Classification

What Are the Major Groups of Plants? Two major groups of land plants arose from ancient green algal ancestors The nonvascular plants (called bryophytes) require a moist environment to reproduce, and therefore they straddle the boundary between aquatic and terrestrial life The vascular plants (called tracheophytes) have been able to colonize dry habitats

Flowers and fruits Seeds and pollen True vascular tissue and lignin Nonvascular plantsVascular plants seed plants Liverworts Ancestral green alga MossesFilocinophytaconiferophytaAngiospermophyta The Evolutionary Tree of Some Major Plant Groups Fig bryophyta

Major Plant Groups

Nonvascular plants Mosses are the most diverse, and some have a waterproof covering that retains moisture, permitting them to invade drier areas, such as deserts, bare rock, and northern and southern climes where humidity is low and water scarce Mosses of the genus Sphagnum are common in northern regions of the world Accumulations of dead mosses form peat, which is used as a fuel

Nonvascular Plants Fig. 21-5

Vascular plants have conducting cells that also provide support distinguished by specialized groups of tube- shaped conducting cells cells are impregnated with the stiffening agent lignin diploid sporophyte generation is dominant Vascular plants can be divided into two groups: seedless vascular plants and the seed plants

seedless vascular plants include ferns have swimming sperm and require water for reproduction do not produce seeds, but propagate by spores

Ferns are broad-leaved and more diverse With 12,000 species, ferns are the largest and most diverse group of seedless vascular plants Their well-developed, broad leaves emerge from coiled fiddleheads Ferns are the only seedless vascular plants that have broad leaves Reproduction in ferns requires water

The seed plants aided by two important adaptations: pollen and seeds Pollen grains are tiny male gametophytes that carry sperm-producing cells They are dispersed by wind or pollinators They eliminate the need for sperm to swim to the egg Seeds have three components: an embryonic sporophyte plant, a food supply for the embryo, and a protective outer coat

coniferospermophyta nonflowering seed plants evolved earlier than the flowering plants first fully terrestrial plants to evolve, most of these early gymnosperms are now extinct

Conifer seeds develop in cones A conifer tree is a diploid sporophyte and develops male and female cones Male cones are relatively small produce pollen pollen grain germinates and forms a pollen tube if it lands near a female gametophyte Female cones consist of numerous woody scales arranged spirally around a central axis Two ovules (immature seeds) are located at the base of each scale Cells within each ovule undergo meiosis to produce haploid female gametophytes The female gametophytes produce egg cells

Angiospermophyta flowering seed plants produce flowers and fruits They are the most diverse and widespread of all plants, with more than 230,000 species broad range in size The smallest is duckweed (3 mm in diameter) The largest is the eucalyptus tree (100 meters in height)

Angiosperms Fig

Angiosperms Three major adaptations have contributed to the dominance of angiosperms Flowers Fruits Broad leaves

Flowers attract pollinators Flowers are reproductive structures in which both male and female gametophytes are formed Flowers are believed to have evolved when gymnosperm ancestors formed an association with animals Animals benefited by eating some of the protein-rich pollen Plants benefited by using animals as pollinators Flowers develop on the dominant sporophyte plant Male gametophytes (pollen) develop inside anthers The female gametophyte develops from an ovule inside the ovary

Fruits encourage seed dispersal Fruits are mature ovaries that contain developing seeds Various fruit adaptations help disperse seeds Edible fruits entice animals to eat them (seeds pass through the digestive tract unharmed) Burrs cling to animal fur Winged fruits are carried through the air

Broad leaves broad leaves of angiosperms collect more sunlight for photosynthesis than the narrow leaves of gymnosperms Temperate angiosperms drop their leaves to conserve water when it is in short supply (fall, winter) Tropical and subtropical angiosperms are evergreen broad, tender leaves are more appealing to herbivores than the tough, waxy needles of conifers Physical defenses (thorns, spines, resins) Chemical defenses (plant tissue is poisonous or distasteful) Many defensive compounds have been exploited by humans for medicinal and culinary uses Medicines (aspirin, codeine) Stimulants (nicotine, caffeine) Spicy flavors (mustard, peppermint)