Ch.23 Flashcards
Land Plants (part of Kingdom Plantae) General Characteristics:
• Land-dwelling (terrestrial)
– some have returned to aquatic (freshwater) habitats
• Eukaryotes
• Multicellular
• Sessile
• Photosynthetic (photoautotrophs)
• Cell walls made of cellulose
• Alternation of generations
•exceptions are ghost and corpse plants.
— lack chloroplasts therefore are parasitic.
• Charophytes and land plants
share a common ancestor
—most likely lived in aquatic environments.
•embryophytes and charophytes are sister clades.
• N.B. today’s Charophytes have also changed from ancestor
Main groups
- Non-vascular plants (e.g., mosses)
Vascular plants: - Seedless vascular plants (e.g., ferns) Seed plants:
- Gymnosperms (e.g., conifers, pines)
- Angiosperms (flowering plants)
Evolution of Land Plants
Ancestral green alga —> 1. Origin of plants (about 470 Mya) (non vascular plants = bryophytes) —> 2. Origin of vascular plants (about 425 Mya) —> 3. Origin of seed plants (about 360 Mya)
Non vascular plants:
— liverworts
—mosses
—hornworts
Vascular plants:
Seedless:
— lycophytes (club mosses, spikemosses, quillworts)
— monikphytes (ferns, horsetails, whisk ferns)
Seed plants:
—gymnosperms
—Angiosperms
• After 1. Able to adapt to shallow waters, still need moist environments.
• colonizations first most likely from cyanobacteria or fungi
•plants are hard to preserve
• when we find fossils they have already colonized the land, so it means they were there before we were able to find them, just hadn’t colonized it yet.
Early land plants
•most likely poikilohydric = able to shut down metabolic rate and vital functions = dormant state.
-do this when there is no water available.
- can do this for a very long time
-considered a primitive state as this was most likely beneficial when plants 1st colonized land
Evolutionary trend in land plants
Baryophytes —> seedless vascular plants —> Gymnosperms —> angiosperms
B: sporophytes (2n), gametophytes (n)
Same for seedless ^
G: sporophyte (2n) —> Microscopic male gametophytes (n) inside pollen cone —> microscopic female gametophytes (n) inside ovulate cone.
A: sporophyte (2n), Microscopic male gametophytes (n) inside parts of flower, microscopic female gametophytes (n) inside different part of flower.
From B—> A
• gametophytes become smaller and less important.
Ancestral green algae (Charophycean algae) —> zygote becomes a multicellular sporophyte that is nutritionally dependent on gametophyte (bryophytes) —> sporophyte becomes free living (not dependent on gametophyte) and fully autotrophic —> transpiration initiated with stomata change (Rhyine chert fossils) —> gametophyte passes through subterranean phase (now dependent on sporophytes) (living vascular plants)
Mycorrhizae = fungi + plant
• Confirmed fossil evidence of mycorrhizae from approximately 450 Mya
• Possibly much older than that!
Benefits of Terrestriality?
Challenges of Terrestriality?
• More sunlight
• More nutrients
• More CO2
• Less competition
• Fewer predators
• Fewer parasites/diseases
• Less water
•Lack support
• UV
• Changing conditions
• Reproduction
Growing upright and away from competition
• Problem: Tissues need to withstand the force of gravity
→ Solution: lignin
-also needed to support water transportation in tissue.
• Problem: Plant needs to transport water in its tissues
→ Solution: Tissues adapted for transporting water
A Series of Evolutionary Innovations Allowed Plants to Adapt to Life on Land
• Sporopollenin
• Cuticle,pores
• Stomata
• Embryophytes
• VascularTissue
• Roots
• True Leaves
• Alternation of generations
Characteristics of land plants:
Sporopollenin Spores
Sporopollenin Spores – unique among Land Plants
- Sporopollenin - tough polymer
• in Charophytes: protects zygote from drying
• in Plants: walls of plant spores
• resistant to drying and physical stresses
• ancestral charophytes with this trait favoured by natural selection – how?
— b/c they were able to have a greater success in reproduction and handle the selective pressure better.
Characteristics of land plants
2. Adaptations for water conservation
• waxy cuticle on epidermis: waterproofing, protection from microbial attack
• stomata (sing., stoma): pores in the epidermis of leaves and other photosynthetic organs
– allow gas exchange between air and leaf interior
– sites for water to exit via evaporation
– closed stomata: minimize water loss
— if mainly closed = dry environment
— if mainly open = moist environment
• stoma open or closed based on presents of water and helps prevent water loss.
Characteristics of land plants
3. Multicellular, dependent embryos
• In contrast to most green algae, zygotes of land plants are retained within tissues of the female parent
• Parent provides nutrients, embryo has specialized placental cells (transfer nutrients)
Why is this important for terrestrial survival?
• The group’s formal name is Embryophyta, literally, the
“embryo-plants”
• The retention of the fertilized egg in embryophytes is analogous to pregnancy in mammals
Characteristics of land plants
4. Lignified vascular tissue for internal transport
• Contain lignin (a complex polymer), which strengthens
• Xylem cells carry water and minerals up from roots
– Dead wall act as microscopic water pipes
— key in transportation of water. The minerals are in the water.
• Phloem cells distribute organic products
– Living cells
— supports the sugars and amino acids.
— photosynthesis —> support sugar —> important b/c plants use sugar for vital functions. But also make enough for other organisms, hence why so many are dependent on plants.
Characteristics of land plants
5. Resources more compartmentalized
• most plants show structural specialization for searching for water & minerals underground (roots) and light and gases aboveground (shoots)
– Note: early land plants did not have roots (that’s why they), relied on symbiotic relationships with mycorrhizal fungi (which helped plants colonize land)
— without fungi, wouldn’t have been able to colonize land.
• elongation and branching maximizes root and shoot exposure to environmental resources —> apical meristems
Apical Meristems
• Undifferentiated tissue (localized) from which new, differentiated cells arise.
– Simple in non-vascular plants
– More complex structures
at tips of shoots and roots in vascular plants
• Cellsproducedby meristems differentiate into various tissues, including epidermis and internal tissues. And later on roots of plants
Characteristics of land plants:
6. Alternation of generations
• Alternation of Haploid (n) multicellular and diploid (2n) multicellular body forms alternate
—spores do not make gametes
— sporophytes are in gametophytes
Gametophytes make gametes through mitosis (n) —Gametes—> fertilization —zygote (2n)—> mitosis (2n) —> diploid multicellular organism (sporophyte) (2n) —sporophytes make spores through meiosis (2n) —meiosis—> spores (n) —mitosis (n)—> haploid multicellular organism (gametophyte) (n) —mitosis—> gametophytes make gametes through mitosis (n) —> and so on.
• All land plants undergo alternation of generations
• Why is alternation of generations an important adaptation?
• Mitosis before meiosis!!
• Delayed Meiosis = many copies through mitosis before making spores
• Amplification, which is good for harsh conditions of land (more spores, more survive)
Differences in alternation of generations
(Deals with increased adaptation to land)
• Bryophytes (mosses) are gametophyte dominant
• Ferns, gymnosperms, angiosperms are sporophyte dominant.
Dispersal
• b/c they can’t move (sessile) must find another way to reproduce it become fertilized.
Ex:
• Malpe tree seeds
• palm seedling
• seeds in poop
• seeds on animals
• mymecochory (like an ant grasping elaiosine(a seed))
Evolutionary History of Land Plants
Adaptive Radiation: Angiosperms
• An adaptive radiation occurs when a single lineage produces a large number of descendant species that are adapted to a wide variety of habitats
• Angiosperms represent one of the great adaptive radiations in the history of life
Decreasing dependence on water
Moist habitats
|
V
Non-vascular plants lack complex conducting tissues found in vascular plants
|
V
Dry habitats
Bryophytes (= Nonvascular plants)
• Not a monophyletic group
• Dotted lines indicate uncertainty of relationships of the phyla
Characteristics of Bryophytes
• Lack true vascular tissue (no lignin)
– limits their size
• Most tissues only one or a few cells thick
• Haploid (1n) gametophytes are the most conspicuous, dominant phase of the life cycle – diploid (2n) sporophytes smaller & dependent
– diploid present only part of the time
• Often entire plants are either male or female
- sperm swim through water film to fertilize eggs
- Must live in moist places
- Not yet fully terrestrial
➢ Parallels with animals?
— amphibians
Bryophyte Life Cycle
Key steps:
1. Spore dispersal
2. Gametophyte germination
3. Gamete production (fertilization)
4. Sporophyte generation
5. Sporangium development
6. Spore production and dispersal
→ New gametophyte is produced, and cycle starts again!
To clarify:
Gametophyte-dominant vs. Sporophyte-dominant
✓Gametophyte-dominant: (ex. fern)
— also considered gametophyte-independent
• gametophyte stage is the dominant and independent stage of the life cycle;
sporophyte stage is reduced and nutritionally dependent on the gametophyte
• gametophyte stage can carry out all of the necessary functions to sustain the organism, including photosynthesis, nutrient uptake, and reproduction
✓Sporophyte-dominant: (ex. flowering plants/angiosperms)
— also considered sporophyte-dependent
• sporophyte stage is the dominant and independent stage of the life cycle
• gametophyte stage is relatively small and dependent on the sporophyte for nutrients and protection
Origin of Vascular Plants
• Bryophytes probably Earth’s only plants for first 100 million years
• Aglaophyton (~425 MYA) first plant to have sporophytes not continuously nutritionally dependent on gametophytes (a feature of vascular
plants)
– did not have leaves or roots – no vascular tissue… yet
• Fossils of other simple plants from early Devonian show evidence of vascular tissue in the stem
• Later came leaves, and then roots
Characteristics of vascular plants
• Complex multicellular roots
– absorb nutrients, provide anchor
• Complex multicellular leaves
– photosynthetic organs
• Microphylls: single strand of vascular tissue, usually small
- no leaf gap, single vein on leaf
• Megaphylls: branching vascular system, usually large
- leaf gap, branching veins on leaf
Vascular plants
• Sporophylls
– Modified leaves that bear sporangia (spore-producing organ)
– sporangia + sporopollenin walls→terrestrial adaptation!
• Sporophyte dominant in life cycle
– increased size, complexity and persistence
– not continuously nutritionally dependent on gametophyte
Characteristics of Seedless Vascular Plants
• Sporophyte–dominant form
• Gametophytes: tiny independent plantson or just below soil surface
• Flagellated sperm: must swim in a film of water to reach eggs
– Similar to bryophytes, seedless vascular plants most common in relatively damp habitats
– Still need water to reproduce
Fern life cycle
Key steps:
1. Spore Production
2. Spore Dispersal
3. Gametophyte
Formation
4. Gamete
Production
5. Fertilization
6. Sporophyte
Development
7. Sporangia Formation → Then Spore
Production again
Seed plants
Seed Plants Shared features:
• seeds!
• heterospory (microspores/male and megaspores/female = no flagellated spores)
• Pollen(non- swimming sperm)
• sporophyte independent
• gametophyte dependent
Main Clades of Seed Plants
• Gymnosperms
✓Gymnos = naked, sperm = seed
• Angiosperms
✓Angio = container, vesse
• Seeds changed the course of plant evolution
• Seed-bearing plants are the dominant producers in most terrestrial ecosystems
— which makes it really successful on land
• A seed consists of an embryo and nutrients surrounded by a protective coat
• Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements
