Plant Evolution & The Conquest of Land

Streptophyte green algae

Green algal relatives most closely related to the ancestors of land plants.

Cytokinesis (plant ancestry)

Division of the cytoplasm during cell division; distinctive cytokinesis is shared by streptophyte algae and land plants.

Plasmodesmata

Channels through cell walls that connect the cytoplasm of adjacent plant cells.

Apical meristems

Regions of actively dividing cells at the growing tips of shoots and roots.

Alternation of generations

A life cycle that alternates between multicellular haploid gametophyte and diploid sporophyte stages.

Sporic life cycle

A life cycle featuring alternation between a spore-producing sporophyte and a gamete-producing gametophyte.

Sporophyte

The diploid (2n) generation that produces haploid spores by meiosis.

Gametophyte

The haploid (n) generation that produces haploid gametes.

Gametangia

Structures in which gametes are produced; in bryophytes, surrounding cells form a protective jacket.

Antheridia

Spherical or elongated gametangia that produce sperm.

Archegonia

Flask-shaped gametangia that each enclose an egg cell.

Matrotrophy

Nourishment and protection of the developing embryo by maternal gametophyte tissues.

Embryophytes

Land plants characterized by matrotrophic embryos.

Sporangium

An enclosure in which meiosis occurs and haploid spores are produced.

Sporopollenin

A tough material in spore walls that helps prevent damage during transport through air.

Bryophytes

Nonvascular land plants, including liverworts, hornworts, and mosses.

Liverworts (Hepatophyta)

A phylum of bryophytes; one of the simplest land-plant groups.

Hornworts (Anthoceratophyta)

A phylum of bryophytes; one of the simplest land-plant groups.

Mosses (Bryophyta)

A phylum of bryophytes; the gametophyte is the conspicuous generation.

Vascular plants

Plants with internal water- and nutrient-conducting tissues that also provide structural support.

Vascular tissue

Plant tissue that conducts water, minerals, and organic materials through the plant.

Tracheophytes

Vascular plants, including lycophytes, pteridophytes, and seed plants.

Tracheids

Water-conducting xylem cells with thick, lignified walls that also provide structural support.

Xylem

Vascular tissue that conducts water and minerals and provides structural support.

Phloem

Vascular tissue that transports sugars and other organic compounds through a plant.

Lignin

A tough polymer in cell walls that strengthens vascular plant tissues.

Stems

Plant organs that bear leaves and branches and support reproductive structures.

Roots

Plant organs specialized for uptake of water and minerals from soil.

Leaves

Plant organs specialized for photosynthesis.

Waxy cuticle

A cutin- and wax-containing surface layer that reduces water loss and helps protect plant surfaces.

Stomata

Adjustable pores in plant surfaces that regulate gas exchange and water loss.

Lycophytes (Lycopodiophyta)

Seedless vascular plants with small leaves called microphylls; many bear sporangia in strobili.

Rhizome

A horizontal stem, often underground, from which roots and shoots can grow.

Strobilus (plural strobili)

A cone-like cluster of sporangia, often at the tip of a shoot.

Apical sporangia

Sporangia located at the tips of lycophyte shoots, often grouped into strobili.

Pteridophytes (Pteridophyta)

Seedless vascular plants that include horsetails, whisk ferns, and true ferns.

Horsetails

Pteridophytes with hollow, silica-rich stems, whorled branches, and cone-like strobili.

Whisk ferns

Leafless pteridophytes that photosynthesize through branched green stems.

Fronds

The divided leaves of true ferns.

Fiddleheads

Young, developing fern fronds that are often coiled.

Sori

Clusters of sporangia, commonly found on the undersides of fern fronds.

Lycophylls (microphylls)

Small leaves with a single, unbranched vein, characteristic of lycophytes.

Euphylls (megaphylls)

Leaves with branched veins that can supply relatively large leaf areas.

Heterospory

Production of two distinct types of spores in two types of sporangia.

Microspores

Spores produced in microsporangia that develop into male gametophytes.

Megaspores

Spores produced in megasporangia that develop into female gametophytes.

Pollen

A tiny male gametophyte enclosed by a protective wall containing sporopollenin.

Ovule

A structure containing a megasporangium and integuments; develops into a seed after fertilization.

Integuments

Protective layers surrounding the ovule that develop into the seed coat.

Wood

Secondary xylem whose cell walls are strengthened by lignin.

Vascular cambium

A meristematic tissue that produces secondary xylem (wood) and secondary phloem (inner bark).

Progymnosperms

Ancient seedless plants in which wood first appeared; early tree forms.

Eustele

A vascular arrangement in which vascular tissue is organized in a ring, allowing vascular cambium development.

Carboniferous period

A geological period associated with extensive forests of ancient vascular plants and major coal deposits.

Coal

A fossil fuel formed largely from accumulated and transformed plant material.

CO2 sequestration

The capture and storage of carbon dioxide, including through plant growth and burial of plant material.

Sphagnum

A moss important in peat formation and carbon storage.

K/T event

The Cretaceous–Paleogene mass extinction event, after which angiosperms became especially prominent.