Plant Reproduction - BIOLOGY JUNCTION

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