How long ago was the evolution of seeded plants?
- About 360 MYA
What were some of the main evolutionary innovations of seed plants?
- Further gametophytes miniaturization
- Produce 2 kinds of spores (“heterospory”)
- Specialised structures to protect the gametophyte
What are the two kinds of heterospory?
- Seed plants produce 2 kinds of spores (“heterospory”):
» Megaspore: gives rise to female gametophyte
» Microspore: gives rise to male gametophyte
What are the specialised structures in seed plants used to protect the gametophyte?
» Megaspore encased in protective and food layers (called “ovule”)
• Megaspore gives rise to female gametophyte, which stays sheltered in the ovule
• Once fertilized, it becomes the “seed”
» Microspore encased within pollen wall, which contains sporopollenin → resistant to drying (called “pollen grain”)
• Microspore gives rise to male gametophyte, sheltered within pollen grain
Which is dominant in non vascular plants, Gametophyte or sporophyte?
- Gametophyte is dominant
- Sporophyte is reduced and so the plant is dependant on gametophyte for nutrition
Which is dominant in seedless vascular plants, Gametophyte or sporophyte?
- Sporophyte is dominant
- Gametophyte is reduced and so the plant is independent (Photosynthetic and free-living)
Which is dominant in seeded vascular plants, Gametophyte or sporophyte?
- Sporophyte is dominant
- Gametophyte is reduced (usually microscopic) and so plant is dependant on surrounding sporophyte tissue for nutrition.
How are the egg and sperm produced in seeded plants?
- Diploid sporophyte produce haploid spore by meiosis:
» Megaspore → egg (within the ovule)
» Microspore → sperm (within pollen grain, generally without flagella)
How does reproduction of seeded plants work?
- Only pollen grain leaves parent plant
» Pollen grain reaches and enters pollen tube of ovule (“pollination”)
» Pollen nucleus reaches egg and fuses (“fertilisation”) → seed
» New sporophyte grows out of the seed - No independent gametophyte organism
Explain the anatomy of an unfertilized ovule.
- A megasporangium is surrounded by a protective layer of tissue called an integument.
- This megasporangium surrounds the spore containing the egg.
- The micropyle is the only opening through the integument and allows entry of a pollen grain.
What is the process of fertilising the egg of a seed?
- A megaspore develops into a female gametophyte, which produces an egg.
- The pollen grain, which had entered through the micorpyle, contains male gametophyte.
- The male gametophyte develops a pollen tube that discharges sperm, thereby fertilising the egg and creating a Gymnosperm seed.
What is a Gymnosperm seed?
- Fertilisation initiates the transformation of the ovule into a seed, which consists of a sporophte embryo, a food supply and a protective seed coat derived from the integument.
- The megasporangium dries out and collapses.
Draw the diagram of the formation of a gymnosperm seed
book
What distiguishes seeded plants?
- Internal fertilisation and protection of the embryonic sporophyte (seed) distinguishes these plants → adaptation to dry spells
» Later innovation, make it appealing for animals → increases dispersal, and discarded in favourable location
What are the benefits of a pollen grain?
- Pollen grain resistant to drying and can travel by wind or hitchhike on animal → removes the dependence on water for sperm transport → allows plants to colonise dry environments
What are the two clades of seed plants?
- Gymnosperms
» “gymno” (bare, naked) and “sperm” (seed) → “naked seed”
» Seeds exposed - Angiosperms
» “angeion” (vessel, container) and “sperm” (seed) → “contained seed”
» Seeds contained in fruit
What are the differences between Gymnosperms and Angiosperms?
- Wood structure
- Different pollen
- Gymnosperms lack flowers
- Gymnosperms dominant flora in Mesozoic (245–65 mya), angiosperms have dominated the Cenozoic, particularly since late Eocene (40 mya)
- Gymnosperms have 4 phyla, angiosperms only 1
- Angiosperms account for over 99.5% of all “higher plants” (seed plants) species
What are the three key features of the Gymnosperm life cycle?
- Miniaturisation of gametophyte, dominance of the sporophyte generation
- Development of resistant, dispersible seeds from fertilised ovules
- The transfer of sperm to ovules by airborne pollen grains
If Pine is a gymnosperm what does it use to reproduce?
- Pine trees are heterosporous: produce male spores (microspores) and female spores (megaspores) by meiosis
how does the reproduction of a pine work?
- Pollen reaches ovule → pollen germinates forming pollen tube that digests its way to megaspore; meanwhile megaspore produces haploid gametophyte; pollen nucleus reaches egg → fertilisation → seed → seedling → mature sporophyte → releases spores
What is the history of Gymnosperms?
- Early gymnosperms lived in moist Carboniferous alongside seedless plants (which require moisture for reproduction)
- As climate dried in the Permian (299-252 mya), gymnosperms thrived, and dominated terrestrial ecosystems until the end of the Mesozoic era (66 mya)
How many Phyla of Gymnosperms are there?
- 4 extant gymnosperm phyla
Provide a summary on Conifers
- Gymnosperms = Phylum Coniferophyta
- Largest phylum (600 species)
- Cone-bearing
- Perennial (long-living) woody plants
- Most are trees, but a few are shrubs
- Highly adaptable (arid lands, high latitudes)
- Conifers
» Seen as more primitive compared to flowering plants, but …
» Grow in all parts of the world (4 families in Australia and NZ)
» Dominate in forests in the wettest, coldest and highest parts of the world
» Total area of coniferous forest exceeds that of flowering plants
What are four examples of Australian Conifers?
- Bunya pine (Araucaria Bidwillii)
- Wollemi pine (Wollemia nobilis)
- Tasmania Cedar (Athrotaxis Cupressoides)
- Pine tree (pinus radiata)
Give a brief summary on Cycads
- Gymnosperm =Phylum Cycadophyta
- 300 species
- Palm-like plants
» True palms are angiosperms - Single upright, usually unbranched trunk
- Long feathery (“pinnate”) leaves
- Large seeds (up to 15kg)
- Separate male and female individuals (“dioecious”)
- Most endangered of all plant group
» 75% of species threatened by habitat destruction
Give a summary of Phylum Gnetophyta
- Gymnosperm = Phylum Gnetophyta
- Poorly known and understood group
- 75 species, 3 genera: Gnetum, Ephedra and Welwitschia
- Group united by very similar floral morphology, and considered the link between gymnosperms and angiosperms
Give a summary of Phylum Ginkgophyta
- Gymnosperm = Phylum Ginkgophyta
- Single living species Ginkgo biloba
- Remnant of once flourishing Cretaceous flora
- Only wild population located in western China
- Tolerates pollution, commonly planted in cities
- Dioecious (individual trees have either male or female organs)
» Female produces edible seeds, but rotting seed has pungent smell (!)
» Usually only pollen producing males planted
Give a summary of Anigosperms
- Most diverse and widespread of all plants
» Approximately 250,000 known species, covering 11,000 genera; 275 families and 49 orders
» Dominate nearly all terrestrial ecosystems - Highest productive capacity
- Recognised by two key adaptations and unique reproductive structures: flowers and fruits
What is a flower?
- Unique structure for sexual reproduction
- Specialised shoot with up to 4 types of modified leaves:
» Sepal
» Petal
» Stamen
» Carpel
How is pollen transferred from one plant to another?
- In many species, insects and other animals transfer pollen from one flower to sex organ of another
» Overcomes randomness of wind dependent pollination of gymnosperms
» Some angiosperms still wind pollinated
What are the four types of modified leaves in a flower?
» Sepal: green, enclose the flower before it opens
» Petal: brightly colored to attract pollinators if not wind-pollinated
» Stamen: produce microspores → pollen grain with male gametophyte (stored in anther)
» Carpel: “container” where megaspores produced → female gametophyte
• Stigma, style (canal) and ovary (contains one or more ovules)
draw the diagram of a flower
book
Where are flowers derived from
- All living flowers ultimately derive from a single ancestor that lived about 140 million years ago, a study suggests.
What is the difference between the seed and the fruit?
After fertilisation:
» Ovule → seed
» Ovary → fruit
What are the different textures of fruit?
Fruit can be:
» Fleshy (eg, peach, tomato, apple)
» Dry (eg, cereal grain of corn, rice, wheat; nuts)
What does the fruit do?
Fruit protects seeds and helps in their dispersal
What is the first stage of the Angiosperm Life cycle?
- Flower = sporophyte composed of male and female structures
- Male gametophytes contained in pollen grains produced by the sporangia of anthers
- Female gametophyte = embryo sac, develops in ovule in ovary at base of stigma
What is the main purpose of a flower?
- Most flowers - mechanisms to ensure cross-pollination between flowers on different plants
What happens in the second stage of the Angiosperm life cycle?
- Pollen grain land on stigma, germinates and pollen tube grows down to the ovary
- Pollen tube discharges two sperm cells into the female gametophyte within an ovule (double fertilisation)
What is the third stage of the Angiosperm life cycle?
- One sperm fertilises egg, while other combines with two nuclei in the central cell of female gametophyte and initiates development of food- storing endosperm
What happens in the fourth stage of Angiosperm life cycle?
- Triploid endosperm nourishes developing embryo
- Within a seed, the embryo consists of a root and two seed leaves called cotyledons
- The ovary develops into a fruit to help seed dispersal
What are the main two groups of angiosperms?
- One phylum, but >250,000 species of living angiosperms
- Two particularly large groups: monocots and eudicots
Provide a summary of Monocots.
- a group of Angiosperms
- “mono” = one “cotyl” = cup or socket
- About a third of angiosperm species (70,000)
- Lilies, grasses and palms
- Predominantly small plants, lacking true wood or secondary thickening of stems/branches
- Large diversity
» From 35 m palms to 0.1 mm floating weed (Wolffia)
What are some examples of Monocots?
» Wheat, corn, grasses, orchids, sedges, papyrus, lilies, pineapples, palm trees and irises
- Wolffia globosa – smallest flowering plant (0.1 mm)
Give a summary of Eudicots
- “eu” = true, “di” = two, “cotyl” = cup or socket
- Two thirds of angiosperm species (180,000)
- The “typical” trees and shrubs, plus many herbs and legumes
- Large diversity also, from Eucalyptus regnans (>90m) to smallest herb
What are some examples of Eudicots?
» Banksias, eucalypts, acacia, figs, roses, daises, peas, apples, pears, cabbage, mangoes, grasses
What is the Australian diversity of Eudicots?
- ANZ flora comprise a few species-rich families
» High degree of endemism (80-84% of species occur only in ANZ)
» 75% of Australia is dominated by eucalyptus and acacia
What are the four main reasons why Angiosperms are successful?
- Traits that made them resistant to drought and cold
- Very efficient vascular system
- Efficient reproduction (flower and fruit)
- Huge amount of diversity in chemistry
What is the first reason why Angiosperms are successful?
- Traits that made them resistant to drought and cold
» Tough leaves that can be lost when it gets cold (“deciduous”), reduced in size, efficient internal systems, tough seed coat
What is the 2nd reason why Angiosperms are successful?
- Very efficient vascular system
» Efficient conduction of sugars throughout the plant in phloem
» Efficient transport of water in xylem (tracheids + special water conducting cells called “vessel elements”)
What is the 3rd reason why Angiosperms are successful?
- Efficient reproduction (flower and fruit)
» Precise method of pollination and seed dispersal, which allows them to exist as widely scattered individuals in many habitats
What is the 4th reason why Angiosperms are successful?
- Huge amount of diversity in chemistry
» Increased defense mechanisms (against herbivores for example)