Chapter 29 Evolution of Land Plants

Overview Plants can be described as multicellular, eukaryotic, photosynthetic autotrophs Four main groups:  Bryophytes (seedless, non-vascular)  Pteridophytes (seedless, vascular)  Gymnosperms (plants with “naked” seeds)  Angiosperms (flowering plants)

How did they get on land? Land plants evolved from green algae Many characteristics of land plants also appear in a variety of algal clades However, land plants share four key traits only with charophyceans (Green Algae):  Rose-shaped complexes for cellulose synthesis  Peroxisome enzymes  Structure of flagellated sperm  Formation of a phragmoplast

How did they get on land? In charophyceans a layer of a durable polymer called sporopollenin  Prevents exposed zygotes from desiccating Accumulation of traits facilitating survival on land may have opened the way to its colonization by plants

How did they get on land? Water conservation  Cuticle A waxy layer made of polymers Seals the cell(s)  Stomata Openings on the undersurface of the leaf Allow the passage of CO 2 and H 2 O

How did they get on land? Five key traits appear in nearly all land plants but are absent in the charophyceans:  Apical meristems  Alternation of generations  Walled spores produced in sporangia  Multicellular gametangia  Multicellular dependent embryos

All plants have a life cycle that consists of two stages (alternation of generations)  This is an alternation between two MULTICELLULAR stages Gametophyte stage (haploid) Sporophyte stage (diploid)

Mitosis Alternation of Generations Spores Mitosis Zygote Gametes Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) MEIOSIS FERTILIZATION

Mitosis Alternation of Generations Spores Mitosis Zygote Gametes Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) MEIOSIS FERTILIZATION

Mitosis Alternation of Generations Spores Mitosis Zygote Gametes Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) MEIOSIS FERTILIZATION

Mitosis Alternation of Generations Spores Mitosis Zygote Gametes Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) MEIOSIS FERTILIZATION

Mitosis Alternation of Generations Spores Mitosis Zygote Gametes Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) MEIOSIS FERTILIZATION

Bryophytes Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants:  Liverworts, phylum Hepatophyta  Hornworts, phylum Anthocerophyta  Mosses, phylum Bryophyta In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes

Bryophyte Life Cycle Bryophyte gametophytes  Produce flagellated sperm in antheridia  Produce ova in archegonia  Generally form ground-hugging carpets and are at most only a few cells thick

Bryophyte Life Cycle Bryophyte sporophytes  Grow out of archegonia  Are the smallest and simplest of all extant plant groups  Consist of a foot, a seta, and a sporangium

Male gametophyte “Bud” Spores develop into threadlike protonemata. Protonemata “Bud” The haploid protonemata produce “buds” that grow into gametophytes. Raindrop Sperm Antheridia Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively. Egg Haploid (n) Diploid (2n) Key A sperm swims through a film of moisture to an archegonium and fertilizes the egg. Archegonia Rhizoid Female gametophyte Gametophore Spores Sporangium Peristome MEIOSIS Meiosis occurs and haploid spores develop in the sporangium of the sporophyte. When the sporangium lid pops off, the peristome “teeth” regulate gradual release of the spores. The sporophyte grows a long stalk, or seta, that emerges from the archegonium. FERTILIZATION (within archegonium) Archegonium Zygote Embryo Calyptra Young sporophyte Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte. The diploid zygote develops into a sporophyte embryo within the archegonium. Capsule (sporangium) Seta Foot Mature sporophytes Capsule with peristome (SEM) Female gametophytes

Ecological/Economic Importance Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat Sphagnum plays an important role in the Earth’s carbon cycle

Ancestral green alga Origin of land plants (about 475 mya) Origin of vascular plants (about 420 mya) Origin of seed plants (about 360 mya) Land plants Vascular plants Seed plants Seedless vascular plantsBryophytes Liverworts HornwortsMosses Lycophytes Pterophytes Gymno- sperms Angio- sperms Charophyceans

Seedless Vascular Plants Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution Began to diversify during the Carboniferous period Dominate most landscapes today

Seedless Vascular Plants Vascular plants have two types of vascular tissue: xylem and phloem Xylem conducts most of the water and minerals  Includes dead cells called tracheids  Lignified (strengthened with a polymer called lignin) Phloem consists of living cells  Distributes sugars, amino acids, and other organic products

Seedless Vascular Plants Evolved roots and leaves These two adaptations increase the surface area through which to take in resources.

Seedless Vascular Plant Life Cycle In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern The gametophytes are tiny plants that grow on or below the soil surface

Seedless Vascular Plant Life Cycle Spore Sperm Antheridium Egg Haploid (n) Diploid (2n) Key Young gametophyte Sorus Sporangium MEIOSIS FERTILIZATION Archegonium Zygote New sporophyte Mature sporophyte Sporangium Gametophyte Fiddlehead

Seedless Vascular Plant Life Cycle Sporophylls are modified leaves with sporangia Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte All seed plants and some seedless vascular plants are heterosporous, having two types of spores that give rise to male and female gametophytes

Seedless Vascular Plant Life Cycle Sporophylls are modified leaves with sporangia Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte All seed plants and some seedless vascular plants are heterosporous, having two types of spores that give rise to male and female gametophytes

Seedless Vascular Plants These adaptations have allowed seedless vascular plants to grow to greater heights and to take advantage of new environments  Accelerated photosynthesis Increased removal of CO 2 Formed the first forests Swamp lands were created Formed thick layers of peat Turned to coal over millions of years