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Evolution and Diversity in Plants I - E col 182 – 4-7-2005. Re-downloaded at 7:10am on 4-7. Big Questions. What have been the important constraints and / or principles that have shaped the evolution of plants. Diversification Form and function.

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Evolution and Diversity in Plants I - E col 182 – 4-7-2005

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big questions
Big Questions
  • What have been the important constraints and / or principles that have shaped the evolution of plants.
    • Diversification
    • Form and function

Important particularities on evolution and speciation in plants

R.A. Fisher (1958)

Fundamental Theorem of Natural Selection

“Rate of increase in the mean fitness of a population is proportional

to the genetic variance in fitness”

In order for there to be evolution there must be genetic variation

Major ways genetic variation is introduced into populations

(1) Mutation (UV, random error)

(2) Genetic recombination (meiosis) – including ‘crossing-over’

(3) Immigration (into population)


But plants ‘do’ two additional ‘tricks’ that enhance genetic variation

(4) Polyploidy – an organism that has more than one complete set of the normal chromosome compliment

- most animals are diploids, many plants are polyploids

- occurs through processes such as chromosome duplication

(5) Hybridization – crossing of closely related taxa (usually between species within a genus)


Multicellularity and plant evolution

Multicellularity evolved more than once!

-for plants, prokaryotic unicellular algae → multicellular algae → embryophytes

Multicellularity has several interesting advantages

Cells can be specialized – division of labor (requires communication and transport)

Organism can increase surface area for environmental exchange (access to more resources)

Organism can increase in size – better buffering of environmental extremes – live longer – access to ‘additional’ resources


When is an organism multicellular?

  • When neighboring cells adhere, interact, and physiologically ‘communicate’
  • Contact is achieved in four ways:
  • (1) Tight junctions – proteins in membranes that bond neighboring cells
  • (2) Desmosomes – intracellular filaments that adjoin cells (often creating a space for material movement)
  • (3) Gap junctions – pores surrounded by transmembrane proteins (direct material movement between cells
  • (4) Plasmodesmata – open channels within the plant cell wall that connect cells directly

Multicellular plant

-Single living protoplast of adjoining cells.

-Cell membranes (which line plasmodesmata)

are continuous from one cell to the next

-Water and small molecules may pass with relative ease (essentially through the whole plant).

Material flow may be modified by altering number and location of plasmodesmata

what is a plant
What is a plant?
  • Plants are photosynthetic eukaryotes
    • including algae
  • A more derived group of plants is called the embryophytes
    • produce an embryo that is protected by tissues of the parent plant
  • Plants appear monophyletic, forming a single branch of the evolutionary tree (so says your book)
    • Please, please remember these endosymbiotic events and the discussion you have had on a ‘tree-like’ phylogeny versus a ‘web-like’ phylogeny
diversity of embrophytes
Diversity of Embrophytes
  • Embryophytes fall out into 10 phyla
    • Seven include members possessing well-developed vascular systems are called the tracheophytes.
    • Three phyla (liverworts, hornworts, and mosses – derived in that order) lack tracheids and are collectively referred to as the nontracheophytes.
    • Table 29.1 in your book lists the groups and their defining characteristics – good source for important knowledge (hint)
unique characteristics of plants
Unique characteristics of plants
  • Alternation of generations is a universal feature of the life cycles of plants.
    • Life cycle includes both multicellular diploid and multicellular haploid individuals.
    • Gametes are produced by mitosis, while meiosis produces spores that develop into multicellular haploid individuals.
The multicellular, diploid plant is called the sporophyte.
  • The sporangia (on the sporophyte) produce haploid, unicellular spores by meiosis.
  • The multicellular, haploid plant formed by mitosis of a spore is called the gametophyte.
  • The gametophyte produces haploid gametes.
  • The fusion of two gametes results in the formation of a diploid cell, the zygote, and the cycle repeats.

Figure 29.2 in the book

Sporophyte generation – from the zygote through the adult, multicellular, diploid plant.

Gametophyte generation - from the spore through the adult, multicellular, haploid plant to the gamete.


Charophytes (a group of green algae) appear to be the closest living relative of Embryophytes

These organisms now occupy the margins of ponds or marshes (meaning that the ‘jump’ to a terrestrial environment was in close proximity)

the conquest of the land
The Conquest of the Land
  • Embryophytes invaded the terrestrial environment approximately 400–500 mya.
  • Invading the land is more like ‘invading the air’, rather than soil.
    • Water not as available and quickly lost from plant in the terrestrial environment
    • Gravity becomes very important
    • Dispersal of gametes is much more difficult outside of an aquatic environment
Some adaptations to life on land:
    • Cuticle-a waxy covering that prevents drying
    • Gametangia-enclosure for gametes to prevent drying
    • Embryos-protected, young sporophytes
    • Pigments-protection against mutagenic UV radiation
    • Spore wall thickening-prevent drying and resist decay
    • Mychorhizzae-mutualisticassociation with a fungus to promotes nutrient uptake from the soil***
    • Stomata–controllable ‘pore’ in tissue that regulate water loss and CO2 uptake
    • Aerenchyma–invaginations in tissue that create moist internal surface area for gas exchange
the conquest of the land17
The Conquest of the Land
  • Evolution of specialized water conducting cells - tracheids allowed for advancement in the terrestrial environment
    • We distinguish between embryophytes that have (tracheophytes) and do not (non-tracheophytes) have tracheids
  • The first plants either lacked vascular tissue or, like some mosses, had very simple conducting tissue that developed from dead cells.
Water and nutrient acquisition by non-tracheophytes (recall, they do not have a vascular system):
    • Many grow in dense masses through which water can move by capillary action.
    • They have leaflike structures that catch and hold water that splashes onto them.
    • They are small enough that minerals can be distributed evenly by diffusion.
nontracheophytes liverworts hornworts and mosses
Nontracheophytes:Liverworts, Hornworts, and Mosses
  • Grow in dense mats in moist habitats, typically they are small in size.
  • Layers of maternal tissue prevent loss of water from the embryo.
  • Have a thin cuticle, though it is not highly effective in retarding water loss.
  • Are widespread across six continents and exist locally on the coast of Antarctica.
Nontracheophytes – visible green structure is the gametophyte.
  • Sporophyte produces unicellular, haploid spores through meiosis within sporangium or capsules.
  • Spores germinate and give rise to a multicellular, haploid gametophyte whose cells contain chloroplasts.
Gametangia are where gametes are formed.
  • The archegonium is a multicellular female sex organ with a long neck and a base that contains a single egg (a above)
  • The antheridium produces sperm (b above)
  • The sporophyte produces a sporangium, or capsule, within which meiotic divisions produce spores and thus the next gametophyte generation.
Liverworts - most ancient surviving plant clade.

Rhizoids absorb water with filaments found on the lower surfaces gametophytes.

Several genera have both sexual and asexual reproduction

Asexual reproduction - by simple fragmentation of the gametophyte.

The hornworts, phylum Anthocerophyta, mosses and tracheophytes, all have unique adaptations to life on land

These groups all possess stomata that allow the uptake of CO2 and the release of O2, but they can be closed to prevent excessive water loss (in some groups).

Two characteristics distinguish hornworts from liverworts and mosses:
    • Cells of hornworts contain a single large, platelike chloroplast, whereas liverworts and mosses contain numerous small, lens-shaped chloroplasts.
  • Cyanobacteria often populate internal, mucilage-filled cavities within hornworts.
    • These cyanobacteria are able to fix atmospheric nitrogen gas into a form that can be used by the hornwort.
The phylum Bryophyta (mosses) are probably sister to the tracheophytes.

Hydroid cells, in many mosses, are a likely progenitor of the water-conducting cells of the tracheophytes.

When hydroid cells die, they leave a tiny channel through which water can flow.

the tracheophytes
The Tracheophytes
  • The sporophyte generation of a now-extinct organism produced a new cell type, called the tracheid.
    • Allowed for the radiation of a novel life form
  • The tracheid is the principal water-conducting element in the xylem in all tracheophytes except the angiosperms.
The tracheophytes have well-developed vasculature, consisting of:
    • Phloem conducts photosynthetic products from production sites to sites where they are used or stored (think ‘source-sink’).
    • Xylem conducts water and minerals from the soil to the aerial parts of the plants, or from one place in the soil to another.
      • Xylem can provide support as it is stiffened by lignin.
The evolution of tracheids had two important consequences:
    • provided a pathway for long-distance transport.
    • provided rigid structural support.
  • Tracheids set the stage for invasion of land by plants.
  • Tracheophytes also feature a branching, independent sporophyte.
  • We break tracheophytes down into at least seven different groups (see fig. 29.10) – with the biggest distinction of those that produce seeds, and those that do not produce seeds
the tracheophytes31
The Tracheophytes
  • Recall plants invaded land about 400-500 million years ago.
  • During the Devonian period club mosses (lycopods), horsetails, and ferns made the environment more hospitable to animals.
  • Trees dominated during the Carboniferous period, resulting in forest that eventually become coal deposits.
  • At the end of the Permian period, the 200-million-year reign of the lycopod–fern forests came to an end as they were replaced by forests of seed plants.
introducing the tracheophytes
Introducing the Tracheophytes
  • The first tracheophytes were in the now-extinct phylum Rhyniophyta.
    • They had the structural features found in all other tracheophyte phyla
  • Club mosses (Lycophyta), appeared in the Silurian period.
  • Ferns, horsetails, and whisk ferns (Pteridophyta) appeared in the Devonian.
  • These groups (Lycophyta and Pteridophyta) had true roots, true leaves, and a differentiation between two types of spores.
the tracheophytes33
The Tracheophytes

Roots had their origins as branches, either as rhizomes or aboveground portion of stems.

Early roots were simple structures that penetrated soil, branching and anchoring the plant (absorbing water and minerals?)

Belowground and aboveground environments are quite different.

the tracheophytes34
The Tracheophytes
  • A leaf is a flattened photosynthetic structure emerging laterally from a main axis or stem and possessing true vascular tissue.
  • There are two leaf types: microphylls and megaphylls.
  • The microphyll has a single vascular strand that has departed from the stem without disturbing the stem’s vascular structure. The club mosses have microphylls.
  • Microphylls may have evolved from sterile sporangia.
the tracheophytes36
The Tracheophytes

The megaphyll is larger, and more complex found in ferns and seed plants.

May have arose from flattening of stems and development of overtopping (one branch differentiates from and extends beyond rest).

introducing the tracheophytes37
Introducing the Tracheophytes
  • Plants that bear a single type of spore are said to be homosporous.
  • The most ancient tracheophytes were all homosporous.
  • Both the gametophyte and the sporophyte are independent and usually photosynthetic.
  • A single type of gametophyte bears both female and male reproductive organs.
introducing the tracheophytes38
Introducing the Tracheophytes
  • Plants with two distinct types of spores evolved later, and are said to be heterosporous.
  • In heterosporous plants, the megaspore develops into a larger, specifically female gametophyte (megagametophyte).
  • The microspore develops into the smaller, male gametophyte (microgametophyte).
  • Heterospory evolved independently and repeatedly, suggesting that it affords selective advantages.
the surviving nonseed tracheophytes
The Surviving Nonseed Tracheophytes
  • The club mosses (phylum Lycophyta) have microphylls, exhibit apical growth, and have roots that branch dichotomously.
  • Sporangia in many club mosses are contained within conelike structures called strobili, clusters of spore-bearing leaves inserted between a specialized leaf and the stem.
  • There are both homosporous and heterosporous species.
  • The Lycophyta and the Pteridophyta were the dominant phyla during the Carboniferous period.
the surviving nonseed tracheophytes42
The Surviving Nonseed Tracheophytes
  • The horsetails, whisk ferns, and ferns form a clade, the phylum Pteridophyta.
  • The horsetails (all are genus Equisetum) have true roots that branch irregularly, and sporangia on short stalks called sporangiophores.
  • The leaves are reduced megaphylls and grow in whorls.
  • Stem growth is from the base of the stem segments.
the surviving nonseed tracheophytes44
The Surviving Nonseed Tracheophytes
  • The whisk ferns are two genera of rootless, spore-bearing plants, Psilotum and Tmesipteris.
  • Psilotum has only minute scales instead of true leaves.
  • Although whisk ferns resemble the most ancient tracheophytes, they are now considered to be highly specialized plants that evolved fairly recently.
the surviving nonseed tracheophytes46
The Surviving Nonseed Tracheophytes
  • The sporophytes of the ferns typically have true roots, stems, and leaves.
  • The ferns first appeared during the Devonian.
  • About 97% of fern species belong to one clade, the leptosporangiate ferns. These ferns have sporangia with walls only one cell thick, borne on a stalk.
  • Ferns are characterized by fronds, large leaves with complex vasculature.
  • Sporangia are found on the undersurfaces of the fronds, clustered in groups called sori.
the surviving nonseed tracheophytes49
The Surviving Nonseed Tracheophytes
  • The sporophyte generation dominates the fern life cycle.
  • Spores germinates and form a gametophyte, bearing antheridia or archegonia (or both).
  • The antheridia release sperm that swim to a nearby archegonium and fertilize an egg.
  • The sperm are guided by chemical attractants released from the archegonia.
  • The resulting diploid embryo forms roots and fronds, and grows into the familiar sporophyte life stage.