PLANT REPRODUCTION All Materials © Cmassengale

PLANT LIFE CYCLES: 

• A life cycle includes all of the stages of an organism's growth and development
• A plant's life cycle involves two alternating multicellular stages - a Diploid (2n) sporophyte stage and a Haploid (1n) gametophyte stage
• This type of life cycle is called Alternation of Generations

Moss Characteristics: 

• Nonvascular (pass water cell-to-cell)
• Seedless (reproduce by spores)
• Low growing
• Phylum Bryophyta (also includes liverworts & hornworts)
• Grow on moist brick walls, in sidewalks, as thick mats on forest floors, and on the shaded side of trees

• Can survive periodic dry spells, reviving when water becomes available
• Require water for fertilization so sperm can swim to egg
• Rhizoids (root like structures) anchor mosses
• Have waxy covering called cuticle on aerial parts to prevent desiccation 

 Moss Life Cycle:

•  Dominant form of a moss is a clump of leafy green gametophytes (photosynthetic)
•  Moss alternates between a haploid (1n) gametophyte and diploid (2n) sporophyte
• Gametophyte generation produces gametes (eggs & Sperm)
• Sporophyte generation forms at the top of the gametophytes and produces spores 
• Stalk-like sporophytes lack chlorophyll
• Capsule at the top of the sporophyte forms haploid (1n) spores

Sexual reproduction in Moss:

• Moss produce 2 kinds of jacketed gametes --- eggs & sperm
• Egg producing organ is called the archegonium
• Eggs are larger and nonmotile
• Sperm producing organ is called the antheridium
• Sperm are smaller, flagellated cells
• Antheridia & archegonia are both part of the gametophyte plant
• Fertilization can occur only during or soon after RAIN  when the gametophyte is covered with Water
• Sperm swim to the egg by following a trail of chemicals released by the egg in the water
• Fertilization produces a zygote that becomes a sporophyte
• Mature sporophytes produce homosporous spores (all the same type)
• Mature capsules open & release spores spread by wind
• Spores landing on moist places germinate into protonema that become new gametophytes

 Asexual Moss Reproduction:

• Small pieces may break off from a gametophyte & become a new plant (fragmentation)
• Small buds called gemmae may be washed off by rain and develop new moss plants

Fern Characteristics & Life Cycle:

• Largest group of seedless, vascular plants
• Grow in moist places
• Goes through alternation of generations 
• Sporophyte phase is the dominant stage
• Fern gametophytes are small, flat plants anchored to the soil by root-like rhizoids
• Antheridia & archegonia form on the underside of fern gametophytes

• Sperm swim to egg through water droplets to form zygote (fertilized egg)
• Zygotes form new sporophytes with roots, stems, & leaves
• Spore cases called sori form on the underside of fern fronds (leaves)

• Ferns are homosporous (single type of spore formed)

• New fronds form from an underground stem called the rhizome

• Vascular tissue carries nutrients & water between the parts of the fern

• Fronds are compound leaves attached by a short stalk called the stipe to the underground stem or rhizome

• Immature fronds or fiddleheads are coiled
   

Characteristics & Life Cycle of Conifers:

• Called gymnosperms

• Have naked seeds that develop on scales of the female cones

• Sporophyte is the dominant stage

• Adapted to cooler climates

• Called evergreens (pine, cedar, spruce, fir...)

• Giant Redwood is one of the Earth's largest organisms

• Bristlecone Pines are the oldest living organisms (some more than 5000 years old)

Giant Redwood	Bristlecone Pine

• Produce 2 types of spores (heterosporous)

• Male spores called microspores grow into male gametophytes

• Female spores called megaspores grow into female gametophytes

• A Pine cone is the female cone on a pine tree

• Male cones on pine trees are smaller & grow in clusters at the tips of branches

• Both male & female cones appear on the same tree

Female Cones	Male Cones

• The pine life cycle takes 2-3 years from the formation of cones until seeds are released

• Female cones have spirally-arranged scales with ovules at their base

• Female cones produce sticky resin

• Ovules contain an egg that will develop into a seed 

• Male cones produce large amounts of pollen in the spring that is spread by wind to the female cones

• Resin traps the pollen so pollination can occur

• A tube from the pollen grain takes a year to grow to the ovule so a sperm can fertilize the egg and form seeds

Angiosperms or Flowering Plants:

• Bright colors, attractive shapes, and fragrant aromas help flowering plants attract their pollinators (insects, birds, mammals...)
• Flowers without bright colors and pleasing odors are usually wind or water pollinated (grasses)
• Called angiosperms
• Flowers, the reproductive part of a plant,  have a swollen base or receptacle to attach to the stem
• Flowers have 4 whorls (modified leaves) attached to the receptacle --- petals, sepals, pistils, and stamen
• Pistils (innermost whorl) are the female part of the flower, while Stamens are the male part
• Sepals (outermost whorl) are found below the petals and may look leaf-like (some may be the same color as petals)
• Sepals enclose the flower bud before it opens 
• Sepals are collectively called the calyx
• Petals are often colorful to attract pollinators
• Petals are collectively called the corolla

• Monocot flower parts are arranged in multiple of THREES, while dicots are in multiples of FOUR or FIVE

• Perfect flowers have both stamens & pistils (rose)

• Imperfect flowers are either a male (pistillate) or female (staminate) flower (pumpkin or melons)

• Some angiosperms have both male & female flowers on the SAME plant (monoecious)

• Other angiosperms have entire male OR female plants (dioecious)

Female Reproductive Structures:

• Called carpals
• Carpals may be fused to form the pistil
• Produce eggs
• Composed of 3 parts --- stigma, style, and ovary
• Stigma is located at the top and may be sticky or have hairs to hold pollen grains landing there
• Style is a stalk-like connection between the stigma and the ovary
• Ovary is the enlarged base containing ovules with eggs

Pistil

Male Reproductive organs:

• Called stamens
• Produce pollen
• Composed of 2 parts --- filament & anther (pollen sac)
• Anthers produce pollen grains containing sperm
• Filament is stalk-like & supports the pollen sacs

Stamen

Angiosperm Life Cycle:

• Undergo alternation of generations

• Sporophyte is dominant phase

• Gametophytes (flowers) form male & female gametes

• Anthers form pollen grains from microspores 

• Pollen grains contain 2 cells --- tube cell & generative cell (sperm)

• Two protective layers called integuments surround the megasporangium

• The entire structure including the integuments is the ovule and becomes the seed

• Each ovule has 4 megaspores (three disintegrate)

• The remaining megaspore undergoes mitosis to produce a large cell & polar nuclei 

• When pollen lands on the stigma, a pollen tube grows through the style to the ovary

• Two sperm travel down the pollen tube --- one fertilizes the egg and the other join with polar nuclei to form endosperm (stored food for Seed)

• Called Double Fertilization

• After fertilization, ovule becomes the seed and the ovary & surrounding tissues form a protective fruit

• A fruit is a ripened ovary with seeds (apple, melon, cocklebur...)

• When seed land on moist soil, they germinate (sprout) and form new sporophyte plants

Pollination:

• Wind, water, and animals help spread pollen

• As pollinators drink nectar or eat the fruit, pollen gets on their bodies and is spread to other flowers

• Self pollination occurs whenever pollen from a flower lands on the stigma of that SAME flower (pea plants)

• Cross pollination occurs whenever pollen is spread to a different flower producing hybrids (more gene combinations)

Seeds & Fruit:

• Fruits are adaptations for dispersing seeds (coconuts float, cockleburs catch onto animal fur, some seeds eaten by birds aren't digestible...)
• More energy is required to produce seeds than spores because they contain stored food
• Seeds may be dormant (inactive) for weeks or years protected by their seed coat
• Seeds contain a plant embryo and endosperm

• Many fruits are fleshy & their seeds aren't digested by the animals that eat them
• Heavy seeds have adaptations such as wing-like structures (maple) or prickly coats (cocklebur) to help them disperse

maple seeds	Cockleburs	Coconut

• Fruits may be dry or fleshy
• Three types of fruits exist --- simple, aggregate, & multiple
• Simple fruits (apple) form from One pistil on a flower
• Aggregate fruits (raspberry) form from several pistils on a flower
• Multiple fruits (pineapple) form several flowers growing close together
• Cotyledons are leaf-like structures of the plant embryo
• Monocot seeds have one seed leaf (Cotyledon), while dicots have two cotyledons

• The epicotyl is the part of the plant embryo ABOVE the cotyledon & becomes the stem
• The radicle is the part of the plant embryo BELOW the cotyledon & becomes the root
• The hypocotyl is the part of the plant embryo BETWEEN the cotyledon &  the radicle
• The hilium is a scar along the seed edge where it was attached to the ovary
• In monocot seeds like corn, a sheath called the coleoptile grows out of the ground to protect the newly emerging plant

Germination:

• Many seeds require environmental factors, such as Water, Oxygen, and Temperature to trigger germination
• Some seeds only germinate after exposure to extreme cold or after passing through an animal's digestive tract
• Water must FIRST be absorbed by the seed to break the seed coat & activate enzymes to change starch in the endosperm or cotyledons into simple sugars for energy
• The radicle emerges first

• Once the seed coat opens, OXYGEN is needed for cellular respiration carried on by the embryo plant
• The shoot (hypocotyl & embryonic leaves) begin to grow, synthesize chlorophyll, and carry on photosynthesis

• After the stored food is used up in dicots, the cotyledons fall off

• In Monocots like corn, the Cotyledon remains underground and transfers nutrients to the growing Embryo.

 Asexual Reproduction in Plants:

• Asexual reproduction is FASTER and produces well-adapted offspring 
• Called vegetative reproduction
• Occurs from non-reproductive parts such as roots, stem, or leaves
• Runners, Rhizomes, Bulbs, and Tubers can be used to produce new plants
• Cutting is taking a piece of Stem or Leaf and growing a new plant 
• Grafting occurs whenever 2 cut ends of plant stems are fused
• Layering occurs when aerial roots touch soil & start growing new plants