Plants Without Seeds: From Water to Land

Plants without Seeds: From Water to Land

Key Concepts

• 27.1 Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes
• 27.2 Key Adaptations Permitted Plants to Colonize Land
• 27.3 Vascular Tissues Led to Rapid Diversification of Land Plants

Investigating Life: A Toxic Spill of Ancient Fossil Algae

• Petroleum is derived naturally from green algae.
• Question: Can humans use green algae to produce oil commercially?
• Petroleum is a fossil fuel derived from ancient phytoplankton, including green algae and other microbial groups.
• Phytoplankton made hydrocarbons for energy storage.

27.1 Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes

• Primary endosymbiosis is a shared derived trait (synapomorphy) of the Plantae.
• "Plants" often refers only to land plants, but many clades are aquatic.
• Algae refers to aquatic photosynthetic eukaryotes, but these groups are not all closely related; it is a convenience term.
• The ancestor of all Plantae may have been similar to glaucophytes, the sister group to all other Plantae.
• Chloroplast membranes have some peptidoglycan, the same as in cyanobacteria.
• Peptidoglycan has been lost from all other photosynthetic eukaryotes.

Glaucophytes

• Sister group to all other Plantae.
• Chloroplast membranes contain peptidoglycan.

Red Algae

• Most are multicellular.
• Red color results from the accessory photosynthetic pigment phycoerythrin.
• Chloroplasts also have chlorophyll a and other accessory pigments.
• Most are marine; a few live in freshwater; most grow attached to a substrate by a holdfast.
• The ratio of phycoerythrin to chlorophyll a depends on light intensity:
  • In deep water (dim light) more phycoerythrin results in red color.
  • In shallow water, the red alga may appear bright green.

Green Plants

• Other algal groups in the Plantae have chlorophylls a and b and store photosynthetic products as starch in chloroplasts.
• All groups that share these two traits are called green plants.

Chlorophytes

• Largest group of green algae; most are aquatic.
• Some are unicellular, others multicellular; great diversity of shapes and body forms.
• Volvox, a freshwater unicellular alga, forms large colonies with some cells specialized for reproduction.
• Multicellular chlorophyte species include filamentous forms.
• Others, like Ulva, grow into thin, membranous sheets a few centimeters across.
• Biologists are exploring several chlorophytes as a source of biofuels.
• One limitation is providing an appropriate growth medium.
• Appropriate nutrients are present in municipal wastewater; removal of these nutrients by growing algae would help clean the water supply.

Investigating Life: Can Chlorella Algae Be Grown in Municipal Sewage Wastewater for Biofuel Production?

• Hypothesis: Chlorella minutissima algae can be grown successfully in municipal wastewater, either with or without dilution from traditional growth medium.
• Method:
  1. Grow replicate cultures of a standard inoculate of Chlorella minutissima algae in growth medium, as well as in 25%, 50%, 75%, and 100% municipal wastewater.
  2. Control temperature and light conditions for all cultures.
  3. Measure growth of the algae after 15 days by measuring the concentration of chlorophyll a present in each culture.
• Conclusion: Chlorella minutissima algae can be grown effectively in municipal wastewater, and growth may even exceed production in standard growth medium.

Streptophytes

• All green algae other than chlorophytes, plus land plants.
• The multicellular coleochaetophytes and stoneworts are the closest relatives of land plants.
• Both groups retain eggs in the parental organism, as land plants do, and the cells are connected by plasmodesmata.
• Stoneworts are thought to be the sister group of land plants.
• They have a branching and apical growth form, as in most land plants.
• The close relationship of stoneworts and coleochaetophytes to land plants has been confirmed by gene sequencing.
• A synapomorphy of land plants is an embryo protected by tissues of the parent plant.
• They are also called embryophytes.
• There are ten major clades of land plants.

Vascular and Non-Vascular Plants

• Vascular plants: Vascular systems transport materials throughout the plant body (7 clades).
  • Also called tracheophytes—they have fluid-conducting cells called tracheids.
• Non-vascular plants: The other 3 clades that lack tracheids.

Classification of Land Plants

27.2 Key Adaptations Permitted Plants to Colonize Land

• Key innovations of Plantae facilitated their transition to land.
• Alternation of generations is a universal trait of the Plantae.
• Land plants first appeared between 400 and 500 mya.
• On land, plants needed:
  • Water transport mechanisms
  • Physical support
  • Mechanism to distribute gametes and progeny
• Adaptations of land plants:
  • Cuticle—waxy coating that retards water loss
  • Stomata—openings in stems and leaves; regulate gas exchange
  • Gametangia—organs enclosing gametes
  • Embryos in a protective structure
  • Pigments that protect against UV radiation
  • Thick spore walls that prevent desiccation and decay
  • Mutually beneficial associations with fungi (mycorrhizae) that promote nutrient uptake from the soil
• Ancient plants colonized land and contributed to soil formation:
  • Acids secreted by plants help break down rock.
  • Organic material from dead plants contributes to soil structure.

Alternation of Generations

• All land plants have alternation of generations:
  • Multicellular diploid and haploid stages alternate.
  • Gametes are produced by mitosis.
  • Meiosis produces spores that develop into haploid organisms.
• The multicellular diploid plant is the sporophyte ("spore plant").
• Cells in sporangia undergo meiosis to produce haploid, unicellular spores.
• Spores develop into a multicellular haploid plant—the gametophyte (“gamete plant”).
• Gametophytes produce haploid gametes by mitosis.
• Fusion of gametes (fertilization) results in a diploid zygote.
• The zygote develops into the multicellular sporophyte.
• There is a trend toward reduction of the gametophyte generation in plant evolution.
• In nonvascular plants, the gametophyte is larger, longer-lived, and more self-sufficient than the sporophyte.
• In plants that appeared later, this is reversed.

Nonvascular Plants

• Liverworts, mosses, and hornworts.
• Thought to be similar to earliest land plants.
• With no vascular transport system, they cannot grow very tall.
• They have a thin cuticle or no cuticle, and most live in moist habitats.
• Nonvascular plants lack true leaves, stems, and roots but have analogous structures.
• Water transport is via diffusion and capillary action.
• Some nonvascular plants can live on bare rock and other marginal habitats because of mutualistic associations with fungi.
• The earliest such association dates to 460 mya.
• It facilitated absorption of water and minerals from the first soils.
• The gametophyte is the familiar, photosynthetic form.
• The sporophyte may or may not be photosynthetic but is always nutritionally dependent on the gametophyte and is permanently attached.
• The haploid gametophyte produces gametes in archegonia and antheridia.
• Sperm must swim or be splashed by raindrops to an archegonium to fertilize an egg.
• Zygote develops into the multicellular, diploid sporophyte.
  • Liverworts:
    • 9,000 species.
    • Sister clade of remaining land plants.
    • Some have leafy gametophytes; some are thalloid.
    • Sporophytes are only a few mm high.
    • A stalk raises the sporangium above ground level to allow spores to be dispersed.
    • Liverworts also reproduce asexually:
      • Fragmentation of the gametophyte
      • Gemmae—clumps of cells in gemmae cups. Gemmae are dispersed by raindrops.
  • Mosses:
    • 15,000 species.
    • Sister clade of the vascular plants plus the hornworts.
    • Mosses have stomata, important in water and gas exchange.
    • Stomata are a synapomorphy of mosses and other land plants, except liverworts.
    • Some mosses have specialized cells called hydroids, which die and form channels through which water can move. Hydroids are functionally similar to tracheids.
    • Sphagnum moss grows in cool, swampy places.
    • The upper layers of moss compress lower layers that are beginning to decompose, forming peat, which can be used as a fuel.
    • Long ago, continued compression of peat led to the formation of coal.
  • Hornworts:
    • 100 species.
    • Gametophytes are flat plates of cells; sporophytes look like small horns.
    • Hornwort cells have a single, large chloroplast.
    • The sporophyte has no stalk but has a basal region capable of indefinite cell division.
    • Hornworts have symbiotic, nitrogen-fixing cyanobacteria in specialized internal cavities.

27.3 Vascular Tissues Led to Rapid Diversification of Land Plants

Vascular plants

• Key synapomorphy of the vascular plants is a vascular system.
• The ability to transport water and food throughout their bodies allowed them to spread to new environments and diversify rapidly.
• The vascular system:
  • Xylem conducts water and minerals from soil up to the rest of the plant.
    • Some xylem cell walls have lignin, which provides support.
  • Phloem conducts products of photosynthesis throughout the plant.
• In the mid-Silurian (430 mya), tracheid cells that conduct water evolved in sporophytes.
• This was critical for the invasion of land.
• Transport of water and minerals and rigid structural support allow plants to grow tall and compete for light and aid in spore dispersal.
• Vascular plants also developed a branching, independent sporophyte.
• A branching sporophyte can produce more spores and develop in complex ways.
• The sporophyte is the familiar, photosynthetic form; it is nutritionally independent from the gametophyte.
• Herbivores were initially absent on land, which helped make the first vascular plants successful.
• These plants then made the terrestrial environment more hospitable to animals, which moved onto land only after vascular plants became established.
• During the Permian, the continents came together to form Pangaea.
• Extensive glaciation occurred in the late Permian.
• Lycophyte–fern forests were replaced by gymnosperms.

Earliest Vascular Plants (now extinct):

• Rhyniophytes (Silurian) had a simple vascular system and dichotomous branching but lacked leaves and roots.
  • They were anchored by rhizomes (horizontal portions of stem) and rhizoids (water-absorbing filaments).

Lycophytes:

• Club mosses, spike mosses, and quillworts; 1,200 species.
• Sister group to the other vascular plant groups.
• Stems and true roots have dichotomous branching.
• Simple leaflike structures (microphylls) are arranged spirally on the stem.
• Some club mosses have sporangia arranged in clusters called strobili.
• In others, sporangia are on upper surfaces of specialized microphylls.

Monilophytes:

• Ferns and horsetails—sister clade to the seed plants.
• Horsetails: 15 species in the genus Equisetum.
  • True roots; sporangia are on short stalks called sporangiophores.
  • Reduced leaves grow in whorls.
  • Silica in cell walls—“scouring rushes.”
• Ferns: 12,000 species
  • Most are terrestrial, a few are aquatic.
  • Large leaves with branching vascular strands, some fern leaves climb and may grow up to 30 m.
  • Fern sporophytes can be large and very long-lived.
  • Most live in moist habitats—water is required to transport male gametes.
  • Sporangia are borne on a stalk in clusters called sori on the underside of the leaves.

Evolution of Leaves

• New features that evolved in lycophytes and monilophytes:
  • Roots probably originated from branches on a rhizome or stem.
  • Microphylls may have developed from sterile sporangia. They are small and have only a single vascular strand.
• Monilophytes and seed plants form a clade called euphyllophytes.
• A synapomorphy is overtopping growth: new branches grow beyond the others (an advantage in the competition for light).
• Overtopping allowed megaphylls (complex leaves) to evolve.
• They may have arisen from flattening of branching stems.
• Flat plates of photosynthetic tissue developed between branches, which increased photosynthetic surface area.
• Small megaphylls appeared in the Devonian.
• Large megaphylls did not appear until the Carboniferous.
• High CO_2 concentrations in the Devonian reduced selection for stomata.
• Fewer stomata were needed to take up CO_2, and so megaphylls remained small.
• Stomata also allow heat to be lost by the evaporation of water.
• If megaphylls had grown large during this time, with few stomata, overheating would have been lethal.
• Recent research supports this hypothesis. Larger megaphylls evolved only as CO_2 concentrations dropped.

Homospory and Heterospory

• Heterosporous plants produce 2 spore types:
  • Megaspores develop into female gametophytes—megagametophytes, which produce only eggs.
  • Microspores develop into male gametophytes—microgametophytes, which produce only sperm.
• Heterospory evolved independently in several groups of vascular plants, suggesting there are selective advantages.
• Subsequent evolution featured ever greater specialization of the heterosporous condition.

Investigating Life: A Toxic Spill of Ancient Fossil Algae

• Biofuels can be produced from green algae, but this is not yet commercially viable.
• Biofuels release CO2 when burned, but algae take up CO2 during photosynthesis, so biofuels should result in less accumulation of CO_2 in the atmosphere.
• Many new methods for growing algae are being developed.
• The main limitations include establishing efficient growing facilities, water needs, harvest and refining methods, and costs of fertilizers, labor, etc.