Overview of Green PlantsChapter 30

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Defining Plants• The kingdom Viridiplantae includes land

plants and green algae– Red and brown algae are excluded

• The green algae split into two major clades– Chlorophytes– Charophytes

• Streptophyta includes the Charophytes and all land plants

• All green plants arose from a single species of freshwater algae

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Defining Plants

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Defining Plants

• Land plants have two major features

1. Protected embryos

2. Multicellular haploid and diploid phases

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Defining Plants

Adaptations to terrestrial life

• Evolution of leaves that increase photosynthetic surface area

• Protection from desiccation by a waxy cuticle and stomata

• Shift to a dominant vertical diploid generation

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Plant Life Cycles

• Humans have a diplontic life cycle– Only the diploid stage is multicellular

Plants have a haplodiplontic life cycle– Multicellular diploid stage = Sporophyte– Multicellular haploid stage = Gametophyte

• Plants have an alternation of generation– sporophyte → gametophyte → sporophyte → etc.

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Plant Life Cycles

• Sporophyte (2N) produces haploid spores (N) by meiosis

• Spores (N) divide by mitosis producing the gametophyte (N)

• Gametophyte (N) produces gametes (N) by mitosis

• Gametes (N) fuse to form the diploid sporophyte (2N)

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The General Plant Life Cycle

process

process

process

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Plant Life Cycles

As more complex plants evolved:

• Diploid stage (sporophyte) became the dominant portion of the life cycle

• Gametophyte became more limited in size

• Sporophyte became nutritionally independent

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Chlorophytes• Chlorophytes, sister taxa of the Streptophytes,

are a diverse group including:

• Chlamydomonas– Unicellular chlorophyte

with two flagella– Have eyespots to direct

swimming– Reproduces asexually as

well as sexually

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Chlorophytes

• Volvox – Colonial chlorophyte

– Hollow sphere of a single layer of 500-60,000 cells

– A few cells are specialized for reproduction

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Chlorophytes

• Ulva – Multicellular chlorophyte– True haplodiplontic life

cycle– Gametophyte and

sporophyte have identical appearance

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MITOSIS

Ulva life cycle

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Land plants

Charophytes

• Charophytes are green algae related to land plants

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Charophytes

• Charales (300 species) – Macroscopic– Plant-like plasmodesmata– Sister clade to land plants

• Choleocaetales (30 species) – Microscopic– Plant-like mitosis– Next closest plant relatives

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Bryophytes

• Bryophytes are the closest living descendants of the first land plants– Called nontracheophytes because they lack

tracheids (specialized transport cells)– Simple, but highly adapted to diverse terrestrial

environments– Non-photosynthetic sporophyte is nutritionally

dependent on the gametophyte– 3 groups: liverworts, hornworts and mosses

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Bryophytes

• Liverworts (phylum Hepaticophyta)– Have flattened gametophytes with liver-like

lobes

– Form gametangia in umbrella-shaped structures

– Also undergo asexual reproduction

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Bryophytes

• Hornworts (phylum Anthocerotophyta)– Sporophyte has stomata

– Sporophyte is photosynthetic

– Cells have a single large chloroplast

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Bryophytes

• Mosses (phylum Bryophyta)– Gametophytes consist of small, leaflike

structures around a stemlike axis– Anchored to substrate by rhizoids– Multicellular gametangia form at the tips of

gametophytes• Archegonia – Female gametangia• Antheridia – Male gametangia

– Mosses withstand drought, but not air pollution

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Moss Reproduction

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Tracheophyte Plants

• Cooksonia, the first vascular land plant, appeared about 420 MYA

– Only a few centimeters tall

– No roots or leaves– Homosporous

(spores are the same size and type)

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Tracheophyte Plants

• Vascular tissues are of two types– Xylem – Conducts water and dissolved

minerals upward from the roots• contains tracheids

– Phloem – Conducts sucrose and hormones throughout the plant

• These enable enhanced height and size in the tracheophytes

• Tracheophytes are also characterized by the presence of a cuticle and stomata

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Tracheophyte Plants

• Vascular plants have gametophytes reduced in size and complexity relative to sporophytes

• Seeds– Highly-resistant structures that protect the plant

embryo– Occur only in heterosporous plants

• Fruits in flowering plants add a layer of protection to seeds – Also attract animals that disperse seeds

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Tracheophytes

• Vascular plants (tracheophytes) include seven extant phyla grouped in three clades

– Lycophytes (club mosses) – Pterophytes (ferns and their relatives) – Seed plants

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Lycophytes

• Club mosses are the earliest vascular plants– They lack seeds

– Superficially resemble true mosses but they are not related

– Homosporous or heterosporous

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Pterophytes• The phylogenetic relationships among ferns

and their relatives is still being sorted out

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Pterophytes

• Whisk ferns– Saprophyte consists of evenly forking green

stems without leaves or roots

– Some gametophytes develop elements of vascular tissue

– Often symbionts with fungi

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Pterophytes• Horsetails

– All 15 living species are homosporous– Constitute a single genus, Equisetum

– Consist of ribbed, jointed photosynthetic stems that arise from branching rhizomes

– High silica content in stems made them useful as “scouring rushes”

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Pterophytes

• Ferns– The most abundant group of seedless vascular

plants with about 11,000 species

– The conspicuous sporophyte and much smaller gametophyte are both photosynthetic

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Pterophytes

• The fern life cycle differs from that of a moss– Much greater development, independence and

dominance of the fern’s sporophyte

• Fern morphology– Sporophytes have rhizomes– Fronds (leaves) develop at the tip of the

rhizome as tightly rolled-up coils then uncoil and expand

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Pterophytes

Uncoiled fronds are called “fiddleheads” and are a delicacy among northern First Nation peoples

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Pterophytes

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Pterophytes

• Fern reproduction– Most fern are homosporous– Produce distinctive sporangia in clusters called

sori on the back of the fronds– Diploid spore mother cells in sporangia produce

haploid spores by meiosis– At maturity, the spores are catapulted by

snapping action

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Seed Plants

• Seed plants first appeared 305-465 MYA– Evolved from spore-bearing plants known as

progymnosperms

• The seed represents an important advance– Protects the embryo– Easily dispersed– Introduces a dormant phase in the life cycle

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Seed Plants

• Seed plants produce 2 kinds of gametophytes– Male gametophytes

• Pollen grains• Dispersed by wind or a pollinator

– Female gametophytes • Develop within an ovule• Enclosed within diploid sporophyte tissue

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• There are two types of seed plants– Gymnosperms are plants with “naked seeds”

• Ovule is exposed on a scale at pollination• All lack flowers and fruits of angiosperms

– Angiosperms are flowering plants• Ovules are enclosed in diploid tissue at pollination• The carpel (modified leaf) covers seeds and

develops into fruit

Seed Plants

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• There are four living groups– Cycadophytes– Gnetophytes– Ginkgophytes– Coniferophytes

Gymnosperms

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Gymnosperms

• Cycads (phylum Cycadophyta)– Slow-growing gymnosperms of tropical and

subtropical regions

– Sporophytes resemble palm trees

– Have largest sperm cells of all organisms!

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Gymnosperms

• Gnetophytes (phylum Gnetophyta)– Only gymnosperms with vessels in their xylem

– Contain three (unusual) genera

• Welwitschia• Gnetum• Ephedra

– ephedrine can be extracted from species of this genus

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Gymnosperms

• Ginkgophytes (phylum Ginkgophyta)– Only one living species remains

• Ginkgo biloba

– Dioecious• Male and female

reproductive structures form on different trees

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• Conifers (phylum Coniferophyta) are the largest gymnosperm phylum and include:– Pines, spruces, firs, cedars and others– Coastal redwood – Tallest tree– Bristlecone pine – Oldest living tree

• Conifers are sources of important products• Timber, paper, resin and taxol (anti-cancer)

Gymnosperms

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• Pines– More than 100 species, all

in the Northern hemisphere

– Produce tough needlelike leaves in clusters

– Leaves have:• Thick cuticle and recessed

stomata• Canals into which cells

secrete resin

Gymnosperms

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Pine reproduction

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Angiosperms• Angiosperm origins are a mystery

• The oldest known angiosperm in the fossil record is Archaefructus

• The closest living relative to the original angiosperm is Amborella

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Angiosperms

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Angiosperms

• Flower morphology– Primordium develops into a bud at the end of

a stalk called the pedicel– Pedicel expands at the tip to form a

receptacle, to which other parts attach

– Flower parts are organized in circles called whorls

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Angiosperms

• Flower morphology– Outermost whorl = Sepals– Second whorl = Petals– Third whorl = Stamens (androecium)

• Each stamen has a pollen-bearing anther and a filament (stalk)

– Innermost whorl = Gynoecium• Consists of one or more carpels that house the

female gametophyte

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Angiosperms

The ovary eventually develops into the plant’s fruit

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Angiosperm Reproduction