Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 29.1: Land plants evolved from green algae Green algae called charophytes are the closest relatives of land plants Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Morphological and Molecular Evidence Many characteristics of plants such as presence chloroplast, storage of starch, cell wall cellulose, alteration of generations also appear in a variety of algal clades, mainly green algae But Algae are not considered to be plants because their life cycle does not show a multicellular dependent embryo that is protected by the parent (embryophyte)

Fig µm 5 mm Chara species, a pond organism Coleochaete orbicularis, a disk-shaped charophyte that also lives in ponds (LM)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Systematists are currently debating the boundaries of the plant kingdom Some biologists think the plant kingdom should be expanded to include some or all green algae Until this debate is resolved, we will retain the embryophyte definition of kingdom Plantae

Fig ANCESTRAL ALGA Red algae Chlorophytes Charophytes Embryophytes Viridiplantae Streptophyta Plantae

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Adaptations Enabling the Move to Land The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO 2, nutrient-rich soil Land presented challenges: a scarcity of water, lack of structural support, dispersing of gametes, herbivores

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Some adaptations to life on land: – The cuticle, a waxy covering that prevents drying – Gametangia, cases that enclose gametes and prevent drying – Embryos, young multicellular embryos contained within a protective structure – Secondary products and Pigments that afford protection against mutagenic ultraviolet radiation and predators – Thick spore walls to Spore walls contain sporopollenin, which makes them resistant to harsh environments prevent drying and resist decay – A mutualistic association with a fungus that promotes nutrient uptake from the soil

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Plant Kingdom Alternation of generations is a universal feature of the life cycles of plants. This life cycle includes both multicellular diploid individuals and multicellular haploid individuals. Gametes are produced by mitosis, not meiosis. Meiosis produces spores that develop into multicellular haploid individuals.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Plant Kingdom The multicellular, diploid plant is called the sporophyte. Cells contained in the sporangia on the sporophyte produce haploid, unicellular spores by meiosis. The multicellular, haploid plant formed by mitosis and cytokinesis of a spore is called the gametophyte. The gametophyte produces haploid gametes. The fusion of two gametes results in the formation of a diploid cell, the zygote, and the cycle repeats.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Plant Kingdom The sporophyte generation extends from the zygote through the adult, multicellular, diploid plant. The gametophyte generation extends from the spore through the adult, multicellular, haploid plant to the gamete. Some protist life cycles also feature alternation of generations, suggesting that the plants arose from one of these protist groups.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 29.2 Alternation of Generations

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Those ancestral species gave rise to a vast diversity of modern plants Land plants can be informally grouped based on the presence or absence of vascular tissue Most plants have vascular tissue; these constitute the vascular plants or tracheophytes Nonvascular plants are commonly called bryophytes or non-tracheophytes

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Seedless vascular plants can be divided into clades – Lycophytes (club mosses and their relatives) – Pterophytes (ferns and their relatives)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A seed is an embryo and nutrients surrounded by a protective coat Seed plants form a clade and can be divided into further clades: – Gymnosperms, the “naked seed” plants, including the conifers – Angiosperms, the flowering plants

Table 29-1

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Bryophytes, pteridiophytes, gymnosperms, and angiosperms demonstrate four great episodes in the evolution of land plants:  the origin of bryophytes from algal ancestors  the origin and diversification of vascular plants  the origin of seeds  the evolution of flowers Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 29.2: Mosses and other nonvascular plants have life cycles dominated by gametophytes Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants: – Liverworts, phylum Hepatophyta – Hornworts, phylum Anthocerophyta – Mosses, phylum Bryophyta Mosses are most closely related to vascular plants

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The non-tracheophytes have developed ways to obtain water and minerals in the absence of a vascular system: Many grow in dense masses through which water can move by capillary action. They have leaflike structures that catch and hold water that splashes onto them. They are small enough that minerals can be distributed evenly by diffusion.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Liverworts, phylum Hepatophyta, may be the most ancient surviving plant clade. The gametophytes are green leaflike layer that lie flat on the grounsd. Rhizoids are water-absorbing filaments that are found on the lower surfaces of the simplest liverwort gametophytes. Liverwort sporophytes have a stalk that connects the capsule and the foot. This capsule can elongate to raise the capsule above ground level, aiding in the dispersion of spores. A most familiar liverwort belong to the genus Marchantia

Fig. 29-9a Thallus Gametophore of female gametophyte Marchantia polymorpha, a “thalloid” liverwort Marchantia sporophyte (LM) Sporophyte Foot Seta Capsule (sporangium) 500 µm

Fig. 29-5d Female gametophyte Male gametophyte Antheridium with sperm Archegonium with egg Archegonia and antheridia of Marchantia (a liverwort)

The hornworts, phylum Anthocerophyta, along with the mosses and the tracheophytes, all have an adaptation to life on land not found in the liverworts. These groups all possess stomata that allow the uptake of CO 2 and the release of O 2, but they can be closed to prevent excessive water loss. They are so called because their sporophytes look like little horns

Fig. 29-9c An Anthoceros hornwort species Sporophyte Gametophyte

The phylum Bryophyta (mosses) are probably sister to the tracheophytes. Hydroid cells, found in many mosses, are a likely progenitor of the water-conducting cells of the tracheophytes. When hydroid cells die, they leave a tiny channel through which water can flow. The sporophytes of mosses and tracheophytes grow by apical cell division, whereby a region at the growing tip provides an organized pattern of cell division, elongation, and differentiation.

Fig. 29-9d Gametophyte Seta Capsule Sporophyte (a sturdy plant that takes months to grow) Polytrichum commune, hairy-cap moss

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Bryophyte Gametophytes In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes Sporophytes are typically present only part of the time

Fig Key Haploid (n) Diploid (2n) Protonemata (n) “Bud” Male gametophyte (n) Female gametophyte (n) Gametophore Rhizoid Spores Spore dispersal Peristome Sporangium MEIOSIS Seta Capsule (sporangium) Foot Mature sporophytes Capsule with peristome (SEM) Female gametophytes 2 mm Raindrop Sperm Antheridia Egg Archegonia FERTILIZATION (within archegonium) Zygote (2n) Embryo Archegonium Young sporophyte (2n)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A spore germinates into a gametophyte composed of a protonema and gamete-producing gametophore Rhizoids (not true roots) anchor gametophytes to substrate The height of gametophytes is constrained by lack of vascular tissues Mature gametophytes produce flagellated sperm in antheridia and an egg in each archegonium Sperm swim through a film of water to reach and fertilize the egg

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animation: Moss Life Cycle

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Ecological and Economic Importance of Mosses Moses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlands Some mosses might help retain nitrogen in the soil

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat Sphagnum is an important global reservoir of organic carbon

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 29.3: Ferns and other seedless vascular plants were the first plants to grow tall Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution Vascular plants began to diversify during the Devonian and Carboniferous periods Vascular tissue allowed these plants to grow tall since now the plant could efficiently obtain nutrition but also served as a supprt system Seedless vascular plants have flagellated sperm and are usually restricted to moist environments

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Living vascular plants are characterized by: Life cycles with dominant sporophytes Vascular tissues called xylem and phloem Well-developed roots and leaves

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Life Cycles with Dominant Sporophytes In contrast with bryophytes, sporophytes of seedless vascular plants are the larger dominant generation, as in the familiar leafy fern The gametophytes are tiny plants that grow on or below the soil surface Animation: Fern Life Cycle

Fig Key Haploid (n) Diploid (2n) MEIOSIS Spore dispersal Sporangium Mature sporophyte (2n) Sorus Fiddlehead Spore (n) Young gametophyte Mature gametophyte (n) Archegonium Egg Antheridium Sperm FERTILIZATION New sporophyte Gametophyte Zygote (2n)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Transport in Xylem and Phloem Vascular plants have two types of vascular tissue: xylem and phloem Xylem conducts most of the water and minerals and includes dead cells called tracheids Phloem consists of living cells and distributes sugars, amino acids, and other organic products Water-conducting cells are strengthened by lignin and provide structural support Increased height was an evolutionary advantage

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Evolution of Roots Roots are organs that anchor vascular plants They enable vascular plants to absorb water and nutrients from the soil Roots may have evolved from subterranean stems

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Evolution of Leaves Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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

Fig Vascular tissue SporangiaMicrophyll (a) Microphylls (b) Megaphylls Overtopping growth Megaphyll Other stems become re- duced and flattened. Webbing develops.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sporophylls and Spore Variations Sporophylls are modified leaves with sporangia Sori are clusters of sporangia on the undersides of sporophylls Strobili are cone-like structures formed from groups of sporophylls

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Heterosporous species produce megaspores that give rise to female gametophytes, and microspores that give rise to male gametophytes

Fig. 29-UN3 Homosporous spore production Sporangium on sporophyll Single type of spore Typically a bisexual gametophyte Eggs Sperm Eggs Sperm Heterosporous spore production Megasporangium on megasporophyll Megaspore Female gametophyte Male gametophyte Microspore Microsporangium on microsporophyll

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Classification of Seedless Vascular Plants There are two phyla of seedless vascular plants: – Phylum Lycophyta includes club mosses, spike mosses, and quillworts – Phylum Pterophyta includes ferns, horsetails, and whisk ferns and their relatives

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts Giant lycophytes thrived for millions of years in moist swamps Surviving species are small herbaceous plants Club mosses and spike mosses have vascular tissues and are not true mosses

Fig a Lycophytes (Phylum Lycophyta) Selaginella apoda, a spike moss Isoetes gunnii, a quillwort Strobili (clusters of sporophylls) 2.5 cm Diphasiastrum tristachyum, a club moss 1 cm

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives Ferns are the most diverse seedless vascular plants, with more than 12,000 species They are most diverse in the tropics but also thrive in temperate forests Horsetails were diverse during the Carboniferous period, but are now restricted to the genus Equisetum Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns

Fig e Pterophytes (Phylum Pterophyta) Athyrium filix-femina, lady fern Vegetative stem Strobilus on fertile stem 1.5 cm 25 cm 2.5 cm Psilotum nudum, a whisk fern Equisetum arvense, field horsetail

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Significance of Seedless Vascular Plants The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carboniferous, forming the first forests Increased photosynthesis may have helped produce the global cooling at the end of the Carboniferous period The decaying plants of these Carboniferous forests eventually became coal

Fig

Fig. 29-UN7

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Describe four shared characteristics and four distinct characteristics between charophytes and land plants 2.Distinguish between the phylum Bryophyta and bryophytes 3.Diagram and label the life cycle of a bryophyte 4.Explain why most bryophytes grow close to the ground and are restricted to periodically moist environments

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1.Describe three traits that characterize modern vascular plants and explain how these traits have contributed to success on land 2.Explain how vascular plants differ from bryophytes 3.Distinguish between the following pairs of terms: microphyll and megaphyll; homosporous and heterosporous 4.Diagram and label the life cycle of a seedless vascular plant