Plants :) Flashcards
True or False
All plants originate from a land anscestor
True
Plants occupy most terrestrial environments, including deserts, grasslands, and forests.
- Some species have returned to aquatic environments, although their ancestors were terrestrial; most in freshwater, a few in marine waters (e.g. ~60 species of seagrasses)
What traits do plants have that are shared with algae?
Plants are multicellular, photosynthetic (photoautotrophic) eukaryotes.
- Many photosynthetic protists (algae) also fit this description.
Plants possess cell walls composed of cellulose.
-This feature is shared among red, green, and brown algae and some dinoflagellates.
Plants have chloroplasts with chlorophyll a and b.
- Chloroplasts containing chlorophyll
a and b are another common trait shared by plants and certain protists such as green algae, euglenids, and a few dinoflagellates.
- The presence of chloroplasts is an ancestral trait for plants.
Traits shared by charophytes and plants
Both have circular rings of proteins embedded in the plasma membrane that synthesize cellulose.
- Cellulose deposition in cell walls is very similar in charophytes and plants.
In plant species with flagellated sperm, the structure of the sperm closely resembles charophyte sperm.
What is the strictest definitoin of plants?
The strictest definition of plants, Kingdom Plantae, is defined as clade Embryophyta, which includes plants with embryos (embryophytes).
- Embryophytes are informally called “plants”
What are the advatages of plants moving onto land
Name all 5
Decreased competition: land provided spacious habitats with reduced competition.
Increased photosynthesis: plants benefitted from bright sunlight unfiltered by water and phytoplankton algae.
Abundant CO2: the atmosphere was rich in carbon dioxide.
Rich soil: land offered soils rich in mineral nutrients.
Few herbivores or pathogens: initially, there were few herbivores or pathogens posing threats to early plants.
Challenges associated with plants’ movement onto land
Scarcity of water: desiccation was a major challenge due to the scarcity of water on land.
Lack of structural support: early plants lacked the structural support necessary for upright growth.
To overcome these challenges, early plants evolved adaptations that enhanced their survival out of water, allowing them to successfully colonize the land.
Sporopollenin
Adaptations enabling colonization of land
Charophyte zygotes secrete sporopollenin, an incredibly durable polymer in the cell wall.
- Sporopollenins are complex, highly crosslinked polymers composed of C, H, and O that are resistant to degradation by enzymes and inorganic chemicals.
- Sporopollenin protects charophyte zygotes from desiccation, UV light, and physical stresses.
- Many charophytes live in ephemeral ponds that dry up.
Sporopollenin is also found in the cell walls of spores and pollen of plants.
- Increases resistance of these structures to desiccation and physical stresses.
Adaptations for water conservation
Adaptations enabling colonization of land
The waxy cuticle acts as a protective layer covering the epidermis helping plants conserve water (avoid desiccation) and prevent microbial attack.
Stomata (singular: stoma), tiny pores in the epidermis of leaves and other photosynthetic organs, facilitate gas exchange and serve as sites for water evaporation.
- Stomata can close to minimize water loss during dry conditions.
Waxy cuticles and stomata likely evolved early in the history of plants.
Lignified vascular tissue for internal transport
Adaptations enabling colonization of land
Xylem transports water and minerals from roots to shoots/leaves via microscopic conduits formed by
dead, lignified cells.
- Lignin, a complex polymer, strengthens and waterproofs xylem cell walls.
Phloem are living cells that distribute soluble organic compounds produced during photosynthesis.
Vascular tissue provides:
- Rigidity for vertical (tall) growth (supported by lignified xylem).
- Water transport, enabling plants to grow in desiccating environments, i.e. air
Functional compartmentalization in terrestrial plants
Most plants exhibit structural and functional specialization, with roots exploring underground for water and minerals, and shoots seeking light and gases aboveground.
- Elongation and branching optimize root and shoot exposure to environmental resources, promoting growth toward resource-rich areas
Shared derived traits of plants
- Alternation of generations.
- Multicellular, dependent embryos.
- Walled spores produced in sporangia.
- Apical meristems
- The cuticle helps plants conserve water in terrestrial environments
Alternation of generations
Plants
The life cycle of plants alternates between two multicellular generations:
The sporophyte (diploid, 2n) is specialized for dispersal.
- The fusion of gametes (fertilization) gives rise to a diploid sporophyte, which produces haploid spores by meiosis.
The gametophyte (haploid, 1n) is specialized for fertilization.
- The gametophyte is haploid and produces haploid gametes by mitosis.
- cf. animals have only unicellular haploid stages (gametes).
Alternation of generations also occurs in brown algae (Stramenopile protist), but not in charophytes.
- The alternation of generations evolved independently in these lineages.
Which stage in alternation of generations becaomes more dominant as plants become “more complex”
Sporophytes (2n) became dominant
Sporophyte
The sporophyte (diploid, 2n) is specialized for dispersal.
- The fusion of gametes (fertilization) gives rise to a diploid sporophyte, which produces haploid spores by meiosis.
Gametophyte
The gametophyte (haploid, 1n) is specialized for fertilization.
- The gametophyte is haploid and produces haploid gametes by mitosis.
- cf. animals have only unicellular haploid stages (gametes).
Multicellular, dependent embryos
Shared derived traits of plants
Plants retain multicellular, diploid embryos within the tissues of the female gametophyte.
- Plants are called embryophytes because of embryo dependency on the gametophyte.
Nutrient transfer occurs through specialized placental transfer cells.
Walled spores produced in sporangia
Shared derived traits of plants
- Spores are produced by the sporophyte within structures called sporangia.
- Diploid cells called sporocytes undergo meiosis to yield haploid spores.
-
Spore cell walls, containing sporopollenin, confer resistance to harsh environments, particularly desiccation.
- Spores that can disperse in dry conditions, a crucial terrestrial adaptation.
Apical meristems
Shared derived traits of plants
Plants exhibit structural specialization in roots and shoots, sustained by apical meristems.
Apical meristems are regions of cell division at the shoot and root tips that enable continual growth.
- Plants “move” by growing from the tips of roots and shoots, i.e. apical meristems sustain continual growth of roots and shoots.
- Meristem-produced cells differentiate into various tissues, including the epidermis and internal structures.
Cuticle
Shared derived traits of plants
The cuticle helps plants conserve water in terrestrial environments.
- The cuticle, a waxy layer produced by the epidermis, minimizes water loss and shields against microbial threats.
- Stomata are formed by guard cells, specialized epidermal cells that regulate water loss by controlling stomatal opening and closing, evolved early in the history of plants but are absent from some non-vascular plants
What are the 2 clades of seedless vascular plants?
- Lycophytes (club mosses and their relatives)
- Monilophytes (ferns and their relatives; also known as pteridophytes)
Seedless vascular plants are paraphyletic
Nonvascular plants
Nonvascular plants are commonly called bryophytes, which includes liverworts, mosses, and hornworts.
- However, their relationships with each other and vascular plants remain unresolved.
- Bryophytes are not a monophyletic group.
Seed plants
Seed plants, characterized by seeds containing an embryo and nutrients enclosed in a protective coat, represent the third clade of vascular plants.
Seed plants consist of two clades:
- Gymnosperms (e.g. conifers)
- Angiosperms (flowering plants), the most diverse and dominant plant group.
Bryophytes
Nonvascular plants
Bryophytes are the earliest lineages to diverge from the common ancestor of plants.
Bryophytes are a paraphyletic group consisting of three phyla of small herbaceous (non-woody), nonvascular plants:
1. Liverworts (phylum Hepatophyta), ~9,000 species.
2. Mosses (phylum Bryophyta) ~12,000 species
3. Hornworts (phylum Anthocerophyta), ~225 species
Characteristics of byrophytes
Bryophytes lack true vascular tissue and lignin.
- Limits size
- DOES NOT FORM ROOTS
- Water is absorbed through their surfaces
Bryophyte sporophytes are unbranched and lack roots and leaves
Sporophytes of mosses and hornworts have stomata for gas exchange
- Sporophytes of liverworts do not have stomata.
- No extant gametophytes have stomata.
Which phase in alternation of generations dominates the cycle in bryophytes
Gametophytes are larger and longer-living than sporophytes in all three bryophyte phyla.
- Bryophytes have a free-living haploid (1n) gametophyte as the most conspicuous, dominant phase of the life cycle.
- Smaller diploid (2n) sporophytes are present only part of the time, and they are dependent on the gametophyte
True or false
Bryophyte life cycles require water for fertilization
True
Ecology of bryophytes
Bryophytes thrive in moist forests and wetlands, with mosses often dominating the ground cover
Example:
The moss Sphagnum (“peat moss”) plays a vital role in regulating water flow, particularly in peat bogs (muskegs) found throughout the Arctic and boreal regions.
- Mosses can lose most cellular water, and then rehydrate and reactivate cells when moisture becomes available.
Sphagnum bogs form extensive
deposits of undecayed organic
material called peat
- Peatlands are characterized by low
temperatures, pH, and oxygen levels that inhibit the decay of moss and other organisms
- Peatlands serve as carbon reservoirs,
stabilizing atmospheric CO2 levels
Origins of Vascular Plants
Bryophytes dominated terrestrial vegetation for first 100 million years of plant evolution.
The earliest vascular plant fossils date to ~425 mya.
- Early vascular plants, such as Aglaophyton, exhibited anatomical features intermediate between bryophytes and fully developed vascular plants.
- They possessed independent, branching sporophytes.
- Sporophytes were not continuously reliant on gametophytes for sustenance.
- However, like bryophytes they lacked leaves or roots and lacked true vascular tissues.
Subsequent fossil findings revealed the gradual evolution of vascular tissue, followed by the development of leaves and roots.
Shared derived triats of vascular plants
- Vascular tissues
- Life cycles with dominant sporophytes
- Well-developed roots and leaves
Xylem
Xylem conducts water and mineral via dead, hollow cells, forming continuous conduits throughout the plant.
- These water-conducting cells, strengthened by lignin, provide structural support.
Phloem
Phloem consists of living cells and facilitates the distribution of nutrients and organic products.
Well-developed roots and leaves
Vascular Plants
Complex multicellular roots anchor vascular plant sporophytes and facilitate the absorption and transportation of water and nutrients from the soil.
- Roots may have evolved from subterranean stems
Complex multicellular leaves increase the surface area of vascular plant sporophytes, capturing more solar energy for photosynthesis.
Bryophyte sporophytes lack roots and leaves.
What are the two types of vascular plant leaves
Microphylls: Small leaves with single veins that possibly evolved as outgrowths of stems.
Megaphylls: Large leaves with highly branched vascular systems that potentially evolved from webbing between flattened branches.
Sporophylls
Vascular Plants
Vascular plants possess sporophylls, modified leaves bearing sporangia (spore- producing organs).
- Sori are clusters of sporangia found on the undersides of sporophylls, e.g. ferns.
- Strobili are cone-like structures formed from groups of sporophylls, e.g. lycophytes and most gymnosperms.
Sori
Vascular plants
Sori are clusters of sporangia found on the undersides of sporophylls, e.g. ferns.
Strobili
Strobili are cone-like structures formed from groups of sporophylls, e.g. lycophytes and most gymnosperms.
True or False
Vascular plants exhibit variation in spore sizes among taxa
Most seedless vascular plants are homosporous, with sporophytes producing a single type of spore that develops into a bisexual gametophyte.
However, all seed plants and a few seedless vascular plants are heterosporous, producing megaspores (developing into female gametophytes) and microspores (developing into male gametophytes).
Flagellated Sperm
Seedless Vascular Plants
Seedless vascular plants have flagellated sperm.
- Seedless vascular plants have flagellated sperm, requiring a film of water for fertilization.
- Similar to bryophytes, seedless vascular plants are predominantly found in relatively damp habitats.
Which phase of alternation of generations is dominant in seedless vascular plants?
The sporophyte is dominant in seedless vascular plants.
The sporophytes of seedless vascular plants are the larger and dominant generation, unlike in bryophytes where
gametophytes dominate.
- Gametophytes in seedless vascular plants are tiny, independent plants growing on or below the soil surface
- Most seedless vascular plants are homosporous.
What are the two clades of seedless vascular plants?
Phylum Lycophyta includes club mosses, spike mosses, and quillworts (~1,200 species).
Phylum Monilophyta includes ferns, horsetails, whisk ferns, and their relatives (~12,000 species).
Lycophytes
Seedless Vascular Plants
Giant lycophyte trees once dominated Carboniferous swamps but became extinct as the climate cooled and dried.
Surviving lycophyte species are small herbaceous plants.
- All lycophytes are microphyllous and are either homo- or heterosporous.
- Club mosses and spike mosses have vascular tissues, and are not mosses (despite their names!).
Monilophytes
Seedless Vascular Plants
Monilophytes are morphologically diverse and include whisk ferns, horsetails, and ferns
- Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns.
- Horsetails, characterized by brushy stems, were diverse during the Carboniferous (grew to 15 m), but are now limited to the genus Equisetum (15 species; equus = horse, seta = whisker).
- Ferns are the most diverse seedless vascular plants (~12k species).
- Ferns have megaphylls.
- Most produce clusters of sporangia (sori) on the undersides of sporophylls
Ferns
Ferns are the most diverse seedless vascular plants (~12k species).
- They are most diverse in the tropics, but also thrive in temperate forests.
- Some tree-sized (tree ferns).
Ferns have megaphylls.
- Large leaves with branched vascular systems.
Most produce clusters of sporangia (sori) on the undersides of sporophylls.
- Sporangia often have spring-like structures that catapult spore release.
- Spores dispersed by wind.
- Most ferns are homosporous
Significance of seedless vascular plants
Ancestors of extant lycophytes, horsetails, and ferns dominated during the Devonian and Carboniferous periods, forming the first forests.
- Partially decayed plant material of Carboniferous forests eventually formed rock strata known as coal.
- Increased plant growth and photosynthesis during this period potentially contributed to global cooling by reducing atmospheric CO2levels.
- 5-fold decreased atmospheric CO2 during the Carboniferous period.
Seeds
Seeds consist of an embryo and nutrients surrounded in a protective coat, allowing for efficient long-distance dispersal.
- The domestication of seed plants began around 8,000 years ago, enabling permanent human settlement
Shared derived traits of seed plants
- Reduced gametophytes
- Heterospory
- Ovules
- Pollen
- Seeds
Reduced gametophytes
Seed Plants
In seed plants, gametophytes develop within spore walls retained within parental sporophyte tissues.
- Gametophytes are protected from environmental stresses.
- Non-vascular plants (bryophytes): independent, dominant gametophyte; gametophyte-dependent sporophyte.
- Seedless vascular plants: dominant sporophyte, independent gametophyte.
- Seed plants: dominant, independent sporophyte; microscopic, sporophyte-dependent gametophyte
A trend in plant evolution is the progressive reduction in gametophyte size and independence, with an increasing role of the sporophyte
True or False
All seed plants are heterosporous
TRUE
Unlike seedless plants, all seed plants are heterosporous, producing two spore sizes: megaspores and microspores.
- Microsporangia produce microspores that develop into male gametophytes.
- Megasporangia produce megaspores that develop into female gametophytes.
Spores are not dispersed in seed plants.
- They are retained within the sporophyte.
Microsporangia
Microsporangia produce microspores that develop into male gametophytes.
Megasporangia
Megasporangia produce megaspores that develop into female gametophytes
Ovules
Seed Plants
An ovule consists of:
- a megasporangium (2n)
- megaspore (1n)
- one or more protective integuments (2n)
- Gymnosperm ovules have one integument; angiosperm ovules usually have two integument
Female Gametophyte
Seed Plants
The megasporangium (2n) produces a megaspore (1n) via meiosis, enclosed in protective integuments (2n)
The megaspore is not dispersed.
- Pollination triggers female (mega)gametophyte (1n) development
- Gymnosperm female gametophytes consist of thousands of cells, while in angiosperms, they are reduced to a few nuclei and cells (embryo sac).
Pollen
Seed Plants
Microspores develop into male gametophytes within pollen grains
- Pollen grain cell walls are coated with a tough outer wall of sporopollenin, which allows pollen to withstand desiccation, UV, and physical damage.
- The male gametophyte is transported to the ovule in the pollen grain.
- Fertilization without the external release of sperm
What are pollen grain cell walls coated with?
Sporopollenin, which allows pollen to withstand desiccation, UV, and physical damage.
Male Gametophyte
Seed Plants
Microsporangium (2n) produces
microspores (1n) via meiosis.
- Microspores are not dispersed; they develop into pollen grains.
- Microspores undergo mitosis to produce tiny male gametophytes (1n).
- Each pollen grain contains a male gametophyte, which consists of at least a generative cell and a tube cell that become sperm nuclei and a pollen tube, respectively, after pollination.
True or False
Pollen grain = Male gametophyte
Seed Plants
False
Pollen grain ≠ male gametophyte
(outer layer of the pollen cell wall (incl. sporopollenin) are deposited by sporophyte)
Seeds
Seed Plants
A seed develops from a fertilized ovule and consists of an embryo, food supply, and outer seed coat.
- A fertilized ovule develops into a seed containing nextgeneration sporophyte [sporophyte embryo in gametophyte tissues in parent sporophyte wrapper]
- Seed size varies greatly and is primarily determined by the amount of gametophytederived storage reserves.
Pollination
Pollination transports pollen to the ovule for fertilization.
- Pollination is a significant change in the evolutionary strategy for fertilization
Pollen eliminates the need for water during fertilization.
- In seed plants, the entire male gametophyte is carried inside the pollen grain.
- Pollen can be dispersed over long distances by wind or animals.
- If a pollen grain germinates, it develops into a pollen tube that delivers sperm nuclei (without flagella) into the female gametophyte within the ovule.
What are the evolutionary advantages of seeds over spores
Seed plants evolved structures to enhance long-distance seed dispersal by air, water, or in/on animals.
- Seeds are larger than spores, so they are not as easily dispersed by wind.
Seeds have external coats that protect the embryo.
- Seeds may remain dormant for days to years until conditions are favourable for germination.
Seeds have an energy-dense supply of stored food that supports early seedling growth.
- There is a trade-off between seed size and seed number:
- Large-seed plants produce few seeds, but with a lot of stored energy, they can germinate and grow in low light, e.g. forest understory.
- Small-seed plants produce many seeds, but successful germination requires optimal conditions.
True or False
Fertilization of seed plants is dependant on water
False
Fertilization occurs independently of water
- The male gametophyte is transported within pollen through the air to reach the female gametophyte (sperm is not released into the environment).
Dispersal occurs through seeds
What are the two clades of seed plants?
Gymnosperms include the conifers; ~1,000 species.
Angiosperms, the flowering plants; ~290,000 species.
What does the work “gymnosperm” mean?
Gymnosperm means “naked seeds”.
- Seeds are exposed on sporophylls that form cones (strobili).
- Angiosperm seeds are enclosed in fruits, which are mature ovaries
When did angiosperms begin to replace gymnosperms?
Angiosperms began to replace gymnosperms near the end of the Mesozoic.
- Angiosperms now dominate most terrestrial ecosystems.
- Today, cone-bearing gymnosperms called conifers dominate in northern latitudes, e.g. Canadian boreal forests
What are the four extant gymnosperms
Cycadophyta (~350 species)
- Cycads.
Gingkophyta
- One extant species: Ginkgo biloba.
Gnetophyta (~75 species)
- Three genera: Gnetum, Ephedra, Welwitschia.
Coniferophyta (~600 species)
- Conifers, such as pine, spruce, and cedar.
While the relationships among these phyla are not yet fully resolved, they do form a monophyletic group
Phylum Cycadophyta
Gymnosperms
Cycads were diverse and dominant in the Mesozoic but are now limited to small populations in the tropics and subtropics.
- Many extant cycad species are endangered.
Cycads grow very slowly, producing large palm-like leaves on short, unbranched stems.
- Cycads produce large cones on separate male and female sporophytes that are insect-pollinated.
Cycads, together with ginkgos, are unique among seed plants in having flagellated sperm
Phylum Ginkgophyta
Gymnosperms
The only living species is Ginkgo biloba, with distinctive bi-lobed leaves.
- Fossils suggest that G. biloba originated ~200 mya.
- G. biloba had a global distribution before the evolution of angiosperms.
G. biloba has separate male and female sporophyte trees that are wind pollinated (sperm flagellated).
Extant G. biloba populations originate from China.
- No wild populations; all populations associated with human habitation.
- Ginkgo is a common urban amenity tree, with attractive foliage (yellow in autumn) and a high tolerance to air pollution.
Phylum Gnetophyta
Gymnosperms
Comprises three genera:
- Gnetum, Ephedra, Welwitschia
- ~75 species.
- Originated ~200 mya.
- Phylum grouping based on molecular evidence.
Gnetophytes are morphologically diverse and are adapted to various environments.
- Some are adapted to tropical
environments, whereas others are found in deserts.
Phylum Coniferophyta
Most species-rich phylum of gymnosperms (~600 species)
- Named for their cones: clusters of sporophylls (cone = cone, fer = to bear)
Most conifers are woody shrubs or trees
- Most are evergreens
- Adapted to cold, dry habitats
- Xylem tissue of conifers is resistant to collapse following freezing
- Conifer leaves have thick waxy cuticles; reduced leaf surface area; leaf retention.
Conifers were widespread before the evolution of the angiosperms.
Key features of gymnosperm life cycles
Gymnosperms have dominant sporophyte generations.
- A mature tree is a sporophyte (2n).
Gymnosperms develop seeds from fertilized ovules.
- Ovules support female gametophytes that are protected by the sporophyte.
Transfer of male gametophytes to ovules by pollen.
- Chemically resistant pollen cell wall (sporopollenin outer layer) protects the male gametophyte during dispersal.
- Sperm is not released into the environment.
Life cycle of a conifer (Pinus)
The pine tree is a sporophyte (2n) that produces sporangia on scale-like leaves (sporophylls) clustered in cones.
Conifers are heterosporous: they have megasporangia and microsporangia in female and male cones (on the same tree or different trees).
Pollination by wind.
- Pollen grains of many conifers have air bladders.
Seed dispersal by wind.
- Conifer seeds typically have ovuliferous scales (‘wings’) that aid wind dispersal.
Fertilization and seed development in
conifers are slow.
- From cone production to mature seed can take up to ~3 yrs
Small cones vs large cones in conifers
Small cones produce microspores that develop into male (micro)gametophytes enclosed in pollen grains.
Familiar larger cones contain ovules, which produce megaspores that develop into female (mega)gametophytes.
Angiosperms
Angiosperms (flowering plants) are seed plants with two key reproductive adaptations: flowers and fruits.
With over 290,000 species, they are the most diverse group of plants, classified under a single phylum, Anthophyta
(anthos = flower).
- This diverse group includes plants ranging from the tiny duckweed, measuring only 2-3 mm, to giants like Eucalyptus regnans, reaching a record height of 131 m.
Angiosperm life cycle
The three “F”s
The life cycle of angiosperms is characterized by what can be termed as the “three Fs”:
- Flowers
- Double Fertilization (resulting in seed endosperm)
- Fruits
These features represent shared derived traits (synapomorphies) of angiosperms, distinguishing them from other plant groups.
Flowers
Flowers are structures specialized for sexual reproduction in angiosperms.
- Flowers facilitate the transfer and receipt of pollen.
- Many species are pollinated by animals, while some are wind-pollinated.
- Flowers attract and reward animal pollinators, which aid in the transfer of pollen
A flower is a specialized shoot with up to four types of modified leaves:
- Sepales (Sterile structure)
- Petals (Sterile structure)
- Stamens (Reproductive organ)
- Carples (Reproductive organ)
Sepal
Flower structure
The leafy bit that encloses the flower before it blooms
- attaches to a part of the stem called a recaptacle
Stamen
Flower Structure
A stamen (microsporophyll) consists of a stalk (filament) topped by an anther containing pollen sacs (microsporangia) that produce pollen.
- Usually 2-4 pollen sacs per anther, and often many stamens per flower.
- Pollen sacs produce microspores (via meiosis) that develop into male gametophytes within pollen grains.
Carpel
Flower Structure
A carpel (megasporophyll) consists of an ovary at the base, and style leading up to the stigma, where pollen is received.
- Flowers may have one or more carpels, and when fused, they form a structure known as the pistil.
- The ovary contains one or more ovules, each of which has the potential to develop into a seed upon fertilization.
- The megaspore develops into the female gametophyte
True or False
The majority of angiosperm flowers are complete
True
Complete flowers contain all four floral organs.
- Approx. 12% of angiosperm species have incomplete flowers, lacking one or more reproductive floral organs
- Flowers may lack either stamens or carpels, forming female or male flowers, respectively.
- Many species with incomplete flowers rely on wind pollination
Inflorescences
A cluster of flowers
e.g. the sunflower, where numerous individual flowers are grouped in a composite inflorescence.
Development of male gametophytes in pollen grains
Angiosperms
A pollen grain consists of a two-celled male gametophyte enclosed in a tough,
sporopollenin-rich outer wall.
Pollen develops from microspores within the microsporangia (pollen sacs) of anthers.
Each microspore undergoes mitosis to produce two-celled male gametophytes:
- Generative cell: develops into sperm nuclei.
- Tube cell: forms the pollen tube necessary for fertilization
Development of female gametophytes (angiosperms)
The female gametophyte (embryo sac) develops within an ovule located within the ovary of the flower.
- Each ovule typically consists of two integuments surrounding the megasporangium.
The megasporocyte within the megasporangium undergoes meiosis forming four megaspores, but usually only one survives.
- The megaspore undergoes mitotic divisions, producing a seven-celled female gametophyte (cf. ~3k cells in gymnosperms).
- The large central cell within the female gametophyte contains two nuclei, known as polar nuclei.
Generative Cell
Male Gametophyte
Generative cell: develops into sperm nuclei.
Tube Cell
Male Gametophyte
Tube cell: forms the pollen tube necessary for fertilization.
Pollination
Angiosperms
Pollination in angiosperms is the transfer of pollen from an anther to a stigma, facilitated by various agents including wind, water, and animals.
If pollination succeeds:
- The tube cell produces a pollen tube.
- The generative cell undergoes mitosis, producing two sperm nuclei.
- The pollen tube grows down into the ovary, releasing two sperm nuclei into an ovule for fertilization to occur
Double Fertilization
Angiosperms
Double fertilization occurs when the pollen tube discharges two sperm nuclei into the female gametophyte (embryo sac) within an ovule.
- One sperm nucleus fertilizes the egg, resulting in the formation of a zygote (2n), which is the initial stage of the embryo.
- The second sperm nucleus combines with two polar nuclei, forming a triploid cell (3n).
- This triploid cell develops into the triploid endosperm (3n), a tissue rich in nutrients that nourishes the developing embryo.
Seed Development
Angiosperms
Typically, endosperm development precedes embryo development.
- The triploid endosperm nucleus undergoes multiple rounds of mitosis to generate starchy food tissues for the embryo.
- Nutrients for the triploid endosperm are supplied by the megagametophyte and surrounding sporophyte during development.
- In most monocots (e.g. corn and rice) and some eudicots, the endosperm serves as a nutrient store for the seedling.
- In other eudicots (e.g. bean seeds), food reserves from the endosperm are transferred to the cotyledons (embryo).
The zygote undergoes mitotic divisions, resulting in the formation of an elongated embryo containing cotyledons (seed leaves), shoots, and roots
Fruit Development
After double fertilization:
- Each fertilized ovule develops into a seed.
- The ovary develops into a fruit enclosing the seed(s).
Fruit formation occurs as the sporophyte ovary wall thickens and matures.
- Fruits consist of mature ovaries containing seeds but may also incorporate other floral parts.
Fruits serve two primary functions:
1. Protect seeds, especially while they are immature.
2. Assisting in seed dispersal through various means such as wind, water, or animals.
What are the two primary functions of Fruits?
- Protect seeds, especially while they are immature.
- Assisting in seed dispersal through various means such as wind, water, or animals.
Dry Fruits vs Fleshy Fruits
Dry fruits result from the drying out of the ovary at maturity, e.g. dandelions.
Fleshy fruits are characterized by an ovary that becomes thick, soft, and often sweet at maturity, e.g. cherries.
Adaptive advantages of angiosperm fertilization
-
Nutrient stores in the seed endosperm develop only after double fertilization.
− Resource conservation: No resources are wasted if the egg remains unfertilized. -
Fruit development is usually initiated by fertilization.
− Resource efficiency: No resources are wasted if fertilization fails to
occur. -
Female gametophytes undergo significant size reduction.
− Minimal resource requirement for their production. -
Rapid development of the small female gametophyte.
− Angiosperm female gametophytes mature within days, unlike gymnosperm female gametophytes, which may take months.
− This rapid development enables angiosperms to complete their life cycle within a single growing season, i.e. annual plant species.
What are the methods of angiosperm pollination
- Abiotic Pollination
- Biotic Pollination
Abiotic Pollination
Water pollination: occurs rarely and is limited to aquatic angiosperms.
- Pollen floats on the water’s surface until it contacts flowers.
Wind pollination: ~20% of angiosperms are wind-pollinated.
- Wind-pollinated species, including grasses and many trees, release large amounts of pollen.
- These flowers are often small and inconspicuous due to their independence from pollinators.
Biotic Pollination
~80% of angiosperm species rely on
animals for pollination.
Floral morphology varies greatly depending on the mode of biotic pollination.
Flowers attract pollinators through scent and colour, often matching rewards and attractants to the metabolic needs and sensory capabilities of their pollinators.
Insect pollinators: ~65% of flowering plants are pollinated by insects.
- Bees are the most important insect pollinators.
- Bees are attracted to bright colours, primarily yellow and blue.
- As bees forage for nectar and pollen, bees inadvertently transfer pollen between flowers.
Coevolution
The simultaneous evolution of interacting species in response to the selection pressures exerted by each other
Advantages of asexual reproduction
Plants
Asexual reproduction allows for rapid growth and establishment in stable environments since clones are initially supported by the parent.
- However, populations of clonal plants are vulnerable to environmental changes due to a lack of genetic variation
Advantages of sexual reproduction
Plants
Sexual reproduction increases genetic diversity, facilitating evolutionary adaptation.
- Typically, only a fraction of seedlings resulting from sexual reproduction survive.
- Seed germination and seedling establishment are the most vulnerable stages in the life cycle of flowering plants.
While some flowering plants can self-fertilize to ensure seed production, this may lead to inbreeding depression and reduced genetic variation.
- Many species evolved mechanisms to prevent self-fertilization, minimizing the risk of genetic homogeneity within populations.
Mechanisms that prevent self-fertilization in angiosperms
- Self-incompatibility
- Floral structure
- Temporal and spatial separation
What percentage of angiosperms have complete flowers?
~88% of angiosperm species have complete flowers, possessing all four floral organs (sepals, petals, stamens, and carpels).
- This increases the potential for selffertilization, as pollination can occur within the same flower or between flowers on the same plant.
Why would it be benificial for plants to be self-compatable?
- Self-compatible (self-fertilization) is advantageous when plants are in isolated environments, or pollinators are rare.
- Many crops and weedy species are self-compatible
Incomplete Flowers
Some species have evolved incomplete flowers.
- Incomplete flowers lack either stamens or carpels, forming male or female flowers, respectively.
- Species with incomplete flowers can be monoecious (male and female flowers on the same plant, e.g. corn) or dioecious (male and female flowers on different plants).
- Incomplete flowers reduce self-fertilization, especially in dioecious species.
Monoecious vs dioecious flowers
Monoecious - male and female flowers on the same plant, e.g. corn
Dioecious - male and female flowers on different plants
Temporal and spatial separation
Flower fertalization
In some species, stamens and carpels mature at different times or are physically arranged to prevent self-fertilization.
- e.g. in alpine woodsorrel, stamens and carpels are positioned to minimize the chances of self-pollination
True or False
Flowering plants (angiosperms) represent nearly 90% of current plant biodiversity
True
True or False
The rise of angiosperms to ecological dominance was gradual
False
Angiosperms suddenly dominated Cretaceous fossil records.
- Angiosperms originated at least 140 mya and began to dominate terrestrial ecosystems by 100 mya.
Gymnosperms previously held dominance among seed plants, but the Cretaceous saw significant adaptive radiation of angiosperms, coinciding with the decline of many gymnosperm groups.
What factors contributed to the rapid adaptive radiation of angiosperms?
Angiosperms developed modified xylem vessels that support high transpiration rates.
- This adaptation facilitated enhanced photosynthesis and growth, contributing to their rapid spread and success.
Rapid speciation: Coevolution with pollinators drove rapid divergence among plant populations, leading to speciation.
- Plant species-specific pollinators enable widespread plant populations to persist, lowering extinction rates.
Rapid reproduction:
- Angiosperm rapid reproduction is facilitated by traits such as small female gametophytes and the absence of seed storage reserves without double fertilization.
- Rapid reproduction enabled angiosperms to diversify into short-lived habitats, e.g. annual species.
What is the basal extant species of flowering plants.
Amborella
Amborella trichopoda, a rare forest shrub from New Caledonia, has traits shared with both angiosperms and gymnosperms
Eudicot vs. monocot seeds
Eudicot seeds contain an embryo with an embryonic axis attached to two cotyledons (seed leaves).
- e.g. common garden bean.
Monocot seeds have one cotyledon and a large endosperm.
- The presence of one cotyledon is a shared derived trait for clade monocots
Cotyledon
Seed leaves
What are the four main catagories of Angiosperms
Basal angiosperms represent early-evolving dicots, comprising <0.1% of angiosperm diversity.
- Notable lineages include the Amborella family (one species: A. trichopoda), the water-lily family, and star anise and relatives.
Magnoliids evolved later and account for ~2% of angiosperm diversity.
- Magnoliids include magnolias, avocado (Persea americana), and spice-bearing species like nutmeg (Myristica fragrans), cinnamon (Cinnamomum spp.), bay leaves (Laurus nobilis), and black pepper (Piper nigrum).
- Magnoliids are more closely related to monocots and eudicots than they are to basal angiosperms.
Approximately 25% of angiosperm species are monocots.
− Major families:
Orchids, ~28k species
Grasses, ~12k species
> 70% of angiosperm species are eudicots; >300 families.
− Major families:
Daisies, ~32k species
Legumes, ~20k species
Rose family, ~5k species
Basal Angiosperms
Basal angiosperms represent early-evolving dicots, comprising <0.1% of angiosperm diversity.
- Notable lineages include the Amborella family (one species: A. trichopoda), the water-lily family, and star anise and relatives.
Magnoliids
Magnoliids evolved later than basal angiosperms and account for ~2% of angiosperm diversity.
- Magnoliids include magnolias, avocado (Persea americana), and spice bearing species like nutmeg (Myristica fragrans), cinnamon (Cinnamomum spp.), bay leaves (Laurus nobilis), and black pepper (Piper nigrum).
- Magnoliids are more closely related to monocots and eudicots than they are to basal angiosperms.
Monocotes
Contain one cotyledon
Approximately 25% of angiosperm species are monocots.
Major families:
Orchids, ~28k species
Grasses, ~12k species
Euidicots
A subset of dicots, known as eudicots, is monophyletic (clade eudicots; eu = true)
> 70% of angiosperm species are eudicots; >300 families.
− Major families:
Daisies, ~32k species
Legumes, ~20k species
Rose family, ~5k species
