Chapter 29: Plant Diversity

Major plant division

• Vascular

  • Lycophyta

  • Monilophyta

• Nonvascular: do not have vascular tissue 

  • Hepatophyta

  • Bryophyta

  • Anthocerophyta 

• Seedless plants 

  • Gymnosperms

  • Angiosperms: biggest category 

• There have been 5 key innovations during plant evolution

• A few trends are evident in plant evolution

  • Get bigger with the evolution of vascular tissue

  • Growth of the sporophyte phase

  • Shrinkage of the gametophyte phase

  • Live in a wider range of habitats with the evolution of seed

• Plants are important to the ecosystem in general

  • Oxygen 

  • Food

  • Create habitat

  • Fuel 

  • Medicine

Archaeplastida (=Plantae)

• Archaeplastida consists of two major groups 

  • Red algae 

  • Green algae + land plants

  • Land plants are nested within green algae (i.e., green algae are paraphyletic with respect to land plants)

What is a Plant?

• Multicellular, photosynthetic organisms

  • About 325,00 spp

  • Cell walls of cellulose 

  • Chloroplasts with chlorophylls a and b

  • These traits found among protists as well

• True land plants (e.g., mosses, ferns, gymnosperms, and angiosperms) are called embryophytes 

• Their embryos are dependent on the maternal plant [all land plants have this is common]

Land plants synapomorphies

• All land plants share 4 key features 

1. Alternation of generations with multicellular dependent embryos 

2. Walled spores that contain sporopollenin 

• Spores produced in structures called sporangium 

• Charophytes lack multi-celled sporangia; no sporopollenin in their spores (but present in their zygotes) 

3. Multicellular gametangia: specialized structures where gametes are found 

4. Apical meristems: areas of growth on shoot and root tips

• Apical meristem cells can divide throughout the plant’s life, enabling its roots and shoots to elongate and increase exposure to environmental resources 

Five Key Events in Land Plant Evolution

• Multicellularity (1.4 Bya) 

• Invasion of land (470 Mya)

• Vascular tissue (425 Mya) 

• Seed plants (360 Mya)

• Flowers (140 Mya)

Grades of plant morphology 

• Nonvascular plants

  • Liverworts, hornworts, mosses

• Vascular plants

  • Free-sporing plants: lycophytes (clubmosses) and monilophytes (ferns and friends)

  • Seed plants: gymnosperms (e.g., pine trees & cycads) and angiosperms (flowering plants)

Earliest Land Plants Lacked Vascular Tissue 

• Vascular tissue: specialized cells that transport water, nutrients, etc.

• Bryophytes are nonvascular plants

  • Three groups

    • Liverworts

    • Hornworts

    • Mosses

• Form a “grade” rather than a clade; “bryophyta” is not monophyletic 

  • Typically form low “carpetes” rather than upright branches 

• Bryophyte life cycles dominated by short gametophyte, with tall, slender sporophyte 

Plant Life Cycle

• Alternation of generations: alternating, multicellular haploid (n) and diploid (2n) phases 

  • Sporophytes (2n) produces spores (n) by meiosis

  • Spores (n) development into gametophytes (n)

  • Gametophytes (n) produce gametes (n) by mitosis 

  • Gametes (n) fuse to produce sporophytes (2n)

Dominant phase varies among plant groups 

Bryophyte life Cycles Dominated by Gametophyte

• Gametophytes (n) develop from spores (n)

• Larger and longer living relative to sporangia

• Gametophytes (n) have either female or male gametangia 

  • Archegonia: produce eggs

  • Antheridia: produce flagellated sperm 

• Sperm must swim thru water to fertilize eggs

  • Limits mosses to moist habitats 

Innovations of Vascular Plants 

• Vascular plant life cycle dominated by sporophyte, with reduction of the gametophyte 

• Vascular tissue: xylem to transport water, phloem to transport organic molecules 

  • Reinforced with lignin: allowed plants to get bigger

• Roots: organs to absorb water & nutrients from soil; not merely anchors 

  • Allowed plants to get bigger

• Leaves: increase surface area for photosynthesis

  • Allowed plants to get bigger

  • Microphyll

    • Small, spine-shaped leaves with unbranched vasculature (lycophytes)

  • Megaphyll

    • Highly branched vasculature

  • Sporophylls: modified leaves bearing sporangia 

    • Many sporangia per plant

Seedless Vascular Plant Life Cycles Dominated by Sporophyte Stage

• Fern life cycle similar to that bryophyte

  • Sporagnia organized into sori on the sporophylls 

• EXCEPT that: 

  • The (hermaphroditic) gametophyte stage is much smaller than the sporophyte 

Two Extant Phyla of Seedless Vascular Plants

• Phylum lycophyta, 1,200 spp. 

  • Club mosses, spike mosses, quillworts

• Phylum Monilophyta, 12,000 spp.

  • fern, horsetails, and whisk ferns

  • More closely related to seed plants: share derived characters related to leaf and root growth  

Lycophyta

• Clubmoss

• Quillwort

• Spikemoss

Monilophyta

• Fern

• Horsetail

• Whisk fern 

Five Key Events in Land Plant Evolution

• Multicellularity (1.4 Bya) 

• Invasion of land (470 Mya)

• Vascular tissue (425 Mya) 

• Seed plants (360 Mya)

• Flowers (140 Mya)

Three Evolutionary Innovations of seed plants

1. Further reduction of the gametophyte stage 

2. Variation in spore size

• Most seedless plants homosporous

• Heterosporous: male and female spores of different sizes 

  • Female megaspores produced by megasporangia 

    • Larger spore

• Male microspores produced by microsporangia 

  • Smaller spore 

3. Seeds: embryo + food supply in protective coat 

Male Gametophyte = Pollen

• Seedless plants: flagellated sperm swim from male gametophyte (antheridium) to female gametophyte (archegonium)

• Seed Plants: tiny male gametophyte transported to female gametophyte 

  • Microspore develops into pollen grain: male gametophyte in sporopollenin coat 

    • Sporopollenin coat: helps prevent drying out

  • Pollination: transport by wind or animal (e.g. bees) to female reproductive structure 

Two Major Extant Clades of Seed Plants

• Gymnosperm, “naked seeds” on cones 

  • Phylum Cycadophyta 

  • Phylum Gnetophyta

  • Phylum Ginkophyta 

  • Phylum Coniferophyta 

• Angiosperms, flowering plants, seeds “contained” in fruits 

  • Phylum Anthophyta 

    • Basal angiosperms (paraphyletic ambarella, water lilies, etc.)

    • Magnoliids 

    • Monocots

    • Eudicots 

The Gymnosperm Life Cycle Allows Them to Live in Drier Habitats

• Pine tree= mature sporophyte 

  • (male) pollen cones with microsporocytes 

  • (female) ovule comes with megasporocyte

• Microsporocyte develops into male gametophyte [pollen (haploid)] than makes sperm  

• Normally transferred by wind for gymnosperms 

• Megasporocyte develops into female gametophyte that makes eggs 

• Pollination leads to fertilization

  • No need for swimming

  • Seed contains embryo (2n), food reserve (n), maternal seed coat (2n) 

• Seed germinates to become seedling

• Whole cycle takes 3 years 

Fertilized Female Ovules Become Seeds

• Ovule = megaspore + megasporangium + integument 

  • Integument: sporophyte tissue (2n)

  • Megasporangium: sporophyte tissue (2n) 

  • megaspore  develops into female gametophyte 

  • Female gametophyte (n) produces egg (n)

  • Egg (n) fertilized by sperm (n) produced by male gametophyte (n) following pollination 

    • Sperm transported  via pollen tube thru the micropyle

    • Seed: sporophyte embryo (2n) encased in maternal sporophyte tissue 

What are the Advantages of Seeds?

• Seeds are better for dispersal than spores; can remain viable longer  

  • Spore single cells

  • Seeds have multicellular coats & a store of food

• Seeds can remain dormant until conditions are right for germination 

• Seeds can be dispersed farther 

Angiosperms take these Advantages Further with Flowers and Fruits

• Flowers: sex-organs made of up to 4 rings of specialized leaves (sporophylls)

  • Sterile 

    • Sepal: outermost ring, usually green 

    • Petals: may be brightly colored to attract pollinators (e.g., bees, hummingbirds)

  • Fertile 

    • Stamens: males flower-parts

      • Anthers on filaments; produce pollen 

    • Carpals: female flower-parts (1+ per flower)

      • Sticky stigma binds pollen; style connects it to ovary (1+ ovules) 

• Fruits: mature ovary

  • Many forms: fleshy, dry, etc. 

  • Aid in dispersal: can be carried by animals 

The Angiosperm life cycle is highly specialized

• The steps of the process should be familiar, with a few interesting modifications

• Female gametophyte has large central cell with 2 nuclei (n+n)

• Pollen releases 2 sperm

  • 1 fertilizes embryo (2n)

  • 1 fuses with 2 nuclei in central cell: endosperm (3n)

• Endosperm is the food supply for the seed 

Angiosperm Phylogeny

• Angiosperms are a monophyletic group

• Bennettitales are an extinct group that had a reproductive structure reminiscent of the flower 

Ancestral Angiosperm Flower

• Sauquet et al. 2017 reconstructed a hypothetical ancestral flower

• Characteristics

  • Hermaphroditic 

  • Petal-life organs in whorls

• Magnolia flowers are probably similar to the ancestral angiosperm flower

Animals are dependent on plants, and vice versa

• Herbivory: animals feeding on plants; plants have various adaptation to keep from being eaten

  • E.g., chemical: caffeine, nicotine, morphine, cyanide

  • E.g., mechanical: thorns, silica crystals 

• Many fruits are adapted for animal transport 

• Plant-pollinator relationships maybe highly specialized 

  • Co-evolution of insects and flowering plants may explain their great diversity 

Plants have many uses

• Food: 80% of our calories come from only 6 crops (mostly rice, corn, and wheat)

• USA: we use corn in everything 

• Food for other food: plants to feed livestock

• Wood (shelter, fuel), fibers (clothing), moss for peat

• Herbal medicines (2° compounds)

• Nutrient cycling

• Fix carbon, use excess CO₂