33.0 Introduction

1. Plants Are Dominant Photosynthetic Organisms on Land fig 33.1
1. Value of Plants: Food, Shelter, Clothing, Medicines
2. Plants Are Multicellular, Photosynthetic Organisms

33.1 Plants are terrestrial with life cycles that alternate between haploid and diploid

1. The Evolutionary Origins of Plants
1. Plants Derived From Multicellular Green Algae
1. Biochemical and morphological similarities
2. Have chloroplasts containing chlorophylls a and b, carotenoids
3. Possess cellulose-rich cell walls, store starch
4. Only organisms that form cell plate during mitosis

2. Plants Divided into Twelve Phyla
1. Terms phylum and division are interchangeable, the former is preferred
2. Vascular plants are subdivided into nine phyla
1. Include ferns, conifers and flowering plants fig 33.2
2. Have food and water conducting strands

3. Remaining three separate phyla
1. Lack or have poorly developed vascular strands
2. Include mosses, liverworts and hornworts

3. The Green Invasion of the Land
1. Plants and fungi occur almost exclusively on land
2. Arthropods, chordates and mollusks also well represented on land
3. Plant cell walls provide protection from desiccation
1. Relatively impermeable outer waxy cuticle
2. Stomata (stoma, singl.) allow passage of CO₂ for photosynthesis fig 33.3
3. Stoma also allow passage of other gases and water

4. Adaptation to terrestrial habitats enhanced by mycorrhizal fungi
1. Symbiotic association in 80% of plants
2. Play important role in assimilation of phosphorus and other ions

5. Features of plants
1. Specialization of parts shoots, roots
2. Expansion of photosynthetic areas: Leaves
3. Specializations in reproductive features

2. Plant Life Cycles
1. Alternation of Generations
1. Also occurs in brown, red and green algae
2. Alternation of diploid sporophyte with haploid gametophyte
3. Most adult animals are diploid
1. Resemble plant's sporophyte stage
2. Produce gametes (egg, sperm) via meiosis, fusion forms zygote

4. Sporophyte generation of plant does not produce gametes via meiosis
5. Generalized plant life cycle fig 33.4
1. In sporangia, meiosis in spore mother cells (sporocytes) produces haploid spores
2. Spores divide by mitosis to produce multicellular haploid gametophyte
3. Gametophyte, by mitosis, produces haploid gametes
4. Gametes fuse forming diploid zygote
5. Zygote mitotically divides to form sporophyte
6. Sporophyte produces sporangia in which meiosis occurs

2. Specialization in the Plant Life Cycle
1. Liverwort, moss and fern gametophytes are green and free living
2. In others gametophyte is nutritionally dependent on sporophyte or saprobic
3. Moss and liverwort fig 33.5a
1. Gametophyte dominates, larger
2. Sporophyte reduced, smaller, yellowish or brownish
3. Attached to or within tissues of gametophyte

4. Vascular plants
1. Are primarily sporophytic, gametophytes are smaller fig 33.5b
2. Gametophytes nutritionally dependent on, enclosed within sporophyte

5. Trends occur during evolution of plant form
1. Progressive reduction in gametophyte
2. Loss of multicellular gametangia that produce sex cells
3. Increased specialization for life on land
4. Culminate in structural adaptations of flowering plants

6. Most living plant groups did not directly give rise to one another

33.2 Nonvascular plants like mosses are relatively unspecialized

1. Mosses, Liverworts and Hornworts
1. Traditionally Grouped Together as Bryophytes
1. Due to similarities in reproduction, form, habitat
1. Relationships of groups assumed
2. Now realized that all three are distinct phyla
3. Are relatively unspecialized and did not arise from a common ancestor

2. Common characteristics
1. Gametophytes green, nutritionally independent of sporophyte
2. Sporophyte attached to gametophyte, partially nutritionally dependent on it
3. Require external water for fertilization, common in moist places
4. Small in size, gametophyte more conspicuous than sporophyte

2. Bryophyta: Mosses
1. Gametophytes small, spiral or alternate arranged leaves on central axis fig 33.6
2. Axis anchored to substrate by rootlike rhizoids
1. Consists of several cells that absorb water
2. Not near the water absorbing capabilities of true roots

3. Leaves only superficially resemble true leaves
1. Have green, flattened blade, slightly thickened midrib
2. Are only one cell thick, lack vascular strands and stomata
3. Cells are all haploid

4. Special central strand of water conducting tissue
1. Most water used by plant travels up on outside, via capillary action
2. Some may have specialized food conducting cells around water conducting ones

5. Form multicellular gametangia at tips of leafy gametophytes fig 33.7
1. Female archegonia and male antheridia may form on same or separate plants
2. Single egg produced in lower part of archegonium
3. Multiple flagellated sperm produced in antheridium

6. Reproductive cycle
1. Released sperm swim to archegonium
2. One sperm unites with single egg, forms diploid zygote
3. Zygote divides by mitosis, develops into sporophyte
1. Composed of slender basal stalk, seta
2. Capsule or sporangium at its apex

4. Base of sporophyte becomes embedded in gametophyte
5. Sporophyte derives energy from gametophyte
6. Sporangium has row of teeth (peristome) at upper end
1. Peristome protected by caplike operculum
2. Operculum covered by hoodlike calyptra
3. All cells of sporophyte are diploid, except calyptra which is haploid

7. Spore mother cells within sporangium undergo meiosis
1. Produce four haploid spores
2. Operculum (and calyptra) peel off at maturity, release spores

8. Spores germinate on suitable damp location
1. Grows into threadlike protonema
2. Each protonema cell has several chloroplasts
3. Certain cells develop rhizoids and buds
4. Buds develop into new gametophyte axis with leaves

9. Most abundant plants in Arctic and Antarctic, rare in deserts
1. Can withstand long periods of drying
2. Mosses are sensitive to pollutants

10. Economic importance of Sphagnum mosses

3. Hepaticophyta: Liverworts
1. Some have flattened, lobed-shaped bodies called thalli (thallus, singl.) fig 33.8
2. Others are leafy and resemble mosses
3. Are less complex than and differ from mosses
1. Gametophytes develop directly from spores
2. Growth is prostrate, not erect
3. Rhizoids are one-celled

4. Thalloid liverworts have air chambers containing photosynthetic cells
1. Chamber as opening at top
2. Pores are fixed, cannot open/close like stomata

5. Leafy liverworts have two rows of overlapping leaves, cells contain oil bodies
6. Liverwort reproduction
1. Sexual reproduction similar to mosses
2. Thalloid forms produce gametangia on upright umbrella-shaped structures
3. Also reproduce asexually of gemmae produced in cuplike structures

4. Anthocerotophyta: Hornworts
1. Sporophytes resemble tiny, green broom handles fig 33.9
2. Arise from thalloid gametophytes
3. Sporophyte base embedded in gametophyte, derives some nutrition from it
4. Unique characteristics of hornwort sporophyte
1. Has stomata
2. Is photosynthetic, makes much of its own energy

5. Description of hornwort cells
1. Single chloroplast with pyrenoid like green algae
2. May indicate closer relationship to green algae than other plants

6. Gametophytes have mucilage-filled cavities
1. Cyanobacterium inhabits cavities
2. Capable of nitrogen fixation

7. Sporophytes elongate and split into ribbonlike segments, releasing spores

33.3 Seedless vascular plants like ferns have well-developed conducting tissues

1. Features of Vascular Plants
1. Fossil Vascular Plants
1. Rhyniophyta: Simple branching axis with sporangia at tips fig 33.10
2. Were homosporous, produce only one type of spore
3. Later plants evolved first leaves and more complex sporangia

2. Evolution of Vascular Tissues
1. Systems capable of efficient conduction of liquids fig 33.11
2. Specialized strands of cylindrical or elongated cells
1. Xylem conducts water and minerals from roots through stems to leaves
2. Phloem conducts carbohydrates in solution away from leaves

3. Cuticle and stomata are also characteristics of vascular plants

3. A Very Successful Group: The Vascular Plants
1. Nine phyla comprise vascular plants tbl 33.1
2. Alternation of generations modified by reduction of gametophyte
3. Accompanied by appearance of seeds
1. Surrounded by tough, drought-resistant, protective seed coat
2. Protects embryo from drying out, from predators, provides food storage
3. Occur only in heterosporous plants (produce two types of spores)

4. Appearance of flowers
1. Induces insects and animals to spread pollen to other flowers
2. Mechanism to overcome lack of motility
3. Secure benefits of outcrossing, promoting genetic diversity

2. Seedless Vascular Plants
1. History
1. Earliest vascular plants lacked seeds
2. Four living and three extinct phyla lack seeds

2. Pterophyta: The Ferns
1. Most abundant group
1. Include small, reduced aquatic ferns
2. Include tree ferns of great size fig 33.12

2. Sporophyte and gametophyte are both photosynthetic
3. Life cycle compared to mosses
1. Much greater development, independence and dominance of sporophyte
2. Fern sporophyte is more complex
3. Has vascular tissue, well-differentiated roots, stems, leaves

4. Unique structures of the fern sporophytes
1. Horizontal, underground stem called a rhizome
2. Leaves referred to as fronds
3. Develop from rhizomes as coiled fiddleheads
4. Many are highly dissected and feathery
5. Marsilea leaves are clover-shaped, still arise from fiddleheads
6. Some have mixture of photosynthetic and nonphotosynthetic, reproductive fronds

5. Nearly all are homosporous with distinctive sori containing sporangia
1. Sori usually protected by umbrella like indusium
2. Shrivels to expose sporangia

6. Reproduction within the sporangium
1. Spore mother cells undergo meiosis, produce haploid spores
2. Spores catapulted from sporangium at maturity
3. Spores germinate on suitable damp location
4. Produce gametophytes called prothalli (prothallus, singl.)

7. Prothalli are usually heart-shaped and one cell thick
1. Have anchoring rhizoids
2. Produce antheridia and archegonia on same or different prothalli

8. Spores produced in antheridium have flagella, swim to archegonia
1. Requires presence of water
2. One sperm unites with single egg to form zygote
3. Zygote develops into new sporophyte, completes life cycle fig 33.13

3. Psilophyta: Whisk Ferns
1. Three other phyla of seedless vascular plants
1. Psilophyta (whisk ferns)
2. Lycophyta (club mosses)
3. Arthrophyta (horsetails)

2. Share many common features with ferns
1. Form archegonia and antheridia
2. Produce free-swimming sperm that require water for fertilization

3. Comparable features of seed plants
1. Have nonflagellated sperm
2. None form antheridia, few form archegonia

4. Two genera of whisk ferns, considered remnants of earliest vascular plants
1. Simply branching green stems, lack roots or leaves
2. Few Psilotum have scalelike enations, lack veins or stomata fig 33.14
3. Tmespiteris has leaflike appendages

5. Gametophytes found in soil beneath sporophytes
1. Colorless, filamentous form
2. Have saprobic or parasitic associations with fungi to obtain nutrients
3. Some develop vascular tissue, the only gametophytes to do so

4. Lycophyta: Club Mosses
1. Worldwide distribution, most common in tropics, moist temperate regions
1. Several treelike extinct genera
2. Four living genera resemble mosses, clearly different by internal structures

2. Are either homosporous or heterosporous, sporphytes have leafy stems
3. Lycopodium is typical homosporous genus
1. Sporangia produced in conelike clusters on stems or in upper leaves
2. Leaves (microphylls) are short, linear and in whorls or spirals fig 33.15

4. Selaginella is typical heterosporous genus
1. Leaves branch freely
2. May be flattened

5. Lycopodium gametophytes are tiny and carrot-shaped
1. Have exposed area at top
2. Location of archegonia and antheridia

6. Gametophytes of Selaginella
1. Male gametophytes develop within minute microspore
2. Female gametophytes develop in larger multicellular megaspores
3. Contain several archegonia

7. Club mosses are used as ornaments, many are now endangered species
8. Quillworts, genus Isoetes, are also homosporous
1. Leaves are elongate microphylls resembling porcupine quills
2. Emerge from hard base containing roots
3. Most live submerged for part of the year
4. Life cycle is similar to Selaginella

5. Arthrophyta: Horsetails
1. Commonly called scouring rushes, are heterosporous and herbaceous
1. A single genus, Equisetum
2. Grow worldwide, mostly in damp places fig 33.16

2. Description of body form
1. Sporophytes are ribbed, jointed photosynthetic stems
2. Arise from branching underground rhizomes with roots at nodes
3. Whorl of scalelike nonphotosynthetic leaves at each stem node
4. Stems are hollow, contain silica deposits in epidermal cells
5. Possess two kinds of canals, outer ones contain air or inner ones water

3. Horsetail species divided into two groups
1. Sporophytes of one group are unbranched, develop cone-shaped strobili at tips
1. Strobili consist of spore-bearing sporangiophores
2. Produced around a central axis

2. Sporophytes of other group have terminal strobili on nonphotosynthetic stems
1. Photosynthetic stems are highly branched
2. Resemble a horse's tail

4. Horsetail spores have two ribbonlike elaters
1. Uncoil to aid in spore dispersal from sporangium
2. Curl around spore when damp, promoting deposition in correct location

5. Gametophytes are small, either male or bisexual
1. Archegonia develop before antheridia in bisexual forms
2. Numerous flagellated sperm swim to archegonia
3. Fertilization occurs, zygote develops into new sporophyte

33.4 Seed plants like pine trees and roses have protected embryos specialized for dispersal

1. Seed Plants
1. Evolution and Ecology
1. Derived from single, common ancestor
2. Ancestors known as progymnosperms, similar to modern gymnosperms
1. Possess xylem and phloem
2. Some species had leaves
3. Reproduction very simple

3. Advancement of seed
1. Protects embryo
2. Enhances dispersal
3. Precludes need for water, seed has survival value

4. Reproduction
1. Male and female gametophytes develop separately within sporophyte
2. Gametophyte completely dependent on sporophyte
3. Immature male gametophytes called pollen grains, arise from microspores
4. Carried to female, no free water required for fertilization
5. Female gametophyte develops from megaspore within an ovule
1. Angiosperms ovules completely enclosed by sporophyte tissue at pollination
2. Gymnosperms ovules not completely enclosed by sporophyte tissue at pollination

6. Pollination is the transfer of pollen by insects, wind or other agents

2. Gymnosperms
1. Basic Characteristics
1. Include conifers, cycads, ginkgoes and gnetophytes fig 33.17
1. Ovule, becomes seed, rests on an exposed scale
2. Not completely enclosed by sporophyte tissue at pollination

2. Name means "naked seed"
1. Ovules naked at time of pollination
2. Seeds may be enclosed by sporophyte tissue at maturity

3. Characteristics of diverse groups
1. Motile sperm in cycads and ginkgo, borne within pollen tube
2. Others have sperm without flagella
3. Female cone varies greatly in size

2. Coniferophyta: The Conifers
1. Conifers are most familiar group fig 33.17a
1. Include pine, spruce, fir, hemlock and cypress
2. Redwood is tallest plant, bristlecone pine is oldest
3. Found in colder temperate, drier regions of world
4. Great economic value, timber, paper, resins, turpentine

2. Pines
1. 100 species native to northern hemisphere
2. Most have needle-like leaves, in clusters of two to five
1. Evolutionary advance to retard loss of water
2. Have canals into which resin is secreted, deters insect and fungal attack

3. Wood different from woody flowering plants
1. Wood consists primarily of tracheids
2. Lacks vessel or fiber members
3. Absence of fibers causes wood to be "soft"
4. Broad leaved tree wood is considered "hard"

4. Thick bark adapted to survive fires and subzero temperatures

3. All seed plants are heterosporous fig 33.18
1. Pine male gametophytes develop from pollen grains
2. Pollen grains produced in male cones, cluster at tips of lower branches
1. Composed of small, papery scales arranged in spiral or whorl
2. Pair of microsporangia form within each scale
3. Microspore mother cells undergo meiosis, form four microspores
4. Microspores develop into 4-celled pollen grains with pair of air sacs

3. Female cones produced on upper branches of same tree
1. Larger than male cones, scales become woody at maturity
2. Two ovules develop toward base of each scale
3. Ovule contains megasporangium embedded in nutritive nucellus
4. Nucellus completely surrounded by thick integument, opening called micropyle
5. One integument layer becomes seed coat
6. Single megaspore mother cells undergoes meiosis, forms row of four megaspores
7. Three break down, one develops into female gametophyte
8. Each gametophyte produces two to six archegonia, each contains an egg

4. Female cones may take two or more seasons to mature
1. During first spring are green, scales spread apart
2. Pollen grains carried by wind, catch on fluid oozing out of micropyle
3. Pollen grains drawn through micropyle to top of nucellus
4. Scales then close
5. Archegonia and other female parts not mature for another year

5. Pollen tube emerges from pollen grain at bottom of micropyle
1. Digests through nucellus into archegonia
2. Pollen's generative cell divides by mitosis, one cell divides again
3. Last two cells function as sperm
4. Mature male gametophyte is germinated pollen grain, pollen tube, two sperm

6. In 15 months pollen tube reaches an archegonium
1. Discharges contents into it
2. One sperm unites with egg forming zygote
3. Other sperm and cells degenerate

7. Zygote develops into embryo within a seed
8. Seed disperses, germinates, grows into new sporophyte tree

3. Cycadophyta: Cycads
1. Slow growing, found in tropics and subtropics
2. Cycads resemble pines and palm-like leaves fig 33.17b
3. Reproduction
1. Produce cones, have life cycle similar to pines
2. Female cones develop upright among leaf bases
3. Sperm have thousands of spirally arranged flagella
4. Sperm still conveyed to archegonium by pollen tube

4. Several species facing extinction

4. Gnetophyta: Gnetophytes
1. Three genera, 70 species living
1. Closest living relative of angiosperms, probably share common ancestor
2. Only gymnosperms with vessels in xylem, same as angiosperms

2. Gnetophytes differ greatly from one another
1. Welwitschia stem shaped like large, shallow cup fig 33.17c
1. Tapers int tap root
2. Two strap-shaped, leathery leaves that grow continuously
3. Reproductive structures are conelike, appear at bases of leaves
4. Produced on separate male and female plants

2. Ephedra comprise more than 35 species
1. Common in arid regions of U.S and Mexico
2. Shrubby plants with jointed stems, scalelike leaves at each node
3. Male and female reproductive structures produced on same or different plants
4. Natural source for drug ephedrine

3. Gnetum species are tree or vinelike
1. Have broad leaves similar to angiosperms
2. One species cultivated for tender shoots

5. Ginkgophyta: Ginkgo
1. Fossil species once widely distributed, only one species remains Ginkgo biloba
1. Historically found in Japan and China, now a cultivar in U.S.
2. No longer exists in wild fig 33.17d

2. Fan-shaped leaves resemble leaflets of maidenhair fern
3. Reproductive features
1. Sperm have flagella
2. Reproductive structures produced on separate trees
3. Fleshy outer coverings of female seeds are foul smelling
4. Male plants generally planted, propagated from shoots

4. Very resistant to air pollution, planted commonly in cities

3. Angiosperms
1. Flowering Plants fig 33.19
1. Ovules enclosed by sporophytic tissue at pollination
1. Vessels of angiosperm are structures called carpels
2. Like an inrolled leaf with seeds along margins
3. Seed develops from ovule within a carpel
4. Carpel is part of an ovary (gynoecium) that develops into fruit

2. Great variety
1. Huge Tasmanian Eucalyptus trees to tiny duckweeds
2. Leaves may be succulent, floating, submerged, cup-shaped, spinelike and so forth
3. Seeds may be nearly microscopic or six meters long
4. Flowers may be simple buttercup blossoms to complex orchids
5. May be parasitic, mycotrophic or epiphytic

2. Structure of Flowers
1. Flowers are modified stems bearing modified leaves
2. Share specific features fig 33.20
1. Originates as a primordium
2. Develops into a bud at the end of a pedicel stalk
3. Expands slightly at base to form receptacle, to which other parts attach

3. Angiosperm flower composed of four whorls
1. Outermost calyx is composed of sepals
1. Three to five in number, green in color
2. Function to protect immature flower
3. May drop off when flower opens

2. Next whorl is corolla is composed of petals
1. Number three to five
2. May be separate, fused or absent

3. Calyx and corolla may be petallike, collectively called perianth
4. Next whorl below is composed of stamens
1. Collectively called the androecium
2. Stamen composed of filament and anther

5. Innermost whorl collectively called gynoecium
1. Composed of one or more carpels, also called pistils
2. Early form may have been leaflike structure with ovules along margins
3. Edges of blade rolled inward, fused together forming carpel
4. May have several to many separate pistils
5. Usually two to several carpels fuse into a compound pistil

6. Pistil has three main regions
1. Ovary contains ovules, develops into fruit
2. Tip is stigma, pollen grains adhere
3. Style connects stigma and ovary

7. Many flowers have nectaries, glands that secrete nectar

4. Angiosperms once thought to be evolved from gymnosperms
1. Simple flowers with missing parts considered primitive
2. Complex flowers considered more advanced
3. Floral simplification now considered to be evolutionary advancement

5. Angiosperms no longer thought to be evolved from existing gymnosperms

3. The Angiosperm Life Cycle
1. Single megaspore mother cell produces four cells via meiosis fig 33.21
1. Three disintegrate, one survives, divides mitotically
2. Each daughter nucleus divides twice resulting in eight haploid nuclei
3. Arranged in two groups of four

2. Integument layers differentiate to become seed coat
1. Leave small opening, micropyle, at one end fig 33.20

3. One nucleus from each group migrate to center
1. Function as polar nuclei
2. May fuse forming single diploid nucleus
3. May form single cell with two haploid nuclei

4. Cell walls form around remaining nuclei
1. Cell closest to micropyle functions as egg
2. Other two nuclei are synergids

5. Three cells at other end now called antipodals
1. No apparent function
2. Eventually disintegrate

6. Large sac called embryo sac
1. Has eight nuclei in seven cells
2. Constitutes the female gametophyte

7. Development of the male gametophyte in the anthers
1. Most anthers have four patches of tissue
1. Form chambers lined with nutritive cells
2. Each patch composed of many diploid microspore mother cells
3. Undergo simultaneous meiosis to produce four microspores each

2. Four microspores remain together as a tetrad
1. Nucleus of each divides once
2. Quartet then separates
3. Two layered wall develops around each microspore

3. Wall between chambers breaks down, leaves two larger sacs
1. Binucleate microspores are now pollen grains
2. Outer layer called exine, is sculpted, contains chemicals
3. May also have apertures through which pollen tube may emerge

8. Pollination is the mechanical transfer of pollen from anther to stigma
1. Pollen carried by various animals
2. Some plants self-pollinate

9. Pollination may be followed by fertilization
1. Requires genetic compatibility between pollen and stigma
2. Pollen grain absorbs substances from receptive stigma
3. Pollen's cytoplasm bulges through aperture forming pollen tube
4. Pollen tube responds to chemicals from embryo sac
5. Tube follows concentration gradient and grows down style into micropyle
6. Generative nucleus lags behind, divides to produce two sperm nuclei
7. Pollen grain now the mature male gametophyte
1. Contains tube nucleus
2. Contains two sperm

10. Angiosperms exhibit unique process of double fertilization
1. Pollen tube enters embryo sac, destroys a synergid, discharges its contents
2. One sperm fuses with egg and forms zygote, develops into embryo
3. Second sperm fuses with polar nuclei
1. Forms triploid primary endosperm nucleus
2. Gives rise to nutritive endosperm

4. Developing embryos derive nutrition from endosperm
1. In grasses like corn, becomes extensive part of seed
2. In bean and pea disappears when seed is mature fig 33.22

5. Integuments harden and become seed coat
6. Haploid cells in embryo sac disintegrate