Land Plants Flashcards
Monophyletic Group
Definition
a group that contains an ancestral species and all of its descendants
Archaeplastida
Evolutionary History
- derived from primary endosymbiosis event
- diversification of marine photosynthesis for 1-2by
- c. 450mya move to land
Archaeplastida
- red algae
- green algae
- land plants
- glaucophytes
Origin of Land Plants
all extant land plants area thought to have a common origin from a common ancestor with charophycean green algae
evolution in water, tidal pools and ponds
since then, a vast diversity of land plants with distinct features have evolved
Key Features of All Land Plants
- alternation of generations
- roots/leaves/flowers
Alternation of Generations
- plants have a complex life cycle involving the alternation of multicellular haploid and diploid forms
- there are variations in the dominance of the gametophyte (haploid) and sporophyte (diploid) forms
Heteromorphic
Definition
gametophyte and sporophyte look structurally different
Isomorphic
Definition
gametophyte and sporophyte look structurally similar
Types of Life Cycle
- asexual
- sexual (3) - involve the fusion of gametes at some stage
What are the three types of sexual life cycle?
1) Meiosis, orgaism is diploid but produces haploid gametes that fuse
2) Alternation of generations, both haploid and diploid organisms in cycle
3) Zygote undergoes mitosis, zygote is the only diploid cell, all other cells in the organisms are haploid
Similarities Between Land Plants and Algae
-eukaryotic
-multicellular
-cell walls contain cellulose
-chloroplasts with chlorophyll A and B
-
Similarities Between Land Plants and Charophytes
- cellulose synthase complexes arranged in complex rosettes
- peroxisome enzymes that minimise photorespiration (loss of H2O)
- structure of flagellated sperm
- cytokinesis
- nuclear and chloroplast genomes
The Transition to Land - Problems
- for the first 3by the terrestrial surface was lifeless
- at the time of colonisation the land was hostile; bare rock, no soil, no shade
- availability and uptake of water and nutrients
- variation in climate
- lack of physical support
Advantages of the Transition to Land
- unfiltered sun
- more carbon dioxide
- nutrient rich rocks and soil
- few herbivores / pathogens
What did plants need to transition to land?
- desiccation tolerance: formation of waxy cuticle layer with stomata
- water uptake mechanism: first plants lacked true roots/leaves so formed symbiotic associations with mycorrhizae
- structural support: cell walls thickened and lignin (a tough polymer) was used
Xerophytes
-plants that have adapted to survive in dry environments
Xerophytes
Strategies for Survival
- timing of life cycle:
- flowering restricted to short periods
- reduced water loss:
- stomatal closure
- sunken stomata
- hairs to trap water
- thick cuticle
- water storage
Cooksonia
- now extinct group of very early land plants
- first to show apical dichotomous branching
- terminal sporangia
- maximises offspring from single fertilisation
- tallest plant at time~5cm
- evidence of stomata
Devonian Vegetation at Rhine
- rich source of fossils
- volcanic deposits c. 375mya
- fossils of reed like vegetation
- preserved in minute detail
- <30cm
- horizontal and vertical stems
- dichotomous branching
- microphylls and rhizoids instead of tru roots and leaves
- terminal sporangia on aerial stems
Non - Vascular Plants
- Byrophytes
- lack of complex vasculature, some have primitive conductive tissues
- small
- rely on capillary movement and diffusion of water
- rhizoids, no true roots
- microphylls, small ‘leaves’
- usually dioecious (male and female plants)
- ground hugging due to absence of cell wall strengthening lignin
- movement of sperm/fertilisation requires moisture
- gametophyte is dominant
- sporophyte is parasitic on the gametophyte
Mosses - Gametophyte
- haploid
- dominant vegetative plant
- independent
- bears gametes
- union of gametes
- sexual reproduction
- host of sporophyte
Mosses - Sporophyte
- diploid
- seta (stalk) & capsule
- dependent on gametophyte
- bears spores
- meiosis
- multiplication
- dispersal
Bryophytes
Mosses
Liverworts
Hornworts
Liverworts
- c. 9000 species
- simple thalllus
- more leafy
- dominant gametophyte
- reduced sporophyte
- complex gamete bearing structures
- some reproduce asexually
Hornworts
- c. 100 species
- gametophyte thallus
- sporophyte has two ‘horns’
- single large chloroplast per cell
- symbiotic with nitrogen fixing cyanobacteria
Seedless Vascular Plants
- first to grow tall
- complex vasculature
- dominant sporophyte independent of the gametophyte
- much reduced gametophyte, a prothallus
- leaves and roots
- mostly homosporous
Homosporous
plants that produce 1 type of spore that develops into a bisexual gamete
Heterosporous
plants that produce 2 types of spores, megaspores that become female gametotypes and microspores that become male gametotypes
Vascular Tissue
Xylem - water conducting cells strengthened by lignin, also provides strong structural support for plant to grow taller
Phloem - living cells that distribute sugars, amino acids and other organic products
Roots
organs that anchor vascular plants
Leaves
microphylls - single vein
megaphylls - many veins
the greater the surface area, the greater the light absorption for photosynthesis
Pteridophytes
- Ferns, horsetails and whisk ferns
- vascular, seedless
Ferns
- 12000 species
- large pinnate leaves or fronds (megaphylls)
- young leaves unfurl as croziers
- spores are bourne in sporangia, on under surface of leaf
- characteristic sporangia with annulus mechanism
- stem often a rhizome
- true roots
- small gamtopohyte, dominant sporophyte
- homosporous
Sporophylls
modified leaved with sporangia
Sori
clusters of sporangia on underside of sporophyte
Strobili
cone like structures, groups of sporophylls
dehiscence
release of spores
Annulus
- dead water filled cells
- thin outer walls
- water evaporation from annulus distorts outer wall
- outer wall peals back and sporangium opens
- as more water is lost, air comes out of solution
- bubble forms in the centre of the annulus
- spores are dislodged a few cm then picked up on air currents
Whisk Ferns
- 13 species
- ‘living fossils’
- simple structure
- dichotomous branching
- may be primitive or reduced
- high chromosome number
- saprophytic gametophyte
Horsetails
- 15 species
- ‘living fossils’
- modern plants are much smaller than carboniferous ancestors
- jointed rib stems
- silica rich
- scale leaves in whorls at nodes
- distinctive strobili/cones
- spores have elators
- homosporous
Elators
- ribbon like appendages attached to spores
- curl up when moist and expand when dry
- they catch in air currents aiding spore dispersal
Lycophytes
- club mosses, spike mosses, quillworts
- 1000 species
- microphylls
- true roots
- reproductive shoots on sporophytes
- similar life cycle to ferns
- some species are heterosporous
Significance of Seedless Vascular Plants
- ancestors of modern lycophytes
- evolution of vascular tisssues, roots and leaves
- increase in photosynthesis, led to a spike in oxygen levels and a reduction in carbon dioxide
- global cooling at the end of the carboniferous
When did see plants first evolve?
350 mya
What is the difference between Gymnosperms and Angiosperms?
- both are seed plants
- gymnosperm seeds are exposed on modified leaves forming cones
- angiosperm seeds are enclosed in fruits
Why have seed plants become dominant?
- embryo and food supply (endosperm) are surrounded by a protective coat
- the seed can be transported away from the mother plant to more favourable conditions
- seeds can retain viability for up to centuries
Heterospory
Definition
plant has distinct male and female producing structures
Seed Plants
Lifecycle Features
- Gamtophyte reduced and protected (multicellular but not independent)
- retention of female gametophyte on sporophyte
- pollination
Advantages of a Miniaturised Gametophyte
- gametophyte develops within sporophyte
- moist environment prevents dessication
- protection from UV
- nutrients supplied by sporophyte
Seed vs Seedless Reproduction
Seedless
FlagellatedSsperm --require aqueous environment to move --travel few cm -sensitive to desiccation Spores --can survive if environment is good --single cells - susceptible to damage --no nutrient supply
Seed vs Seedless Reproduction
Seeds
Unflagellated Sperm --use wind/animals to move --can survive for long periods of time Seeds --can survive for centuries --multicellular with protective coat --nutrient supply
When did gymnosperms first evolve?
305 mya
When did Angiosperms first evolve?
105 mya
Gymnosperm Phyla
- Ginkgophyta
- Cycadophyta
- Gynetophyta
- Coniferophyta
Gymnosperm Phyla
Ginkgophyta
- separate male and female plants
- one extant member
- living fossils (no close living relatives)
- female cone splits to reveal seeds
Gymnosperm Phyla
Cycadophyta
- c.100 species
- ancient plant group
Gymnosperm Phyla
Gynetophyta
- closest extant relatives of angiosperms
- 69 species
Gymnosperm Phyla
Coniferophyta
- 610 species
- tallest and longest lived organisms on earth
Pine Life Cycle
fertilisation is usually more than a year after pollination
Angiosperms
- dominant plants on earth today
- colour
- increased transpiration
- superior vascular system
- pollination -> coevolution of birds and insects
- animals/humans depend on fruit and seeds
What are the main adaptations of angiosperms?
- flowers
- double fertilisation
- fruit
Flowewrs
Stamen
- made up of filament and anther
- produces microspores which develop into pollen grains
Flowers
Carpel
- made up of stigma, style and ovary
- produces megaspores which develop into female gametes
Double Fertilisation
- pollen grain germinates after landing on stigma
- pollen tube (with 2 nuclei) grows through the style towards the ovaries
- pollen tube penetrates through the micropyle
- pollen tube discharges 2 nuclei into female gametophyte (egg sac)
- one fertilises the egg forming a diploid zygote
- the other fuses with 2 polar nuclei present in the ovary forming a triploid cell that divides to form the endosperm
Mechanisms Promoting Cross Pollination
- self incompatibility, male and female parts of the plat mature at different times
- male and female flowers on different plants
Pollination Mechanisms
- responsible for transferring pollen from one flower to another
- insect attracting mechanisms: colour, shape, smell
- pollen can also be transferred by mammals e.g. bats
- wind pollinated plants have no flowers e.g. grasses
What do seeds form from?
ovules
What do fruits form from?
ovaries
Seed Energy Storage
-found in endosperm or cotyledons/embryo
-
Why is seed dormancy an advantage?
- it increases the chance that germination will occur at a time and place most advantageous to the seedling
- dormancy is broken in response to a change in the environmental conditions e.g. temperature
Seed Dispersal
By Wind -parachute e.g. dandelion -tumbleweed By Animals -barbed fruit -'buried in caches -carried to ant nest -seeds in faeces
Diversity of Angiosperms
- terrestrial / aquatic
- development and dispersal mechanisms
- germination, branching, leaf shape
- annuals, biennials, perenials
- trees, shrubs, herbs
- hot / cold
- wet / dry
- sun / shade
Grasslands
- ~20% of Earth’s surface area
- 10 000 species
New Uses for Plants and Algae
- phytopharmaceuticals
- materials
- biofuels
- cell factories
- ‘environmental managers’
- biolubricants
- biopolymers
