-3, p. 470
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Gymnosperms
Angiosperms
Gnetophytes
Ginkgoes
Conifers
Cycads
Stepped Art
Fig. 24-3, p. 470
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KEY TERMS

• CONIFER
• Any of a large phylum of gymnosperms that are
  woody trees and shrubs with needlelike, mostly
  evergreen, leaves and seeds in cones

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Conifers
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Drought adaptations of a pine needle. The pine
needles recessed stomata, the thick epidermis,
the hypodermis, the endodermis surrounding the
vascular bundle, and the transfusion tissue are
all adaptations that prevent water loss.
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Conifer Leaves
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(a) In white pine (Pinus strobus), leaves are
long, slender needles that occur in clusters of
five.
Fig. 24-5a, p. 472
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(b) In American arborvitae (Thuja occidentalis),
leaves are small and scalelike (see inset).
Fig. 24-5b, p. 472
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LEARNING OBJECTIVE 5

• Contrast monoecious plants and dioecious plants

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KEY TERMS

• MONOECIOUS
• Having male and female reproductive parts in
  separate flowers or cones on the same plant
• DIOECIOUS
• Having male and female reproductive structures on
  separate plants

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Monoecious or Dioecious

• Most conifers are monoecious
• Cycads, ginkgo, and most gnetophytes are dioecious

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LEARNING OBJECTIVE 6

• Trace the steps in the life cycle of pine
• Compare the sporophyte and gametophyte generations

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Pine

• A pine tree is a mature sporophyte
• Pine gametophytes are extremely small and
  nutritionally dependent on sporophyte generation
• Pine is heterosporous
• Produces microspores and megaspores in separate
  cones
• Each cone has sporophylls

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Male and Female Cones
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Pollen Cone
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Pollen grains (immature male gametophytes)
Tip of scale
Microsporangium on scale in male cone
Fig. 24-8, p. 474
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Pine Spores

• Male cones produce microspores
• Develop into pollen grains, carried by air
  currents to female cones
• Female cones produce megaspores
• One of four megaspores produced by meiosis
  develops into a female gametophyte within an
  ovule (megasporangium)

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Pollination

• Pollination
• The transfer of pollen to female cones
• After pollination
• A pollen tube grows through the megasporangium to
  the egg within the archegonium

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KEY TERMS

• POLLEN TUBE
• In seed plants, a tube that forms after the
  germination of a pollen grain and through which
  male gametes (sperm cells) pass into the ovule

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Development

• After fertilization
• The zygote develops into an embryo encased in a
  seed adapted for wind dispersal

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Animation Pine Life Cycle
CLICKTO PLAY
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Life Cycle Pine
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What are some selective advantages of conifers in
tolerance of cold weather and dry winds?

• Leaves are narrow, which expose less surface to
  the air and are therefore less susceptible to
  damage by freezing or by dry winds.
• The stomata are recessed and therefore lose water
  less readily.
• Since they lack vessel elements, they are not
  prone to permanent disruption of water flow by
  freezing.
• Their vascular bundles are surrounded by an
  endodermis, which prevents water loss by
  directing water and mineral transport through
  cell membranes.
• Between the vascular bundles and the endodermis,
  a region of transfusion tissue moves liquid
  efficiently from the xylem into the mesophyll.
• The area leaf between the epidermis and
  endodermis, known as the hypodermis, has
  thick-walled cells that prevent water loss.

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Microsporangium
Microspores, each of which develops into a pollen
grain
Each scale bears two microsporangia
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Pollen grains are transferred to the female cone
by wind
Scale from a male cone
Male cone
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Scale from a female cone
Female gametophyte
Megasporangium
Each scale bears two ovules (megasporangia)
Megaspore
Growing pollen tube
Ovule
HAPLOID (n) GAMETOPHYTE GENERATION
Meiosis
Fertilization
DIPLOID (2n) SPOROPHYTE GENERATION
Immature female cone
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Zygote
Second sperm nucleus
Papery wings
Sperm nucleus united with egg nucleus
Pollen tube
Seed coat
Male cones (pollen cones)
1
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Mature female cone (seed cone)
Embryo
Pine (mature sporophyte)
Two seeds on the upper surface of the scale
Newly germinated seedling
Female gametophyte (nutritive tissue)
Fig. 24-6, p. 473
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LEARNING OBJECTIVE 7

• Describe the ecological and economic significance
  of gymnosperms

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Ecology

• Conifers are the predominant trees in about 35
  of the worlds forests
• Their roots hold soil in place, reducing soil
  erosion
• Conifer forests are important watersheds and
  provide habitat for many organisms

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Economy

• Recreational uses of forests
• Camping, backpacking, picnicking, observing
  nature
• Products
• Lumber, medicinal products, turpentine, resins
• Conifers grown commercially
• Landscape design, Christmas trees

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Amber
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Commercially Important Conifers
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(a) Douglas fir (Pseudotsuga menziesii ) grows
along the Pacific coast and in the Rocky
Mountains.
b) Red spruce (Picea rubens), found in eastern
Canada and the northeastern United States, also
extends southward to the Great Smoky Mountains.
(c) Loblolly pine (Pinus taeda) is
widely distributed through the southeastern United
States.
Fig. 24-15, p. 480
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LEARNING OBJECTIVE 8

• Trace the evolution of gymnosperms from seedless
  vascular plants

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Evolution 1

• Seed plants evolved from seedless vascular plants
  
• Progymnosperms were seedless vascular plants that
  had megaphylls and modern woody tissue

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Evolution 2

• Progymnosperms probably gave rise to conifers and
  seed ferns
• Which likely gave rise to cycads and ginkgo
• Evolution of gnetophytes is unclear
• Molecular data indicate they are closely related
  to conifers

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Evolution of Seed Plants
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(a) Progymnosperm. Archaeopteris, which existed
about 370 mya, had some features in common
with modern seed plants but did not produce seeds.
Fig. 24-13a, p. 478
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(b) Seed fern. Emplectopteris produced seeds on
fernlike leaves. Seed ferns existed from about
360 mya to 250 mya.
Fig. 24-13b, p. 478