Download Unit 14 Plants Gymnosperms Notes

VASCULAR SEED PLANTS
GYMNOSPERMS
GYMNOSPERMS
Although Earth’s ancient forest were dominated by spore-producing vascular plants,
early seed bearing plants also evolved.
280 mya Earth’s climate changed
Long periods of drought and freezing weather caused many spore bearing plants to
become extinct.
A few seed-bearing plants had adaptations to survive
WHAT ARE GYMNOSPERMS?
Gymnosperm = vascular plants that produce seeds on the scales of woody cones
Seeds are not protected by a fruit
They do not produce flowers
4 Divisions that bear naked seeds:
1. Cycadophyta
2. Ginkophyta
3. Gnetophyta
4. Coniferophyta
REPRODUCTION:
Gymnosperm produce spores
Microspores = produced in the male cone and give rise to the male gametophyte
Eventually develop into pollen grains
Megaspores = produced in female cone
Give rise to female gametophyte
Each contained in an ovule and produces archegonia with egg cells
Pollen is carried by the wind to the ovule
The gametophyte contained in the pollen grain produces a pollen tube
This grows into the archegonium and provides a path for the sperm to reach the egg.
Occurs without water
After fertilization, the zygote develops into an embryo
Embryo = an organism at an early stage of growth and development
In plants, the embryo is the young, diploid sporophyte of a plant
A embryo can have many cotyledons
Cotyledons = food storage organs in a plant embryo that becomes the plants first leaves.
As the embryo develops, the tissues of the ovule form the food supply and seed coat of
the pine seed.
Advantages of Seeds Over Spores:
A seed consists of an embryo with a food supply enclosed in a tough protective coat
The seed contains a supply of food to nourish the young plant in early growth stages.
This food is used until the first leaves develop (photosynthesis)
Food supply is stored in cotyledons
Embryo is protected by a tough seed coat
Seeds are also adapted for easy dispersal
Young don’t have to compete with parent: sunlight, water, nutrients, space
Advantage:
Fertilization does not require water
Spore producing plants need a film of water for sperm to swim
In gymnosperms, male gametophyte develops inside a “pollen grain”
Pollen grain = includes sperm cells, nutrients, and a protective outer covering
Female gametophyte develops inside a ovule
Ovule = contains megaspore cell, one or 2 layers of tissue and a protective covering
Sperm is carried by the wind instead of water
GINKOPHYTA
Most numerous during the Jurassic Period
200 mya
Most died out about 65 mya (with dinosaurs)
Only one living species
Ginko biloba
Tree size
Sacred in China and Japan
Asian temple gardens seem to have prevented the tree from becoming extinct
Male and Female cones on separate plants
Male = pollen in strobilus like cones
Grow near bases of leaf clusters
Female = bears seeds
Orange-yellow “cherries”
Seed coat has an unpleasant smell
Most gardeners plant the male
CONIFEROPHYTA
Conifers
Largest and most diverse divisions of gymnosperms
Trees and Shrubs
Needle or scalelike leaves
Most produce seeds in woody cones
Pine, Spruce, Juniper, Cedar, Redwood, Sequoia
Fist emerged around 280 mya
During Jurassic (150 mya) prominent forest
Remain prominent today
Adapted to cold climates
Form large forests
Dry climates
Under dry freezing conditions, roots cannot absorb water from soil
Needlelike leaves are covered with a thick cuticle
Cuticle = protective, waxy layer (prevent water lose)
Have sunken stomata (openings for gas exchange)
Help retain water
Bark also helps reduce water loss by forming a protective covering over the stem
EVERGREENS:
= retain their leaves all year
Most are evergreens
Individual leaves drop off as age or damaged
Tree never loses all its leaves at once
Pine needles remain 2-40 years
Benefit = begin photosynthesis in early spring as soon as warms up
Usually have heavy coating of cutin
Often found where warm growing season is short
Keeping leaves gives a head start on growth
They are abundant where nutrients are scarce.
Thus, eliminate the need to grow a whole new set of leaves each year
Branches and needles are flexible
Allow snow and ice to slide off
No buildup = no breakage
DECIDUOUS:
A few conifers are deciduous (Bald Cypress tree)
= lose all their leaves at same time
Adaptation for reducing water loss when water is unavailable during winter
Plants lose most of water through the leaves
Very little is lost through the bark or roots
Cannot photosynthesize and must remain dormant during winter
HOW GROW SO TALL?
Tissues that make up much of the trunk of a conifer “wood” is composed of thick-walled,
nonliving cells = Tracheids
Tracheids = form the xylem and vascular systems
Provides support and transport water and dissolved minerals from the roots to all other
parts of the plant
Life Cycle of a Pine
1. Adult develops male and female cones on separate branches
Male is smaller
2. Female develops 2 ovules on upper surface of each cone scale
Each contains haploid megaspores
3. Male cone produces microspores = pollen grains
Pollen grains have hard water resistant coverings
4. As the female produces the egg = cone closed
5. Pollination = wind borne pollen falls near the opening of the ovule on female
cone
Each male pollen forms a pollen tube (penetrates tissue of female gametophyte)
6. Sperm fertilizes the egg
7. Zygote develops into an embryo and a mature seed is produced
8. Female cone opens and releases seeds
9. Seed germinates into a new seedling
CONIFERS ARE NAMED FOR CONES:
Reproductive structures produced in cones
Male and Female cones
Male = pollen grain
Female = eggs  fertilization  seed
Male = small and easy to overlook
Usually drop off the tree soon after pollen is released
Female cones are much larger
Stay on the trees until seeds are mature (few months to 2 years)
Most conifers have male and female cones on different branches of the same tree.