Alternation of Generations
• Basic pattern of life cycle is same for all plants
• 2 phases of plant life cycle
– Gametophyte stage
– Sporophyte stage
Sporophyte Generation
• Cells of sporophyte are diploid (2n)
• Certain cells undergo meiosis and produce haploid spores (n)
• These spores grow into the gametophyte
– Mitotic division
Gametophyte Generation
• Some cells will differentiate and form haploid gametes
• Female gamete – egg
• Male gamete - sperm
Fertilization
• When sperm fertilizes an egg, a diploid zygote is formed
• Zygote divides by mitosis
• Development of embryo into mature sporophyte allows life cycle to begin again
Dominant Generation
• The bigger, more obvious plant is the dominant generation
• Lives longer
• Can survive independently of the other generation
• Usually the sporophyte is dominant
Vegetative Reproduction
• Asexual reproduction
• New plant is produced from an existing vegetative structure
• Example: Liverworts
• New plants have same genetic make-up as the original plant
– As if they were cloned
Life Cycle of Mosses
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One of few plant divisions where the gametophyte generation is dominant
Need continuous film of water
Some can reproduce asexually by vegetative reproduction
– Break up into pieces when plant is dry and brittle
Reproductive Structures
• Archegonium
– Female structure
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– Where eggs are produced
Antheridium
– Male structures
– Where sperm is produced
Protonema
• Structure created by haploid spore
• Small green filament of cells
• Develops into either a male or female gametophyte
– In some mosses, the gametophyte can produce both kinds of reproductive
structures
Fertilization
• Motile sperm are released from antheridium and swim to archegonium
– Through continuous film of water
• Sperm fertilizes egg inside archegonium
– Forms the diploid zygote
• Zygote divides by mitosis to form new sporophyte
Sporophyte
• A stalk with a capsule at the top
• Grows out of archegonium
• Remains attached to gametophyte
– Receives nutrition from gametophyte
• Meiotic division within capsule provides haploid spores
Capsule
• Ripens
• Bursts and releases spores
• Spores can be carried by air
• Spore itself can germinate if it lands in a favorable environment
Life Cycle of Ferns
Sporophyte
• Dominant stage of fern life cycle
• Initially dependant on gametophyte for nutrition
• Fronds
– Sporophytes
– Grow from rhizome (underground stem)
Sporangia
• Sori
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Underside of some fronds
Clusters of sporangia
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Meiotic division within sporangia produces spores
When conditions are right, sporangia burst to release haploid spores
Prothallus
• Heart shaped gametophyte
• Spore germinates to form this
• Produces antheridia and archegonia
Fertilization
• Sperm swim through water to archegonium where egg is fertilized
• Fertilization forms diploid zygote
• Zygote develops into sporophyte
Mature Sporophyte
• Once sporophyte produces its green fronds, it is able to photosynthesize
• Prothallus disintegrates as sporophyte matures
– Produces rhizomes that support fronds
• New sporangia develop on pinnae of fronds
Life Cycle of Conifers
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Dominant generation is sporophyte
Adults produce male and female cones on separate branches
Cones
• Contain sporangia (spore-producing structures) on their scales
• Female cones
– Larger than male
– Develop 2 ovules on upper surface of each cone scale
Megaspores
• Produced by sporangium in ovule
• Female spore that eventually becomes the female gametophyte
• Four megaspores produced
• One will survive
– Will grow by mitotic cell divisions into female gametophyte
Female Gametophyte
• Consists of hundreds of cells
• Dependant on sporophyte for protection and nutrition
• Within female gametophyte there are two or more archegonia
– Each contain an egg
Microspores
• Male spores produced by male cones
• Each develops into pollen grain
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Male gametophyte
Has hard, water-resistant outer covering
Pollination
• Transfer of pollen grain from the male cone to the female cone
• Happens when wind-borne pollen grain falls on female cone
• Pollen grain adheres to sticky drop of fluid that covers the opening of ovule
Micropyle
• The opening of the ovule
• Pollen grain is drawn close to micropyle
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Fertilization will not happen for at least a year
Pollen grain and female gametophyte mature during this time
Pollen Tubes
• Produced as pollen grain matures
• Grows through the micropyle and into the ovule
• Transports sperm cell from male gametophyte to egg
– Where fertilization occurs
Zygote
• Nourished by female gametophyte
• Develops inside embryo with several cotyledons
• Ovule produces the seed coat
Seed
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Released when female cone opens
Seed germinates into new, young sporophyte in good conditions
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