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

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

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

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

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

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

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

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

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

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

The End