Diversity of Life : Plants: UNIT 7

Topic 7: Diversity of Life – Plants

Learning Objectives

• List differences between algae and plants and the adaptations plants needed to evolve to live on land

• Identify different plant organs and structures

• Explain the lifecycle of plants including sporophyte and gametophyte generations

• Describe, list defining characters, and give examples of bryophytes, seedless vascular plants, and seed vascular plants

• Describe male and female reproductive parts of seed plants

• Identify the parts of a flower anatomy and explain the reproductive purpose of a flower and a fruit

• Explain flower adaptations that increase pollination

• Describe past and future evolution of plants in terms of humans and agriculture

Plant Ancestry

Origin of Plants

• Plants and green algae evolved from a common ancestor.

• Algal ancestors likely inhabited coastal salt marshes, facing challenges such as shallow water and periodic drying.

• Natural selection favored algae that could survive dry periods.

• Early ancestors may have resembled Coleochaete, found at lake edges.

How do Algae Differ From Plants?

Key Differences

• Algae are anchored by a holdfast rather than true roots.

• No rigid tissues; supported by water pressure.

• Entire body absorbs CO2 and minerals from water.

• Most cells can perform photosynthesis.

• Flagellated sperm swim to fertilize eggs; offspring dispersed by water.

Adaptations to Life on Land

Advantages and Challenges

• Oldest land plant fossils: 470 million years ago.

• Advantages: Unlimited sunlight, abundant CO2, fewer pathogens/herbivores.

• Challenges:

  • Maintaining moisture inside cells

  • Supporting body in air

  • Reproducing and dispersing offspring without water

  • Anchoring in soil

  • Obtaining resources from both soil and air

Key Adaptations

• Maintaining moisture: Waxy cuticle covers aboveground parts; stomata allow gas exchange.

• Obtaining resources: Roots absorb water/minerals; leaves/stems gather light and CO2.

• Support: Cell walls reinforced by lignin, especially in xylem tissue.

• Reproduction and dispersal:

  • Gametangia: Protective cell jackets around gamete-producing cells.

  • Pollen grains: Structures containing sperm-producing cells (in seed plants).

  • Embryophytes: Zygote nourished by parent plant.

Plant Organs and Structures

Major Organs

• Roots: Anchor plant, absorb water and minerals.

• Stems: Support and bear leaves.

• Leaves: Photosynthesis, absorb CO2 and sunlight.

Other Structures

• Apical meristem: Region of cell division at tips of roots and stems; enables growth and branching.

• Vascular tissue: Network of tubes for transport.

  • Xylem: Dead cells forming pipes for water/mineral transport.

  • Phloem: Living cells distributing sugars.

Life Cycles: Alternation of Generations

Basic Cycle

• Gametophyte (N): Haploid generation producing gametes (eggs and sperm).

• Sporophyte (2N): Diploid generation producing spores in sporangia.

• Spore: Cell that develops into a new organism without fusing with another cell.

• Early land plants (mosses, ferns) rely on spores; seed plants use seeds for dispersal.

Evolution of Plant Diversity

Key Adaptations

• Dependent embryos

• Lignified vascular tissues

• Seeds for reproduction and dispersal

• Flowers to house seeds (in angiosperms)

Plant Diversity

Bryophytes

• Nonvascular plants (e.g., mosses, liverworts, hornworts)

• Have apical meristems, retain embryos on parent plant

• Lack true roots, leaves, and lignified cell walls

• Grow in dense mats; fertilization facilitated by water films

Seedless Vascular Plants

• Lycophytes: Club mosses

• Monilophytes: Ferns

• Require moist conditions for fertilization

• Spores dispersed by air; ferns have well-developed roots and stems

Seed Plants

• 90% of living plants

• Seed: Protective covering for embryo and food supply

• Gymnosperms: Naked seeds (e.g., conifers)

• Angiosperms: Flowering plants; seeds develop within protective chambers

Alternation of Generations

Stage	Chromosome Number	Main Function
Gametophyte	N (haploid)	Produces gametes
Sporophyte	2N (diploid)	Produces spores

Pollen and Seeds

Pollen

• Pollen grains carry sperm-producing cells through the air.

• Sporophyte has specialized structures for all reproductive stages.

• Cones in gymnosperms: modified shoots with scales containing sporangia.

Reproductive Structures

Male Structures

• Haploid spores develop into pollen grains (male gametophytes).

• Pollen grains are enclosed within a tough wall.

• Pollination: pollen lands on a compatible female structure, undergoes mitosis to produce sperm (nucleus only).

Female Structures

• Haploid spores develop into ovules (egg-producing female gametophytes).

• Pollen tube enters ovule, releases sperm nucleus.

• Fertilization: fusion of sperm and ovule nuclei forms diploid zygote, which becomes sporophyte embryo.

• Ovule and surrounding tissue mature into seed; seed coat may aid dispersal.

Flowers and Flower Anatomy

Structure and Function

• Flowers are the reproductive centerpiece of angiosperms.

• Sites of pollination and fertilization; contain male and female sporangia and gametophytes.

• Composed of modified leaves attached to a receptacle.

Parts of a Flower

Part	Function
Sepals	Usually green, protect flower bud
Petals	Attract animal pollinators
Stamens	Male reproductive structure (filament + anther)
Carpel	Female reproductive structure (stigma, style, ovary, ovule)

Angiosperm Lifecycle

• Sporophyte (2N) dominates, with gametophyte (N) within its body.

• Pollination and fertilization lead to seed and fruit formation.

Advantageous Adaptations of Angiosperms

• Flowers attract pollinators, increasing reliability of pollen transfer.

• Rapid reproduction: fertilization can occur within hours; seeds produced quickly.

• Fruits protect and help disperse seeds.

Fruits

Fruit Function and Dispersal

• Fruit: Ripened ovary of a flower.

• Dispersal methods: wind (e.g., dandelion), animals (e.g., cocklebur), ingestion and excretion by animals.

• Bright colors attract animals when ripe; tough seeds survive digestion.

Humans and Angiosperms

Importance to Humans

• Main food sources: dry fruits (corn, wheat, rice, other grains).

• Fleshy fruits: apples, cherries, oranges, tomatoes, squash, cucumbers.

• Spices: nutmeg, cinnamon, cumin, cloves, ginger, licorice, black pepper.

Additional info:

• Plant diversity is crucial for agriculture, offering genetic resources for crop improvement and resilience.

• Domestication has reduced food diversity to a few genotypes, increasing vulnerability to pests and diseases.

• Biotechnology and artificial selection are used to improve crop traits and adapt to changing environments.