Bio102 Chapter 29

Derived Traits of Plants

• Five key traits appear in nearly all land plants but are absent in the charophytes - Alternation of generations - Multicellular, dependent embryos - Walled spores produced in sporangia - Multicellular gametangia - Apical meristems - Additional derived traits exist

Derived Traits of Plants

Additional derived traits include Cuticle, a waxy covering of the epidermis Stomata are pores between specialized cells that allow for gas exchange (CO2, O2 and H2O) between the outside air and the plant Mycorrhizae, symbiotic associations between fungi and land plants that may have helped plants without true roots to obtain nutrients 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

Derived Traits (adpatations) of Plants [not found in the ancestor or sister taxa] [trait=character state]

Five key traits appear in nearly all land plants but are absent in the charophytes [derived character = not a shared ancestral character, see chapter 26] Alternation of generations Multicellular, dependent embryos Walled spores produced in sporangia Multicellular gametangia Apical meristems [-angia/-angium is a multicellular structure enclosing something}

The Greening of Earth

For much of Earth's history, the terrestrial surface was lifeless Cyanobacteria and protists likely existed on land 1.2 billion years ago Around 500 million years ago, small plants, fungi, and animals emerged on land

4. Multicellular Gametangia

Gametes are produced within organs called gametangia Female gametangia, called archegonia, produce eggs and are the site of fertilization Male gametangia, called antheridia, produce and release sperm

Land plants evolved from green algae

Green algae called charophytes are the closest relatives of land plants (charophytes = stoneworts) Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants Note that land plants did not descend from modern charophytes, but share a common ancestor with modern charophytes

Adaptations Enabling the Move to Land

In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out (charophytes cannot live above the waterline) Sporopollenin is also found in plant spore walls (inside which the zygote forms) The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO2, and nutrient-rich soil Land presented challenges: a scarcity of water and lack of structural support (in water they can float) (on land they need to be stiff)

Adaptations Enabling the Move to Land

Land plants diversified as adaptations evolved that enabled them to thrive despite challenges The placement of the boundary dividing land plants from algae is the subject of ongoing debate Until this debate is resolved, we define plants as embryophytes, plants with embryos Sporopollenin is polymerized from mixed monomers by an oxidative process and is resistant to acids and bases, including sulfuric, chromic, and hydrofluoric acids

Morphological and Molecular Evidence

Many characteristics of land plants also appear in some algae However, land plants share the following traits with only charophytes (shared ancestral characters) Rings of cellulose-synthesizing proteins (enzyme rosette) (non-morphological character) Structure of flagellated sperm Formation of a phragmoplast (in the telophase of cytokinesis)

1. Alternation of Generations

Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations The gametophyte is haploid and produces haploid gametes by mitosis Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis

5. Apical Meristems

Plants sustain continual growth in their apical meristems Cells from the apical meristems differentiate into various tissues The tips keep growing, extending the plant, and the laid down cells differentiate

The Greening of Earth

Since colonizing land, plants have diversified into roughly 290,000 living species Land plants are defined as having terrestrial ancestors, even though some are now aquatic Land plants do not include photosynthetic protists (certain algae such as kelp, red algae, green algae, including Charophytes) Plants supply oxygen and are the ultimate source of most food eaten by land animals

2. Multicellular, Dependent Embryos

The diploid embryo is retained within the tissue of the female gametophyte Nutrients are transferred from parent to embryo through placental transfer cells Land plants are called embryophytes because of the dependency of the embryo on the parent

3. Walled Spores Produced in Sporangia

The sporophyte produces spores in organs called sporangia Diploid cells called sporocytes undergo meiosis to generate haploid spores Spore walls contain sporopollenin, which makes them resistant to harsh enviro