Overview of Green Plants Chapter 30

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

Defining Plants

Defining Plants Land plants have two major features Protected embryos Multicellular haploid and diploid phases

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

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.

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)

The General Plant Life Cycle process

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

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

Chlorophytes Volvox Colonial chlorophyte Hollow sphere of a single layer of 500-60,000 cells A few cells are specialized for reproduction

Chlorophytes Ulva Multicellular chlorophyte True haplodiplontic life cycle Gametophyte and sporophyte have identical appearance

Ulva life cycle MITOSIS

Charophytes Charophytes are green algae related to land plants

Charophytes Charales (300 species) Choleocaetales (30 species) Macroscopic Plant-like plasmodesmata Sister clade to land plants Choleocaetales (30 species) Microscopic Plant-like mitosis Next closest plant relatives

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

Bryophytes Liverworts (phylum Hepaticophyta) Have flattened gametophytes with liver-like lobes Form gametangia in umbrella-shaped structures Also undergo asexual reproduction

Bryophytes Hornworts (phylum Anthocerotophyta) Sporophyte has stomata Sporophyte is photosynthetic Cells have a single large chloroplast

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

Moss Reproduction

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)

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

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

Tracheophytes Vascular plants (tracheophytes) include seven extant phyla grouped in three clades Lycophytes (club mosses) Pterophytes (ferns and their relatives) Seed plants

Lycophytes Club mosses are the earliest vascular plants They lack seeds Superficially resemble true mosses but they are not related Homosporous or heterosporous

Pterophytes The phylogenetic relationships among ferns and their relatives is still being sorted out

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

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”

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

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

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

Pterophytes

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

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

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

Seed Plants 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

Gymnosperms There are four living groups Cycadophytes Gnetophytes Ginkgophytes Coniferophytes

Gymnosperms Cycads (phylum Cycadophyta) Slow-growing gymnosperms of tropical and subtropical regions Sporophytes resemble palm trees Have largest sperm cells of all organisms!

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

Gymnosperms Ginkgophytes (phylum Ginkgophyta) Only one living species remains Ginkgo biloba Dioecious Male and female reproductive structures form on different trees

Gymnosperms 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 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

Pine reproduction

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

Angiosperms

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

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

The ovary eventually develops into the plant’s fruit Angiosperms The ovary eventually develops into the plant’s fruit

Angiosperm Reproduction