Homework to be emailed to    valenciabiologyhw@gmail.com

1. Provide evidence to defend the position that plants evolved from green algae
2. Describe 2 adaptations that made a bryophyte move onto land possible
3. Explain how Bryophytes are still tied to water
4. Explain how vascular plants differ from bryophytes.
5. Describe the production and dispersal of fern spores.

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

•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

•Land plants evolved from green algae

•Researchers have identified green algae called charophyceans as the closest relatives of land plants

Plants

•Plants appeared on land about 425 million years ago, and the evolutionary history of the plant kingdom reflects increasing adaptation to the terrestrial environment. There are about 290,000 known plant species. (Food agriculture is based on only about two dozen species.)

. The Invasion of the Land is really the Invasion of the Atmosphere!!!

The Protoplasm of Individual Plant Cells is surrounded by a Cellulose Wall. While Cellulose is strong and prevents mechanical damage to the cell contents, it is extremely hydrophilic and readily absorbs water.However, Cellulose easily loses water via evaporation.

Land plants

Land plants

Land plants

Nontrcheophytes

Mosses

Mosses

Nontrcheophytes

Tracheophytes

Vascular system

Plants

•A. General Characteristics of Plants

•Plants are multicellular eukaryotes that are photosynthetic autotrophs. They share the following characteristics with their green algal ancestors:

•·      Chloroplasts with the photosynthetic pigments: chlorophyll a, chlorophyll b, and carotenoids.

•·       Cell walls containing cellulose.

•Food reserve is starch that is stored in plastids.

Plants

•As plants adapted to terrestrial life, they evolved complex bodies with cell specialization for different functions.

•·      Aerial plant parts are coated with a waxy cuticle that helps prevent desiccation.

•Though gas exchange cannot occur across the waxy cuticle, CO₂ and ₀₂ can diffuse between the leafs interior and the surrounding air through stomata, microscopic pores on the leafs surface.

Plants

•With the move from an aquatic to terrestrial environment, a new mode of reproduction was necessary to solve two problems:

•1. Gametes must be dispersed in a nonaquatic environment. Plants produce gametes within gametangia, organs with protective jackets of sterile (nonreproductive) cells that prevent gametes from drying out. The egg is fertilized within the female organ.

Plants

•2. Embryos must be protected against desiccation. The zygote develops into an embryo that is retained for awhile within the female gametangia's jacket of protective cells. Emphasizing this terrestrial adaptation, plants are often referred to as embryophytes.

. Cellulose is like a sponge. If you drop a sponge in water, it saturates instantaneously. A wet sponge readily loses water when it is placed on a dry substrate. In order for an isolated plant cell, like a unicellular alga, to survive, it must be in constant contact with water.

In order to withstand periodic dry spells, plant cells needed a water protective coating.

. One of the most important plant adaptations is the Cuticle. It is a waxy material that is secreted to the outside of the plasma membrane. It fills in the spaces between cellulose fibrils and forms a continuous external waxy layer to the outside of the cell wall. This makes the cell watertight!

. This cell can be called an "all purpose" cell because it Regulates its water balance and performs Photosynthesis. 

The Cuticle keeps water inside but it also prevents water uptake. The Cuticle is usually thicker on the side of the cell facing the light. Consequently, water could enter the bottom of the cell where the cuticle is thin and where water is more

. abundant, and be retained within the cell by the thick cuticle on its upper side. This could lead to the formation of colonies. The first multicellular forms could be filaments. These might be followed by flat sheets.

The Chlorophyta (Green Algae) is algal group which probably gave rise to land plants. The genus Coleochaete is regarded as the

Plant Evolution

Adaptations

•Cuticle

•alternation of generations

•specialized tissues

Cuticle

•Waxy coating on surfaces

•resists drying out

•stomata exist to allow necessary gas exchange

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

–Peroxisome enzymes

–Structure of flagellated sperm

–Formation of a phragmoplast

Genetic Evidence

•Comparisons of both nuclear and chloroplast genes

–Point to charophyceans as the closest living relatives of land plants

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

•: 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

•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 embryo

•Apical meristems and alternation of generations

•Walled spores; multicellular gametangia; and multicellular, dependent embryos

•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

Whatever the age of the first land plants

–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

•An overview of land plant evolution

•: 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

•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

•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

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

•: 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

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

Transport in Xylem and Phloem

•Vascular plants have two types of vascular tissue

–Xylem and phloem

•Xylem

–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

Sporophylls and Spore Variations

•Sporophylls

–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

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

•Ferns

–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

•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

•Enough for now!