Chapter 29 Plant Diversity I How Plants Colonized Land Plants vs. Algae • Land plants evolved from Charophytes (Chara) green algae • 4 key traits plants share with Charophytes (Morphological and Biochemical Evidence): 1. Rose-shaped complexes for cellulose synthesis 2. Peroxisome enzymes 3. Structure of flagellated sperm 4. Formation of a phragmoplast 5 key traits in nearly all land plants but are absent in the charophytes: 1. apical meristems 2. multicellular dependent embryos 3. alternation of generations 4. walled spores produced in sporangia 5. multicellular gametangia females – archegonia & males – antheridia Fig. 29-5e Apical meristem of shoot Shoot Developing leaves 100 µm Apical meristems Apical meristem of root Root 100 µm Fig. 29-5b 2 µm Embryo Maternal tissue Wall ingrowths 10 µm Placental transfer cell (outlined in blue) Embryo (LM) and placental transfer cell (TEM) of Marchantia (a liverwort) Fig. 29-5a Gametophyte (n) Mitosis n n Spore Gamete from another plant Mitosis n n Gamete MEIOSIS FERTILIZATION 2n Mitosis Sporophyte (2n) Alternation of generations Zygote Fig. 29-5c Spores Sporangium Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte Sporophytes and sporangia of Sphagnum (a moss) Fig. 29-5d Female gametophyte Archegonium with egg Antheridium with sperm Male gametophyte Archegonia and antheridia of Marchantia (a liverwort) Fig. 29-7 1 Origin of land plants (about 475 mya) 2 Origin of vascular plants (about 420 mya) 3 Origin of extant seed plants (about 305 mya) Hornworts 1 Mosses Pterophytes (ferns, horsetails, whisk ferns) 3 Angiosperms 450 400 350 300 Millions of years ago (mya) 50 0 Seed plants Gymnosperms Vascular plants 2 Seedless vascular plants Lycophytes (club mosses, spike mosses, quillworts) 500 Land plants ANCESTRAL GREEN ALGA Nonvascular plants (bryophytes) Liverworts Characteristics of all land plants: • • • • eukaryotic, multicellular, autotrophic cell walls made mostly of cellulose chlorophylls a & b Domain Eukarya, Kingdom Plantae In many plants, additional terrestrial adaptations, such as vascular tissues and secondary compounds, also evolved. Nonvascular plants 1. Represented by three phyla: a. phylum Hepatophyta – liverworts b. phylum Anthocerophyta – hornworts c. phylum Bryophyta - mosses Liverworts Hornworts Moss Fig. 29-9a Thallus Gametophore of female gametophyte Sporophyte Foot Seta Marchantia sporophyte (LM) 500 µm Marchantia polymorpha, a “thalloid” liverwort Capsule (sporangium) 2. The gametophyte is the dominant generation in the life cycle • gametophyte - mass of green, branched, one-cell-thick filaments • sporophytes are smaller; only present part of the time • spores germinate in favorable habitats sporophyte gametophyte sporophyte gametophyte Raindrop Fig. 29-8 The life cycle of a moss. Sperm “Bud” Antheridia Male gametophyte (n) Key Haploid (n) Diploid (2n) Protonemata (n) “Bud” Egg Spores Gametophore Female Archegonia gametophyte (n) Spore dispersal Rhizoid Peristome FERTILIZATION Sporangium MEIOSIS Mature sporophytes Seta Capsule (sporangium) Foot (within archegonium) Zygote (2n) Embryo 2 mm Archegonium Capsule with peristome (SEM) Young sporophyte (2n) Female gametophytes 3. Bryophyte sporophytes disperse enormous numbers of spores • sporophytes remain attached to gametophyte throughout the lifetime – depends on the gametophyte for sugars, amino acids, minerals and water. • sporangium (site of meiosis and spore production) can generate over 50 million spores. 4. Bryophytes provide many ecological and economic benefits • distributed worldwide • common and diverse in moist forests and wetlands • Some common in extreme environments (mountaintops, tundra, and deserts) • Sphagnum, a wetland moss, is especially abundant and widespread. • forms extensive deposits of undecayed organic material, called peat • Wet regions dominated by Sphagnum or peat moss are known as peat bogs Fig. 29-11 (a) Peat being harvested (b) “Tollund Man,” a bog mummy Bog People THE ORIGIN OF VASCULAR PLANTS 1. Two conducting tissues of the vascular system A. Xylem – Dead tissue, water-conducting B. Phloem – Living tissue, food-transporting 2. Water-conducting cells are strengthened by lignin and provide structural support 3. Sporophyte generation is dominant in vascular plants. Seedless vascular plants 4. Two modern phyla: a. phylum Lycophyta – club mosses b. phylum Pterophyta - ferns, whisk ferns, and horsetails Club moss Whisk fern Horsetail Fern Fig. 29-15a Lycophytes (Phylum Lycophyta) 2.5 cm Isoetes Strobili (clusters of gunnii, a quillwort sporophylls) 1 cm Selaginella apoda, a spike moss Diphasiastrum tristachyum, a club moss Fig. 29-15e Pterophytes (Phylum Pterophyta) Athyrium filix-femina, lady fern Equisetum arvense, field horsetail Psilotum nudum, a whisk fern Vegetative stem 2.5 cm 1.5 cm 25 cm Strobilus on fertile stem 5. Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte • both archegonia (female sex organs) and antheridia (male sex organs) • Eg., ferns eggs sporophyte Single type of spore Bisexual gametophyte sperm Fig. 29-13 The life cycle of a fern. Key Haploid (n) Diploid (2n) MEIOSIS Spore dispersal Spore (n) Sporangium Sporangium Antheridium Young gametophyte Mature gametophyte (n) Archegonium Egg Mature sporophyte (2n) New sporophyte Zygote (2n) Sorus Gametophyte Fiddlehead FERTILIZATION Sperm 6. seedless vascular plants are most common in damp habitats 7. ferns produce clusters of sporangia, called sori, on the back of leaves Seedless vascular plants formed vast “coal forests” during the Carboniferous period • These plants left not only living representatives and fossils, but also fossil fuel in the form of coal.