Chapter 30: Plant Evolution and Classification

30-1 Overview of Plants

30-2 Nonvascular Plants

30-3 Vascular Plants

Revisiting Evolution

• The first true plant is thought to have been similar to a green alga, with adaptations that enabled it to survive the dry conditions on land.

Assessing Prior Knowledge

• Explain how successive generations become adapted to different conditions and evolve into different species.

• Describe what types of data are used to construct a phylogenetic tree.

I. Adapting to Land (from green algae?)

• Terrestrial life evolved ~ 430 m.y.a. (following ozone) and at one point small, club-shaped plants began to grow in the mud at water’s edge.

30-1 Overview of Plants

• Like green algae, plants possess chlorophyll, store energy as starch, and have cellulose making up their cell walls.

(A) Preventing Water Loss

• Benefits of migrating? More exposure to sunlight, increased CO2 levels, and greater supply of nutrients. Tradeoff? Desiccation. (1) Cuticle (an early adaptation to life on land)• A waxy protective covering on plant surfaces that prevents water loss (balanced out by stomata to exchange gases through cuticle)

(B) Reproducing by Spores and Seeds

• Evolved to aid terrestrial plant reproductive cells from drying out.

(1) Spore (widespread & water-resistant—primitive dispersal strategy)• Haploid reproductive single cell surrounded by a hard outer wall.(2) Seed (more evolved dispersal strategy and is multicellular)• Embryo surrounded by a protective coat and contains an endosperm.(3) Endosperm• Tissue that provides nourishment for the developing plant embryo.

(C) Transporting Material throughout the Plant • As a multicellular organism, differentiated plant cells need to share resources to grow, divide, and carry out metabolic activity.

(1) Vascular Tissue (2 types: more evolved terrestrial plant species)• Specialized plant tissue that transports water and dissolved substances.(2) Xylem• Vascular tissue carrying water and inorganic nutrients in one direction, from the roots to the stems and leaves.

(3) Phloem• Vascular tissue carrying organic compounds (carbohydrates), and some inorganic nutrients in ANY direction, depending on the plant’s needs.

(4) Woody Tissue (advantage in growing to greater heights, more sunlight)• Formed from several layers of xylem, usually concentrated in the center of the stem (generally brown and rigid) (5) Herbaceous Tissue (shorter plants, non-woody plants)• Vascular tissue that is NOT surrounded by schlerenchyma cells, making the stems of herbaceous plants soft, green, and flexible.

II. Classifying Plants

• Kingdom Plantae is divided into 12 phyla (a.k.a. divisions) divided into 3 groups based upon the presence of vascular tissue.(1) Nonvascular Plants (3 phyla, less diversified, no xylem or phloem)• More primitive plants that lack true vascular tissue, roots, stems, and leaves.

(2) Vascular Plants (9 phyla, more diversified)• More recently evolved plants that possess vascular tissue, roots, stems, and leaves.

Non-Vascular Plants

Mosses 10,000

Liverworts 6,500

Hornworts 100

Vascular Plants - Seedless

Whisk ferns 10 - 13

Club-mosses 1,000

Horsetails 15

Ferns 12,000

Seed Plants - Gymnosperms

Conifers 550

Cycads 100

Gingko 1

Gnetae 70

Angiosperms

Flowering plants 235,000

(3) Seed-Bearing Plants• More evolved than seedless plants (spore-bearing plants), include four phyla of gymnosperms and one phylum of angiosperms.

(4) Gymnosperms (non-flowering plants)• Class of seed-bearing plants that produce seeds that are NOT enclosed in fruits (e.g., pine trees)

(5) Angiosperms (flowering plants)• Class of seed-bearing plants that produce seeds within a protective fruit (e.g., apple and orange trees)

(A) The Fossil Record of Plants

• Strong evidence suggest modern plants evolved from an ancestor of green alga

(e.g., BOTH have the same photosynthetic pigments, chlorophylls a and b, both store energy as starch, and both have cell walls made of cellulose)• Suggested Evolutionary Order:

Algal Ancestors

Nonvascular Plants

Seedless Vascular Plants

Gymnosperms

Angiosperms

(B) Alternating Life Cycles • All plants maintain a life cycle that involves TWO phases, named after the type of reproductive cells the plant produces in each phase.(1) Sporophyte (diploid plant—2N, pine & oak trees are large sporophytes)• Plant producing spores, known as the sporophyte generation/phase and is the dominant phase in VASCULAR plants.(2) Gametophyte (haploid plant—1N) • Plant producing gametes (egg and sperm cells), known as the gametophyte generation/phase and is the dominant phase in NONVASCULAR plants.

(3) Alternation of Generations (NOT found with green algae)• A type of life cycle which alternates between the sporophyte and gametoyphyte phases.

• NOTE: The gametophyte makes gametes, which combine to make a diploid zygote, of which divides to form a sporophyte plant (2N).• Sporophyte (through meiosis) produces haploid spores (1N)—these spores are released by most seedless plants BUT are retained by seed-bearing plants.

• The life cycle begins again when spores divide by mitosis to form new gametophytes.

I. Classifying Bryophytes (the 3 phyla of primitive nonvascular plants)• Require water to reproduce sexually because the sperm must swim through water to an egg. (asexual production of haploid spores does not require water)

30-2 Plants and the Environment

(A) Phylum Bryophyta (dominant-gametophyte, water-dependent reprod.• Includes about 10,000 species of mosses (gametophytes).

(NOTE: The leafy carpet of moss gametophytes is topped by sporophytes that, when mature, release haploid spores which grow into a new generation of gametophytes.)

(1) Rhizoids (lack true vascular tissue)• Attaches the bryophyte to the soil (anchoring) and assists with the absorption of water and inorganic nutrients. (2) Sphagnum (peat moss)• Produces an acid that slows down decomposition in the swamp-like bogs. (also widely used by florists for packing bulbs and flowers for shipping)

(B) Phylum Hepatophyta and Anthocerophyta

• Includes about 6,500 and 100 species of liverworts and hornworts that also grow in moist, shady habitats.

(1) Thalloid (some liverwort species)

• A flat body with distinguishable upper and lower surfaces (most liverworts have thin, transparent leaflike structures arranged along a stemlike axis).

I. Seedless Vascular Plants

• Four phyla (fern allies and ferns), dominated the Earth until about 200 m.y.a. relying upon spores as the mobile sexual reproductive cells.

30-3 Vascular Plants

(A) Phylum Psilotophyta (e.g., whisk ferns)

• Include “whisk ferns” which are epiphytes, growing on other plants in a commensalistic manner.

• Produce reproductive structures on the ends of forked branches and lack true roots or leaves (can grow up to a foot tall).

(B) Phylum Lycophyta (e.g., club mosses, miniature pine trees)

• Include “resurrection plant” (Selaginella lepidophylla) native to the American Southwest, which turns brown and curls up in a ball during a drought but comes back alive within a few hours of becoming moist.

• Evergreens that produce spores in cones and have true roots (can grow only about 2 inches tall).

(1) Strobilus (“cone”)

• A cluster of sporangia-bearing modified leaves.

(C) Phylum Sphenophyta (e.g., Horsetails or Equisetum)

• Jointed stemmed plants with outer cells of stems containing silica, a major component of sand, SiO2 (can grow about 2-3 feet tall)

(D) Phylum Pterophyta (e.g., ferns)

• A diverse group including some floating plants as well as some inhabitants of the Arctic Circle and desert regions. (evolved around 350 m.y.a.)

• Leaves with spores on the underside and an underground stem are present with true ferns (can grow from 1 inch to 82 feet tall—tree ferns).

(1) Rhizome (“horizontal root”)• Fibrous stem that grows horizontally underground

(2) Fiddleheads• Tightly coiled new (immature) leaves of ferns

(3) Fronds

• Mature leaves that develop sori (pl. of sorus) on the underside where haploid spores develop.

II. Vascular Seed-Bearing Plants

• The mobile sexual reproductive part of seed plants is the multicellular seed, an evolutionary success story of adaptation and germination.(1) Germination

• Abiotic conditions favor growth, a seed sprouts as the embryo begins to divide and grow into a new seedling.

(2) Seedling

• New young plant that has emerged as an embryo through the seed coat.(3) Cone (found among the 4 phyla of gymnosperms)

• Reproductive structure made of hard scales, bearing the seeds on the open surface. (serve similar roles in gymnosperms as flowers do for angiosperms)

(A) Phylum Cycadophyta (~100 species remaining)• Gymnosperms native to the tropics and have fern-like, leathery leaves at the top of a short, thick trunk. (male and female plants bear large cones)

• Some cycads are suggested to have been alive for over a thousand years, and grow very slowly yet sometimes large—60 feet in height.

(B) Phylum Ginkgophyta (~1 species remaining, Ginkgo biloba)• Fan-shaped leaves that fall from the tree at the end of each growing season, with large seeds and tall heights—80 feet. (NOTE: Most gymnosperms are evergreens and retain their leaves year-round)• Ginkgos are known as the living fossil because it closely resembles fossil ginkgoes that are 125 million years old. (1) Deciduous

• Trees that lose their leaves at the end of the growing season.

(C) Phylum Coniferophyta (~1,000 species remaining)• Woody plants, most with needles or scale-like leaves, and usually bearing both male (staminate) and female (ovulate) cones.• Conifers include pines, cedars, redwoods, firs, spruces, junipers, cypresses and sequoias.

• Most massive tree is Sequoiadendron giganteum, the Giant Sequoia, which reaches heights of 350 feet tall and weighs approximately 6,200 tons.

(D) Phylum Gnetophyta (~20 species remaining, Ephedra genus)• Unusual group of gymnosperms that have vascular systems that more closely resemble angiosperms with jointed stems.• Examples include Ephedra, a genus of desert shrubs and Welwitschia, a another genus found in the deserts of southwestern Africa. (1) Ephedrine

• Decongestant drug (also a stimulant) derived from the cones of plants belonging to the Ephedra genus.

(E) Phylum Anthophyta (~240,000 species, the flowering plants)• Angiosperms, the other class of seed-bearing plants, are characterized by the presence of a flower and fruit.

• Angiosperms can be herbaceous plants like violets or shrubs like rose bushes. Still others are vines, like grapes and ivy.

(1) Ovary • The female part of the flower that encloses the egg, and when ripened, becomes a fruit of the plant.

• Oak, aspen, and birch trees are all flowering plants that have woody stems, although the small flower are almost undetectable. Likewise, grasses are also angiosperms, but their small, highly modified flowers are rarely noticed…(World’s Largest Flower is made by Rafflesia arnoldii)

(F) The Evolution of Angiosperms (why such success?)

• In angiosperms, seeds germinate and produce mature plants, which in turn produce new seeds, all in ONE growing season—huge advantage over gymnosperms which often take ten or more years to reach maturity and produce seeds.• More efficient vascular system (and more mycorrhizae) and an advanced and diversity of seed dispersal strategies—often with animal carriers.

(G) Monocots and Dicots (all angiosperms are divided into two classes)• Flowering plants are classified based upon the number of cotyledons present in the plant embryo, as well as a few other features.(1) Cotyledons

• Seed leaves found in in the plant embryo, either one (monocot) or two (dicot) leaves are present. (NOTE: By comparison, gymnosperms typically have two or more cotyledons)(2) Parallel Venation (monocot leaves)

• Most mature monocot leaves have several main veins or bundles of vascular tissue running roughly parallel to each other.(3) Net Venation (dicot leaves)

• Most dicots have one or more non-parallel main veins that branch repeatedly, forming an interconnected network.