Plant Diversity 1: How Plant Colonized Land

The evolutionary history of plants begins as the story of adaption to land life.

What we can plants, truly land-adapted multicellular photoautotrophs , are descended from a lineage of one of the “green Algae” and thus are in the clade Archaeplastida.

(1) Evidence supporting that photo-autotrophs plants, are descended from “green Algae”

Chlorophytes, Charophytes & Plantae share:

• multicellularity

• cell walls with cellulose

  • chloroplasts with same pigment (chlorophyll. a & b). Storage molecule is starch.

(2) Evidence supporting that photo-autotrophs plants, are descended from “green Algae”

Charophytes also share with Plantae:

• Similar flagellated sperm

• Cellulose synthesis proteins (rings)

• Cytokinesis process (cell plate formation)

(3) Evidence supporting that photo-autotrophs plants, are descended from “green Algae”

Sporo-pollenin - a durable polymer that protects:

• zygotes in charophytes

• spores in seedless plants

• pollen grains in seed plants

How did selection for land life start?

By 470 mya, multicellular green algae expanded from shallow seas into river & lakes. Green algae along lake shores meant more resources; less competition (=drying), subjected to periodic dehydration, and population with land adaptations thrive.

Advantages of land:

• [CO2] higher

• light intensity higher

• more minerals

• no “herbivores”

• no competition

Disadvantages of Land:

• Risk of desiccation (both adult & gametes)

• Divided resources (air vs. land)

• No “support” in air

8 Mechanisms explain how plants derived their characteristics:

1. Alternation of generations

2. Walled haploid spores

3. Multicellular gametangia

4. Sporophyte embryos grow protected with female gametophyte

5. Apical Meristems

6. Waxy cuticle

7. Secondary Compounds

8. Mycorrhizae

Mechanism #1: Alternation of generations

Plant sporophyte (2n) makes huge # unique diverse offspring per fertilization.

Mechanism #2: Walled haploid spores

Dispersal thru air: sporo-pollenin protects. Made within multicellular sporangium.

Mechanism #3: Apical Meristems (SCs)

Continuously dividing cells. Roots & shoots grow toward resources.

Mechanism #4: Multicellular gametangia

Protecting sperm within antheridium. Egg protected with archegonium.

Mechanism #5: Sporophyte embryos grow protected within female gametophyte.

Nourished via placental transfer cells.

Mechanism #6: Waxy cuticle

Waxy coats stop desiccation (=drying). Pore needed to allow CO2/O2 exchange - controlled by stoma in most plants.

Mechanism #7: Secondary Compounds

Chemicals that deter, repel or poison competitors, herbivores, & parasites. First products such as fruits.

Mechanism #8: Mycorrhizae

Mutualism with fungi; helps water & mineral absorption. Dates back to first land plants (before true roots).

Major Plant Groups

• Non-vascular Plants

• Vascular (seedless) Plants

• Vascular (seed) Plants

  • Gymnosperms

  • Angiosperms

What is the ecological important of Bryophytes?

“Pioneer” species in poor soils. Moss are major 1° producers in cold or high regions. Sphagnum “peat moss” bogs: important wetlands. Also harvested for fuel & gardening.

What are Bryophytes?

Lack vascular & supportive tissue. Very thin body: depends on diffusion. With rhizoids for attachment (not true roots).

Bryophyte Characteristics:

Haploid gametophyte is dominant form:

• Dominant = longest lasting or largest

• Make eggs & flagellated sperm

• Most are small, low growing moist areas

Diploid sporophyte depends on --- for food and water.

gametophyte

Phloem

Move sugar & organic products.

Xylem

Carries water & materials - most vascular plants have tracheids with walls strengthened by tough polymer: lignin.

Evolution of Vascular Plants

Unbranched sporophyte → branched sporophyte → supportive vascular tissue → Sporophyte photosynthesis increases (no longer need food from gametophyte)

Roots evolve from --- ---.

underground stems

Leaves evolved from --- ---.

branched stems

Megaphylls

Branched veins

Sporophylls

Sporangia shift to leaves

How did the success of seedless vascular plants change the Paleozoic world?

Covered the earth, increasing organic C on land (for food); further increases in O2 levels; reduced CO2 levels; all favoring success of land animals.

Lycophytes: Relict Group

Closest group to ancestral vascular plants. All now very small. Include club moss & spike moss. Little economic/ecological importance today.

Lycophytes: Whisk ferns (Psilotum)

Dichotomous branching. No true leaves or roots (lost secondarily).

Lycophytes: Horsetails (Equisetum)

Jointed hollow stem with tiny leaves. Strobili.

Lycophytes: Ferns

Most widespread & diverse magnoliophytes. Large megaphylls

Fern Life Cycle

The life cycle of fern is different from other land plants as both the gametophyte and the sporophyte phases are free living.