Plant Evolution

What are plants? Multicellular Eukaryotic Photosynthetic autotrophs Cell Walls made of cellulose

Plant Ancestors Plants are the descendants of protists Plants are most closely related to charophycean algae, a type of green algae

Unique Plant Characteristics (not shared with Carophyceans) Apical Meristems—localized regions of cell division at the tips of shoots (stems) & roots Muticellular plant embryos Alternation of Generations Walled spores produced in sporangia Multicellular Gametangia (archegonia & antheridia)

Alternation of Generations—The Plant Life cycle

Life on Land and Conserving Water Leaves and other plant parts are covered with water repellant cuticle (made of lipids!) Stomata support photosynthesis and regulate transpiration Xylem tissue for transporting water from roots to leaves Phloem tissue for transporting sugars from leaves to stems and roots

Plant Groups Bryophytes (mosses)—first descendants of Charophyceans & first land plants Pteridophytes (ferns)—descendants of bryophytes; first plants with vascular tissue Gymnosperms (“naked seed”)—descendant of pteridophytes; first plants with seed Angiosperms (enclosed seed)—descendants of gymnosperms; first plants with flowers

Bryophytes

Do not have vascular tissue Tend to be short and must live close to water Sperm have to swim to the egg; requires a water medium

The Origin of Vascular Plants Contain Xylem and Phloem Have a dominant sporophyte generation Pteridophytes (seedless vascular plants) are oldest vascular plants

The Origin of Seed Plants Reproductive Adaptations: Continued reduction of the gametophyte The advent of the seed The evolution of pollen

Spores Vs. Seeds Single Celled Haploid Multicellular & More Complex Diploid Sporophyte Embryo Food Supply Protective Coat

Formation of Seed Seed develops from the megasporangium The megasporangium is encased with layers of integuments for protection The megaspore becomes one or more eggs The integument, megasporangium, and megaspore are the ovule

Pollen Microspores develop into pollen grains Pollen grains mature into the male gametophyte In mosses & ferns the sperm is flagellated and must swim to the egg In gymnosperms & angiosperms, the sperm travels via the wind and then through a pollen tube to the egg

Gymnosperm & Angiosperm differences Gymnosperms (naked seed) lack the enclosed chambers (ovaries) in which angiosperm ovules & seeds develop Gymnosperm ovules & seed develop on surfaces of specialized leaves called sporophylls Gymnosperms are much older than angiosperms

Angiosperms Produce flowers & fruit Seed is encased in the fruit All belong to phylum Anthophyta Angiosperm xylem has fiber cells which specialize for support Vessel elements in xylem are more efficient in transporting water

The Flower—The Angiosperm Invention

The Fruit—To Aid Seed Dispersal Fruit = Mature ovary Fruits protect seed & aid in dispersal Can act as kites or propellers; can have burrs that cling; or can be delicious to eat