Plant Evolution

What are plants?

MulticellularEukaryoticPhotosynthetic autotrophsCell Walls made of cellulose

Plant Ancestors

Plants are the descendants of protistsPlants 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) & rootsMuticellular plant embryosAlternation of GenerationsWalled spores produced in sporangiaMulticellular 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 transpirationXylem tissue for transporting water from roots to leavesPhloem tissue for transporting sugars from leaves to stems and roots

Plant Groups

Bryophytes (mosses)—first descendants of Charophyceans & first land plantsPteridophytes (ferns)—descendants of bryophytes; first plants with vascular tissueGymnosperms (“naked seed”)—descendant of pteridophytes; first plants with seedAngiosperms (enclosed seed)—descendants of gymnosperms; first plants with flowers

Bryophytes

Bryophytes

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

The Origin of Vascular Plants

Contain Xylem and PhloemHave a dominant sporophyte generationPteridophytes (seedless vascular plants) are oldest vascular plants

The Origin of Seed Plants

Reproductive Adaptations:Continued reduction of the gametophyte The advent of the seedThe evolution of pollen

Spores Vs. Seeds

Single Celled

Haploid

Multicellular & More ComplexDiploidSporophyte EmbryoFood SupplyProtective Coat

Formation of Seed

Seed develops from the megasporangiumThe megasporangium is encased with layers of integuments for protectionThe megaspore becomes one or more eggsThe integument, megasporangium, and megaspore are the ovule

Pollen

Microspores develop into pollen grainsPollen grains mature into the male gametophyteIn mosses & ferns the sperm is flagellated and must swim to the eggIn 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 developGymnosperm ovules & seed develop on surfaces of specialized leaves called sporophyllsGymnosperms are much older than angiosperms

Angiosperms

Produce flowers & fruitSeed is encased in the fruitAll belong to phylum AnthophytaAngiosperm xylem has fiber cells which specialize for supportVessel elements in xylem are more efficient in transporting water

The Flower—The Angiosperm Invention

The Fruit—To Aid Seed Dispersal

Fruit = Mature ovaryFruits protect seed & aid in dispersalCan act as kites or propellers; can have burrs that cling; or can be delicious to eat