Evolution of Land Plants Flashcards
What is the Latin name for the plant kingdom?
Plantae
What is the name for land plants?
Embryophytes
What is the name for chlorophytes, charophytes and other green algae and land plants?
Viridiplantae
How many species of land plants are there?
350-400,000 species
What is the name for flowering plants?
Angiosperms
What is symbiosis?
A mutualistic or common relationship between plants and other organisms such as algae or fungi
What are some benefits of symbiosis?
Nutrition Disease prevention Pollination Seed dispersal Habitat
What did plants descend from?
Heterotrophic eukaryotes, which engulfed a cyanobacterium and underwent primary endosymbiosis, forming red and green algae
What are the closest relatives to land plants?
Chara
Coleochaete
What are the features of Chara and Coleochaete that make them similar and different to land plants?
They have parenchymatous bodies (basic tissue of plants) that grow by an apical meristem (rapidly dividing cells at the of shoot and root). However, they have haplontic life cycles
What is a haplontic lifecycle?
Exist as a haploid being most of the time. Gametes are haploid and produced by mitosis. They fuse to form a diploid zygote, but this zygote divides by meiosis straight away. No mitosis in diploid phase
What type of life cycle do all land plants have?
Haplodiplontic
What is a haplodiplontic life cycle?
The plant undergoes both haploid and diploid mitosis - multicellular diploid and haploid stages occur, and meiosis is ‘sporic’
What is a diplontic life cycle?
Mitosis only occurs in the diploid phase
Exist as a multicellular diploid being most of the time, gametes carry genetic information to the next generation. Includes animals and some fungi
What is produced from meiosis in land plants?
Spores, which germinate into a haploid multicellular stage, producing gametes for sexual reproduction
Reproduction in Chara
Oogmaous - large egg cells born in multicellular ‘megagametangia’ and sperm born in multicellular ‘microgametangia’
During fertilisation sterile cells around the zygote thicken to form a protective layer around the egg
What is a gametophyte?
A haploid multicellular organism that develops from a haploid spore that has one set of chromosomes - it is the sexual phase of the alternation of generations
What is the sporophyte?
The asexual and diploid phase of the alternation of generation in plants. It is the dominant form in vascular plants
What and when we the first fossil evidence of Embryophytes?
450 million years ago
‘Microfossils’ - spores with ‘Trilete mark’ found indicating meiotic tetrads and sporophytes
What is a meiotic tetrad?
A group of four chromatids formed from each pair of homologous chromosomes that split longitudinally during the prophase of meiosis
What was the defining feature of the microfossils?
Thick walls made of sporopollenin
What was the first complete fossil, and what form of its life cycle was it in?
Cooksonia
Sporophyte form
What new adaptations did plants require for the shift from aquatic lifestyle to living on land, and what forms did they come in?
They had to avoid desiccation
1) They needed access to water and transport of water around the plant - this came in the form of a vascular system
2) Thick coatings around spores to avoid desiccation, but still allowing gas exchange. Plants evolved stomata and waxy cuticles / sporopollenin
What are the specialised egg-producing structures possessed by all Embryophytes?
Archegonia (specialised megagametangia)
What are transfer cells?
They line the archegonium, and are able to shuttle nutrients to the developing sporophyte embryo in the archegonium
What was the big change that separated land plants from their haplontic ancestors?
The mutation that caused diploid mitosis to occur - the sporophyte becomes a much bigger part of their lifecycle
What do all extant land plants undergo?
A heteromorphic alternation of generations - their life cycle takes them through both haploid and diploid forms
What are the Bryophytes?
An informal group consisting of three divisions of non-vascular land plants
What are the three Bryophyte divisions?
Liverworts
Hornworts
Mosses
What is an example of a genus of liverworts?
Marchantia
What is the sperm-producing (male sex organ) structure of algae, mosses, ferns, fungi and other non-flowering plants?
Antheridium
What is the morphology of liverworts? (6 features)
1) Parenchymatous body
2) Very thin cuticle on upper surface
3) No stomata
4) No roots
5) No true leaves
6) No vascular system
What is the most primitive group of extant Embryophytes?
Liverworts
What reproductive structures does Marchantia (liverwort genus) possess?
Archegonium and antheridium
What are the two types of specialised conducting cells that mosses have?
Hydrom - dead cells which transport water and provide stem support. Found in the centre of stem
Leptom - living cells which transport photosynthate
What type of Bryophyte has a rudimentary transport system in advanced species?
Mosses
What are moss stems surrounded by?
The cortex
What grows from the apical meristem in gametophytes?
The gametophyte
What does each sporangium contain in mosses?
Many spores
What is a sporangium?
A receptacle in ferns and lower plants in which asexual spores and formed
Features of hornworts
Their antheridia and archegonia are buried Cells have just one chloroplast Symbiotic with cyanobacteria Some species have stomata on sporophyte No discrete sporangia
Adaptive features of Embryophytes
Archegonia
Cuticle (cutin - waxy, water-repellant substance)
Stomata
Thick-walled spores (sporopollenin)
Vascular tissue - xylem (lignin for rigidity)
What is the name for vascular plants?
Tracheophytes
What is the dominant form in tracheophytes? What is it supported by?
Sporophyte is the dominant form, supported by tracheids
What are tracheids?
A type of water-conducting cell in the xylem which lacks perforations in the cell wall
Thickened with lignin so provide support for stems and make wood
Present in all vascular plants
Dead cells - no protoplasm
What is the trend in sporophytes of tracheophytes?
They are increasingly more complex and branched
What genus represents a possible intermediate between Bryophytes and Tracheophytes?
Cooksonia
What features of Cooksonia suggest that it is an intermediate between tracheophytes and bryophytes?
They have tracheid-like thickened cells, a possible intermediate between moss hydrom cells and tracheids
Dichotomous branching sporophyte with terminal sporangia
Rhizomes
No fossil gametophytes
What is dichotomous branching?
The branches form as a result of equal division of a terminal bud into two equal branches
What are rhizomes?
Continuously growing horizontal underground stems which puts out lateral shoots and roots at intervals
What is protoplasm?
The colourless material comprising the living part of a call, including the cytoplasm, nucleus and other organelles
What was one of the first tracheophytes, which was similar to Cooksonia but also possessed tracheids? When did it evolve?
Aglaophyton major
400 million years ago
What tracheophyte evolved 400 million years ago and had unequal dichotomous branching (pseudomonopodial)?
Rhynia Gwynne-vaughanii
What tracheophyte evolved around 400 million years ago and had lateral sporangia?
Zosterophyllum
What are lateral sporangia?
Sporangia placed on the side of stems
What are terminal sporangia?
Sporangia on the tips of stems
What tracheophyte evolved 390 million years ago and had a more advanced vascular system with pitted tracheids and underground branching / rhizomes?
Psilophyton dawsonii
How did the branching of sporophytes evolve, allowing plants to grow to much greater heights?
From dichotomous (equal dichotomy) To pseudomonopodial (unequal dichotomy) To monopodial (grows upward from a single point)
What is the stele of a plant?
The central core of the stem and root of vascular plants, composed of primary vascular tissues - the xylem and phloem
What are the names of the 3 arrangements of xylem and phloem within the stele?
Protostele
Siphonostele
Eustele
What plants have a protostele?
Early lycophytes and arthrophytes
E.g. Cooksonia, Aglaophyton, Rhynia
When did the siphonostele evolve, and what plants possess it?
Evolved 395 million years ago
Possessed by Lycophytes, Arthrophytes and ferns
When did the eustele evolve, and what plants possess it?
380 million years ago
Found in advanced tracheophytes, gymnosperms and angiosperms
What are the 5 ways a tracheid wall can thicken?
Annular Helical Scalariform Reticulate Pitted
Evolutionary trends in early tracheophytes
Increased branching of sporophytes Increased size of sporophyte Reduced size of gametophyte Increased complexity of vascular system Evolution of leaves Evolution of roots
When did leaf-like structures and true leaves evolve?
Mid-late Devonian period
390-354 million years ago
What are microphylls?
‘Stem-hugging’ leaves with single vascular strands and no stem
What plants possess microphylls and what type of stele are they associated with?
Lycophytes
They are associated with possessing protosteles
What are megaphylls?
Leaves with branched vascular strands and attached to the main stem by a petiole
What is a petiole?
The stalk that joins a leaf to a stem
What plants are megaphylls found in and what type of stele are they associated with?
Found in ferns and all flowering plants
Associated with stems which have siphonostele or eustele
What may have been the selective force for the evolution of leaves?
Reduced CO2 levels, as plants needed a more efficient way to collect CO2 for photosynthesis when levels were lower
Give examples of Lycophytes
Club mosses
Spike mosses
Quillworts
What are features of Lycophytes?
Lateral sporangia
Microphylls
What are features of Euphyllophytes?
Terminal sporangia in pairs
megaphylls
What plants are Euphillophytes?
Monilophyton
Seed plants
What is the Latin name for ferns?
Monilophyton
When did Carboniferous forests evolve?
Throughout the Devonian-late Carboniferous period (390-290 million years ago)
What trends occurred during the evolution of Carboniferous forests?
Sporophyte size increased dramatically
Increased branching
Increase in leaf size and number
Cause of trends during evolution of Carboniferous forests
1) Competition for space and light
2) Decreasing CO2 levels
3) More effective spore dispersal to moist environments
4) Avoidance of inbreeding
Innovations of Carboniferous forests
1) Advanced vascular systems with elaboration of siphonosteles and eusteles
2) Secondary thickening from a vascular cambium (meristem) - xylem, phloem and cortex tissues making wood
3) The evolution of ‘tree trunks’ - better support
4) Advanced branching (monopodial growth), root systems and leaves
What is the cambium?
A vascular meristem which allows lateral expansion of the stem through ‘secondary growth’. It predominantly produces new xylem and phloem
What types of steles can the cambium be associated with?
Siphonosteles and haplosteles (a type of protostele)
What is transpiration? What is it mainly controlled by?
The movement of water through the plant primarily in the xylem, 95% of which is controlled by stomata movements
What environmental signals regulate the opening and closing of stomata?
CO2 Humidity Light Water availability at soil level Stress factors
How can you determine atmospheric CO2 levels from a plant?
Calculate the stomatal density and index
How are stomata closed?
A high K+ concentration in the cells surrounding the guard cells causes water to leave the guard cells by osmosis, causing them to close the stomatal pore as they lose turgidity
List 3 climatic factors during the Carboniferous period
1) Warm and humid, tropical climates emerge
2) Dramatic fall in CO2 levels, which is what causes the dramatic elaboration of leaves
3) More vegetation than could be decomposed - formation of coal
What is the name for tree lycophytes?
Clubmosses
Features of tree lycophytes
Advanced siphonostele with secondary thickening from vascular cambium
Most support from primary and secondary cortex
Possess strobili consisting of many sporangia situated on sporophylls
What is a sporophyll?
A leaf that possesses sporangia
What is the name for tree arthrophytes?
Horsetails
Features of tree arthrophytes
Advanced siphonostele with secondary thickening from vascular cambium
Lower Euphillophytes
Possess sporangiophore bearing sporangia
What is a Euphillophyte?
A true leaf plant
What is a sporangiophore?
A structure that bears one or more sporangia
What are tree monilophytes? (include latin name)
Ferns (pterophytes)
Feature of tree monilophytes
Massive primary stem - no secondary thickening
What height could tree lycophytes grow to? (extinct now)
35m
What height could tree arthrophytes grow to? (extinct now)
18m
What height could tree monilophytes grow to? (extinct now)
8-10m
Features of progymnosperms (extinct now)
Advanced eustele with secondary thickening from vascular cambium
Similar to extant gymnosperms
What were progymnosperms?
Woody, spore-bearing plants
Extinct now
Three types of reproduction in spore forests
Homospory
Heterospory
Anisospory
What is homospory?
Spores of only one type produced - one type of gametophyte (with antheridia and archegonia) or two types of gametophyte (one with antheridia, one with archegonia)
What plants use homospory?
All bryophytes, all extant arthrophytes and most extant lycophytes and ferns
What is heterospory?
Spores of different types are produced from different types of sporangia - always two types of gametophytes produced
What plants is heterospory found in?
Many extinct lineages of lycophytes, arthrophytes, ferns, progymnosperms, some extant lycophytes and water ferns and all seed plants
What is anisospory? What lineages is it found in?
Large and small spores produced in the same sporangium. Only found in extinct lineages
Evolution of heterospory through the Devonian and Carboniferous period
Lower Devonian: small spores only
Mid Devonian: larger spores appear
Late Devonian: very large spores appear
Carboniferous: many heterosporous fossils
What are the 3 selective advantages of heterospory?
Resource allocation
Increased potential for outbreeding (heterosis)
Protection of gametophytes
How does heterospory help with resource allocation?
More nutrients for fewer, larger spores so better chance of their survival, and lots of small microspores produced so they can find the eggs in archegonia in gametophytes
How does heterospory help reduce inbreeding?
Aerodynamic sorting of spores - as microspores will travel further than megaspores and the gametophytes are unisexual
How does heterospory help with the protection of gametophytes?
It allows endosporic development - development of the gametophyte within the spore
What evolution is analogous to the evolution of heterospory from homospory?
The evolution of oogamy from isogamy
What is oogamy?
A form of anisogamy in which the female gamete is much larger than the male gamete and is non-motile
What is isogamy?
Sexual reproduction by the fusion of similar gametes, differing in general only in allele expression
What is a seed?
When the megaspore is not released and germinates inside the megasporangium to produce the female gametophyte, an ovule (seed)
What did seeds evolve from?
A line of heterosporous progymnosperms that retained their megaspores on the sporophyte
What were the possible steps in ovule evolution?
1) Free sporangia with sterile stems
2) Sterile stems cluster around the sporangium
3) Sterile stems ‘fuse’ around the sporangium with single megaspore inside (megagametophyte)
In a seed, how many megaspores does each megasporangium produce?
One (the other three degenerate)
What is the nucellus?
The central part of an ovule, containing the embryo sac
Seeds are a modified…
megasporangium
What is the structure of a seed?
The one megaspore produced germinates within the modified megasporangium (nucellus) to produce a megasporangium on the sporophyte. The nucellus and megagametophyte are enclosed and protected by layers of sporophyte tissue (integuments) and the whole structure is called an ovule
What are the layers of sporophyte tissue called?
Integuments
What are the 3 benefits of sporophytes retaining megaspores?
1) Reduces significantly the requirement for external water to fertilise eggs
2) Protection of female gametophyte and embryo sporophyte
3) Nourishment of female gametophyte and embryo sporophyte
When did the first seed plants appear in Carboniferous spore tree forests?
350 million years ago
What is a gymnosperm?
A seed plant bearing ‘naked’ ovules/seeds
What gymnosperm groups appeared in the Early Carboniferous period? (350 Ma)
Pteridosperms
Cordaitales
What shift in plant species happened in the Early Permian period? (approx 300 Ma)
Number of gymnosperms dramatically increased.
Pteridosperms and Cordaitales expand
What new groups formed in the Early Permian period?
Cycadales (cycads - stout woody trunk and stiff, evergreen leaves)
Ginkgoales (ginkgos - one extant species)
Bennettitales (extinct)
Glossopterids (extinct)
By when did gymnosperms make up 60% of the flora?
By the upper Permian period - 260 million years ago
When the supercontinent Pangaea formed, what changes did this bring about to the climate?
Widespread aridity in continental interiors
High seasonal temperature fluctuations
Equatorial aridity and monsoons
Why were seed plants more successful than spore plants on Pangaea?
They were better adapted to reproduction in arid conditions
Features of ginkgos
Appear in Early Permian period
There were over 16 genera present
Only one extant species - ginkgo tree
Stem construction similar to conifers - eustele with vascular cambium and lots of secondary xylem
Features of cycads
Appear in Early Permian period Fossils up to 15m long 10 extant genera 100 extant species Found in warm, tropical regions Plants are dioecious - individuals sexually distinct Sperm are motile
when did conifers become common?
Appear in Carboniferous period
Underwent major radiation in Triassic period
8 conifer families
Characteristics of conifers (10 points)
- Arborescent (all trees)
- Pyramidal growth form
- Small simple ‘needle’ leaves
- Stem composed of secondary wood due to secondary thickening
- Tracheids frequently arranged into distinct annual rings with resin canals
- Roots have tap-root simple branching
- Reproductive structures are cones
- Mostly monoecious (male and female cones on different parts of same tree)
- Pollen with air bladders
- Pollen produces tube to deliver non-motile sperms to ovule
What adaptation does the pollen of conifers have and how does it benefit the conifers?
They produce saccate pollen (with air sacs), which increases the dispersal of pollen and therefore reduces the chance of self-fertilisation
What do female conifer cones possess?
Ovuliferous scales - each bears two ovules
What is the life cycle of Pinus (genus of conifer)?
- Naked ovule pollinated
- The pollen consists of 3 cells: tube cell, stalk cell and body cell
- Next spring, the pollen germinates
- The body cell divides to make two sperm cells of equal size
- The largest sperm cell fertilises the egg - the remaining cells degenerate
- Next spring, the winged seeds are dispersed by the wind
What is pyriscence?
Some plants (including some conifer species) rely on fire to mature and disperse their seeds
Features of gnetales
Very derived group of gymnosperms
Poor fossil record - close to pines
All dioecious
What features do gnetales share with flowering plants?
Xylem contains vessels
Reproduction structures (strobili) resemble primitive flowers
Insect pollinated
Female gametophyte reduced to an ‘embryo sac’
What genera of Gnetales have a form of double fertilisation but no endosperm forms?
Gnetum
Ephedra
When are the first Angiosperm fossils from?
Early Cretaceous period (120 Ma)
What part of an angiosperm were the first fossils and why?
Pollen
Because sporopollenin is easily fossilised
What are the major innovations of angiosperms?
- Ovules enclosed within a carpel / pistil
- Presence of a ‘flower’
- Double fertilisation
- Presence of a double integument
- Presence of complex pollen wall
- Presence of xylem ‘vessels’
- Presence of phloem companion cells
- Net-veined leaves (eudicotyledons)
- Fruits developing from flower parts
What group were thought to be the closest living relatives of angiosperms, but are dioecious so similarities are just a result of convergent evolution?
Gnetales
What two orders are the closest relatives to angiosperms?
Bennettitales
Glossopterids
When were Bennettitales abundant, and what type of cones did they possess?
Abundant from early Triassic to late Cretaceous period
They had bisexual cones, unlike cycads
When were Glossopterids abundant and what features did they have?
Dominated flora of Southern Hemisphere during the Permian period
Had advanced net-veined leaves, but unisexual cones
What are the reproductive structures in gymnosperms?
Unisexual cones
What does unisexual mean in terms of cones?
The plant either produces a male or female cone but never a cone with both male and female reproductive structures
What is the carpel and what is it comprised of?
Female sex organ
Comprised of the stigma, style and ovary
What is the stamen and what is it comprised of?
Male sex organ
Comprised of anther and filament
How is the ovule enclosed in angiosperms?
The ovule is enclosed within the carpel, surrounded by a double integument. The gametophyte is greatly reduced to only 8 cells. There are no archegonia
What is observed in the endosperm of angiosperms?
Triploidy, the result of a double fertilisation event. The egg is fertilised by one sperm and the two haploid maternal nuclei in the endosperm get fertiliser by the single haploid sperm nucleus
Describe the fight over the quantity and quality of resources that are laid down for the developing embryo in the endosperm
As there are two haploid maternal nuclei and only one haploid sperm nuclei, the final ratio gives the maternal parent more control, and they have a greater contribution to the genetic constitution of the endosperm
What is the collective word for the petals?
A corolla
What is the male gametophyte?
Two or three cells within a pollen grain
What did stamens evolve from?
Leaf- or petal-like structures bearing microsporangia
How does pollination and double fertilisation occur?
- Pollen tube penetrates a synergid (cell in female gametophyte) and releases sperm nuclei
- One sperm fuses with the egg to form a diploid zygote
- A second sperm fused with the two polar nuclei to form a triploid endosperm which nourishes the developing embryo sporophyte
What happens to the triploid endosperm following fertilisation?
The endosperm nucleus divides repeatedly to form a mass of protoplasm and nuclei without cell division. It does this by acquiring nutrients from the sporophyte. As the seed matures the endosperm undergoes cellularisation (cell walls form)
How do angiosperm seeds utilise maternal resources more effectively than gymnosperm seeds?
They have many fewer cells
The endosperm food reserves aren’t laid down until after fertilisation, so resources aren’t wasted on a seed that is never fertilised
What is a fruit?
A structure formed from part of a flower or inflorescence (the complete flower head of a plant including stems, stalks and flowers) that contains the seeds
What are fruits adapted to encourage?
Seed dispersal by animals
What are dicotyledons (dicots)?
Embryonic plants with 2 seed leaves (cotyledons)
What are the features of dicotyledons?
- Leaves net-veined, opposite or alternate
- Vascular bundles of stem arranged in circle
- Stems often woody
- Flower petals usually grow in multiples of 4 or 5
What are monocotyledons (monocots)?
Embryonic plant with 1 seed leaf (cotyledon)
Features of monocotyledons
- Leaves typically parallel-veined
- Vascular bundle of stem closed and scattered
- No secondary thickening
- Flower petals usually grow in multiples of 3
What is a Eudicotyledon (eudicot) and what pollen does it produce?
A ‘true’ dicot
Produces pollen with 3 apertures
What is pollen with 3 apertures called?
A tricolpate
What type of pollen do monocots and primitive dicots produce?
Pollen with a single aperture - a monocolpate
What are the 5 primary attractants to pollinators?
Pollen - rich in protein, starch and fats Nectar - sugary exudate from nectaries Oil - fat oil from oil glands Protection and brood-place Sexual attraction
What are the oil glands of angiosperms called?
Elaiophores
What are the 4 secondary attractants of specialised plants to pollinators?
- Odour
- Visual attraction - colour, shape, reflection
- Temperature
- Motion - flickering of large influorescences
How can the odour of a plant attract pollinators?
Night-flowering plants use odours
‘Rotting meat’ plants attract some insects
Sex pheromones released
How might temperature attract pollinators?
Heliotropic (growth towards or away from sunlight) flowers in cold climates - attracts pollinators as flower is warmer than environment
What are the 4 adaptations to avoid self-fertilisation?
- Dichogamy
- Herkogamy
- Dicliny
- Self-incompatibility
What is dichogamy?
Separation of sexual organs in time - the stamens mature before carpels/pistils or vice versa
What is a pistil?
A carpet that is fused into one large structure containing many ovules
What is herkogamy?
Separation of sexual organs in space - style length polymorphisms
What are 2 examples of style length polymorphisms?
Distyly and tristyly
What is dicliny?
Sexual polymorphisms - plants are dioecious or monoecious
What is self-incompatibility?
Plants are able to recognise and reject their own pollen before the pollen tube can reach the ovule
What are examples of insect pollinators?
Beetles Flies Bees Wasps Butterflies Moths Ants
Beetles as insect pollinators
Believed to be first pollinators of angiosperms
Attracted to dull, open, accessible flowers with a strong odour
Vertebrate pollinator examples
Birds Bats Possums Mice Lemurs Lizards
What is the Madagascan long-spurred orchid pollinated by and how does it attract the pollinator?
Pollinated by the long-tongued moth
It’s pale colours are visible at night, and it is strongly scented with copious nectar in long tubes/spurs
What species of orchid attracts pollinator males using sexual deception and what pollinator does it attract?
Ophrys speculum (‘mirror orchid’) Attracts Campsoscolia ciliata (wasp) Pseudocopulation occurs, which transfers pollen onto the wasp
What 3 deceptions does Ophrys speculum use to attract the male wasp?
Odour - female bee sex pheromone
Visual - glistening speculum (mirror)
Tactile - hairs on labellum
