Plant Diversity I How Plants Colonized Land Chapter 29 Plant Diversity I How Plants Colonized Land

Overview: The Greening of Earth Looking at a lush landscape It is difficult to imagine the land without any plants or other organisms Figure 29.1

For more than the first 3 billion years of Earth’s history The terrestrial surface was lifeless Since colonizing land Plants have diversified into roughly 290,000 living species

Concept 29.1: Land plants evolved from green algae Researchers have identified green algae called charophyceans as the closest relatives of land plants

Morphological and Biochemical Evidence Many characteristics of land plants Also appear in a variety of algal clades

There are four key traits that land plants share only with charophyceans Rose-shaped complexes for cellulose synthesis 30 nm Figure 29.2

Peroxisome enzymes Structure of flagellated sperm Formation of a phragmoplast

Comparisons of both nuclear and chloroplast genes Genetic Evidence Comparisons of both nuclear and chloroplast genes Point to charophyceans as the closest living relatives of land plants Chara, a pond organism (a) 10 mm Coleochaete orbicularis, a disk- shaped charophycean (LM) (b) 40 µm Figure 29.3a, b

Adaptations Enabling the Move to Land In charophyceans A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out The accumulation of traits that facilitated survival on land May have opened the way to its colonization by plants

Concept 29.2: Land plants possess a set of derived terrestrial adaptations Many adaptations Emerged after land plants diverged from their charophycean relatives

Defining the Plant Kingdom Systematists Are currently debating the boundaries of the plant kingdom Plantae Streptophyta Viridiplantae Red algae Chlorophytes Charophyceans Embryophytes Ancestral alga Figure 29.4

Some biologists think that the plant kingdom Should be expanded to include some or all green algae Until this debate is resolved This textbook retains the embryophyte definition of kingdom Plantae

Derived Traits of Plants 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

Apical meristems and alternation of generations of shoot Developing leaves 100 µm Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the tips of a shoot and root. Apical meristem of root Root Shoot Figure 29.5 Haploid multicellular organism (gametophyte) Mitosis Gametes Zygote Diploid multicellular organism (sporophyte) Alternation of generations: a generalized scheme MEIOSIS FERTILIZATION 2n n Spores ALTERNATION OF GENERATIONS Figure 29.5

Walled spores; multicellular gametangia; and multicellular, dependent embryos WALLED SPORES PRODUCED IN SPORANGIA Sporangium Sporophyte and sporangium of Sphagnum (a moss) Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte MULTICELLULAR GAMETANGIA Female gametophyte Archegonium with egg Archegonia and antheridia of Marchantia (a liverwort) Antheridium with sperm Male gametophyte MULTICELLULAR, DEPENDENT EMBRYOS Embryo Maternal tissue 2 µm Embryo and placental transfer cell of Marchantia 10 µm Figure 29.5 Wall ingrowths Placental transfer cell

Additional derived units Such as a cuticle and secondary compounds, evolved in many plant species

The Origin and Diversification of Plants Fossil evidence Indicates that plants were on land at least 475 million years ago

Fossilized spores and tissues Have been extracted from 475-million-year-old rocks Fossilized spores. Unlike the spores of most living plants, which are single grains, these spores found in Oman are in groups of four (left; one hidden) and two (right). (a) Fossilized sporophyte tissue. The spores were embedded in tissue that appears to be from plants. (b) Figure 29.6 a, b

Whatever the age of the first land plants Those ancestral species gave rise to a vast diversity of modern plants Table 29.1

Land plants can be informally grouped Based on the presence or absence of vascular tissue

Seedless vascular plants An overview of land plant evolution Land plants Vascular plants Bryophytes (nonvascular plants) Seedless vascular plants Seed plants Mosses Liverworts Hornworts Charophyceans Gymnosperms Angiosperms Pterophyte (ferns, horsetails, whisk fern) Origin of seed plants (about 360 mya) Lycophytes (club mosses, spike mosses, quillworts) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga Figure 29.7

Concept 29.3: The life cycles of mosses and other bryophytes are dominated by the gametophyte stage Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants Liverworts, phylum Hepatophyta Hornworts, phylum Anthocerophyta Mosses, phylum Bryophyta

Debate continues over the sequence of bryophyte evolution Mosses are most closely related to vascular plants

Bryophyte Gametophytes In all three bryophyte phyla Gametophytes are larger and longer-living than sporophytes

The life cycle of a moss Figure 29.8 4 3 8 6 5 7 1 2 Mature sporophytes Young sporophyte Male gametophyte Raindrop Sperm Key Haploid (n) Diploid (2n) Antheridia Female gametophyte Egg Archegonia FERTILIZATION (within archegonium) Zygote Archegonium Embryo Female gametophytes Gametophore Foot Capsule (sporangium) Seta Peristome Spores Protonemata “Bud” MEIOSIS Sporangium Calyptra Capsule with peristome (LM) Rhizoid Mature sporophytes Spores develop into threadlike protonemata. 1 The haploid protonemata produce “buds” that grow into gametophytes. 2 Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively. 3 A sperm swims through a film of moisture to an archegonium and fertilizes the egg. 4 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. 8 The sporophyte grows a long stalk, or seta, that emerges from the archegonium. 6 The diploid zygote develops into a sporophyte embryo within the archegonium. 5 Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte. 7 Figure 29.8

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 Some mosses Have conducting tissues in the center of their “stems” and may grow vertically

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 Hornwort and moss sporophytes Have stomata

Bryophyte diversity Figure 29.9 LIVERWORTS (PHYLUM HEPATOPHYTA) HORNWORTS (PHYLUM ANTHOCEROPHYTA) MOSSES (PHYLUM BRYOPHYTA) Gametophore of female gametophyte Marchantia polymorpha, a “thalloid” liverwort Foot Sporangium Seta 500 µm Marchantia sporophyte (LM) Plagiochila deltoidea, a “leafy” liverwort An Anthoceros hornwort species Sporophyte Gametophyte Polytrichum commune, hairy-cap moss Figure 29.9

Ecological and Economic Importance of Mosses Sphagnum, or “peat moss” Forms extensive deposits of partially decayed organic material known as peat Plays an important role in the Earth’s carbon cycle (a) Peat being harvested from a peat bog Sporangium at tip of sporophyte Gametophyte (b) Closeup of Sphagnum. Note the “leafy” gametophytes and their offspring, the sporophytes. Living photo- synthetic cells Dead water- storing cells 100 µm (c) Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality. (d) “Tolland Man,” a bog mummy dating from 405–100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum canpreserve human or other animal bodies for thousands of years. Figure 29.10 a–d

Bryophytes and bryophyte-like plants Concept 29.4: Ferns and other seedless vascular plants formed the first forests Bryophytes and bryophyte-like plants Were the prevalent vegetation during the first 100 million years of plant evolution Vascular plants Began to evolve during the Carboniferous period

Origins and Traits of Vascular Plants Fossils of the forerunners of vascular plants Date back about 420 million years

These early tiny plants Had independent, branching sporophytes Lacked other derived traits of vascular plants Figure 29.11

Life Cycles with Dominant Sporophytes 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

The life cycle of a fern Figure 29.12 Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte. 1 The fern spore develops into a small, photosynthetic gametophyte. 2 Although this illustration shows an egg and sperm from the same gametophyte, a variety of mechanisms promote cross-fertilization between gametophytes. 3 Key Haploid (n) Diploid (2n) Antheridium Spore Young gametophyte MEIOSIS Sporangium Archegonium Sperm Mature sporophyte Egg New sporophyte Zygote Sporangium FERTILIZATION Sorus On the underside of the sporophyte‘s reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. 6 Fern sperm use flagella to swim from the antheridia to eggs in the archegonia. 4 Gametophyte Fiddlehead A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte. 5 Figure 29.12

Transport in Xylem and Phloem Vascular plants have two types of vascular tissue Xylem and phloem

Xylem Phloem Conducts most of the water and minerals Includes dead cells called tracheids Phloem Distributes sugars, amino acids, and other organic products Consists of living cells

Evolution of Roots Roots Are organs that anchor vascular plants Enable vascular plants to absorb water and nutrients from the soil May have evolved from subterranean stems

Evolution of Leaves Leaves Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis

Leaves are categorized by two types Microphylls, leaves with a single vein Megaphylls, leaves with a highly branched vascular system

According to one model of evolution Microphylls evolved first, as outgrowths of stems Vascular tissue Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue. (a) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems. (b) Figure 29.13a, b

Sporophylls and Spore Variations 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

Classification of Seedless Vascular Plants Seedless vascular plants form two phyla Lycophyta, including club mosses, spike mosses, and quillworts Pterophyta, including ferns, horsetails, and whisk ferns and their relatives

The general groups of seedless vascular plants LYCOPHYTES (PHYLUM LYCOPHYTA) PTEROPHYTES (PHYLUM PTEROPHYTA) WHISK FERNS AND RELATIVES HORSETAILS FERNS Isoetes gunnii, a quillwort Selaginella apoda, a spike moss Diphasiastrum tristachyum, a club moss Strobili (clusters of sporophylls) Psilotum nudum, a whisk fern Equisetum arvense, field horsetail Vegetative stem Strobilus on fertile stem Athyrium filix-femina, lady fern Figure 29.14

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts Modern species of lycophytes Are relics from a far more eminent past Are small herbaceous plants

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives Are the most diverse seedless vascular plants

The Significance of Seedless Vascular Plants The ancestors of modern lycophytes, horsetails, and ferns Grew to great heights during the Carboniferous, forming the first forests Figure 29.15

The growth of these early forests May have helped produce the major global cooling that characterized the end of the Carboniferous period Decayed and eventually became coal