Early plants Flashcards
Archaeplastidia
Red algae and green algae are the closest relatives of plants
Charophytes
A type of green algae that is the closest relative of plants
- Chara sp.: Known as muskgrass or skunkweed because of odor
- Shallow freshwater dwelling, branched multicellular algae that can extend above the water surface
What are some key traits that only charophytes share with plants?
Rings of cellulose-synthesizing proteins
- Distinctive cellular rings of proteins in plasma
membrane that synthesize cellulose
microfibrils of the cell wall; linearly arranged in
non-charophytes
Peroxisome enzymes
- Specific peroxisomal enzymes that help to
minimize the loss of organic products resulting
from photorespiration
Structure of flagellated sperm
- Land plants and charophytes have flagellated
sperm of similar structure
Formation of a phragmoplast
- During cell division a set of microtubules known
as a phragmoplast forms between the nuclei;
the location of the phragmoplast becomes the
new cell wall
Presence of sporopollenin-like polymer
- Charophytes produce a durable polymer called
sporopollenin; prevents exposed zygotes from
aerial desiccation
- Sporopollenin is a major component of the
walls of spores and pollen of many plants
Sequence similarities
- Share similar nuclear and chloroplast DNA
sequences
- Does not mean that plants are descended from
living charophytes but that charophytes may
resemble the algal ancestors or plants
- Three possible “plant” kingdoms
Why would first plants want to live permanently above the waterline?
Advantages
- More sunlight
- More CO2
- More nutrients
- Possibly fewer herbivores
Disadvantages
- Scarcity of water
- Desiccation
- Gravity - lack of structural support
What are the four key traits that appear in nearly all land plants but are absent in the charophytes?
- Alternation of generations and multicellular, dependent embryos
- Walled spores produced in sporangia
- Multicellular gametangia
- Apical meristems
Gametophyte
A haploid that produces haploid gametes by mitosis
Sporophyte
Created through the fusion of gametes
- A diploid that produces haploid spores by meiosis
Embryophytes
A name for land plants since they have dependent embryos
- diploid embryo is retained within the tissue of the female gametophyte
- nutrients are transferred from parent to embryo through placental transfer cells
Gametangia
Gametes that are produced within multicellular organs
Archegonia vs. Antheridia
- Archegonia are female gametangia which produce eggs and are the site of fertilization
- Antheridia are male gametangia which produce and release sperm
Sporangia
A sporophyte that produces spores in multicellular organs
Sporocytes
Diploid cells within the sporangia that undergo meiosis to generate haploid spores
Sporopollenin
Contained in the spore walls, makes the walls resistant to harsh environments
Apical Meristems
Where plants sustain continual growth, localized regions of cell division
- Cells from the apical meristems differentiate into various tissues, gives rise to specialized below and aboveground organs
Cuticle
Present in many plants
- helps prevent desiccation and resistance to microbial attack
Epidermis
Covered with cuticle which consists of wax and other polymers
Stomata
Present in many plants
- Stomata have to be open for leaves to receive CO2
- Opening of stomata allows water to exit the leaf
- About 95% of the water a plant loses escapes through stomata
Mycorrhizae
- Fossils from 420 mya reveal presence of endomycorrhizae, haustoria
- Probably allowed first plants to absorb nutrients and water from soil
Production fo secondary metabolites
Products arising from the side-branches off the primary metabolic pathways eg. alkaloids, terpenes, tannins, flavonoids, capsaicin
- purpose is to deter herbivores
Where do we see the origin of the diversification of plants?
- Fossil evidence indicates that plants were on land at least 475 mya
- Fossilized spores and tissues have been extracted from 475 million-year-old rocks
- Spores of ancient plants appear to be grouped rather than single grains
How is plant lineage decided?
- Grouping based on the presence of absence of vascular tissue
- vascular plants, nonvascular plants - bryophytes
- Bryophytes are not a monophyletic group, relationships unresolved
A seed
- An embryo and nutrients surrounded by a protective coat
- Seed plants form a clade and can be divided into further clades
Gymnosperms
The “naked seed” plants, including the conifers
Angiosperms
The flowering plants
What are nonvascular plants life cycles dominated by?
Gametophytes
Bryophytes
Represented today by three phyla of small herbaceous (nonwoody) plants
- Liverworts, phylum Hepatophyta
- Mosses, phylum Bryophyta
- Hornworts, phylum Anthocerophyta
What is the dominant life in all three bryophyte phyla?
Haploid gametophytes are the dominant life history stage
- Larger and longer lived than sporophytes
Sporophytes
- Typically present only part of the time
- Remain attached to the gametophyte and are dependent on it
Are gametophytes tall or short?
They form in ground-hugging carpets because their structure is too delicate to support tall plants
What is the height of gametophytes constrained by?
It is constrained by a lack of vascular tissue
Rhizoids
Anchor gametophytes to the substrate
Archegonia
What bryophyte sporophytes grow out of
What do sporophytes consist of?
A foot, a seta (stalk), and a sporangium (capsule), which discharges spores through a peristome
What is the importance of mosses?
- Common in cold and wet environments
- Moss bofs cover 3% of earth’s land surface
- Peat moss, extensive deposits of partially decayed organic material
- peat moss is an important global reservoir of
organic carbon, contains 30% of soil carbon
What were the first plants to grow tall?
Ferns and other seedless vascular plants
- Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution
- Vascular plants began to diversify during the Devonian and Carboniferous periods (425-300 mya)
- Vascular tissue allowed these plants to grow tall
- Seedless vascular plants have flagellated sperm and so are usually restricted to moist environments
What is the origin of vascular plants?
- First fossil recorded of vascular plants ~425 mya
- Early vascular plants ~15cm tall
- Did not have roots
- Appearance of branching may have opened the way to more complex body plans
- Competition for space and light probably drove evolution of form and function
Why is life cycles with dominant sporophytes different than with bryophytes?
In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation
Sporangium
The tissue where spores are formed
Sporophylls
Modified leaves where sporanium are found
Sorus
The clusters of sporangia on the underside of sporophylls
Homosporous
Produces one type of spore that develops into a bisexual gametophyte
- most seedless vascular plants are homosporous
Heterosporous
Produce megaspores, which give rise to female gametophytes, and microspores, which give rise to male gametophytes
- all seed plants are heterosporous
What do vascular tissues allow for?
Transport of water and nutrients between distant roots and shoots
Xylem
Conducts most of the water and minerals and includes dead cells called tracheids
Lignin
Strengthen water-conducting cells and provide structural support
Phloem
Consists of living cells and distributes organic products
What are the advantages of plants growing taller?
- Taller plants could compete with other plants for sunlight
- Spores of tall plants could disperse farther than those of short plants, tall plants could colonize new environments rapidly
- Competition among vascular plants also increased, and taller plants were favored by natural selection, such as the trees that formed the first forests about 385 million years ago
Roots
Organs that anchor and provide stability for taller vascular plants
- enable vascular plants to absorb water and nutrients from the soil
- roots may have evolved from subterranean stems
Leaves
Organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis
- Leaves are categorized by two types
- Microphylls, leaves with a single vein
- Megaphylls, leaves with a highly branched
vascular system
Microphylls
- According to one model of evolution, microphylls evolved as outgrowths of stems
- Appear in fossil record about 410 mya
Megaphylls
- Megaphylls may have evolved as webbing between flattened branches
- Appear in fossil record about 370 mya
What plants formed the first forests?
The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carniferous
